Personality is a fundamental tool for both survival and reproduction. Most of the decisions that are fundamental for an individual life are influenced by one’s own and others’ personality traits. Individuals choose their partners and decide to reproduce based on their own and their partner’s agreeableness, conscientiousness, and openness. People respond to threats based on their conscientiousness and emotional stability. Individuals determine alliances, and decide to follow or lead a group, all based on their own personality and that of others. Personality differences characterize individuals not only in humans; such differences have also been discovered in a constantly growing number of animals (Uher, 2008).
The diffusion of lethal epidemics in certain regions and certain times, such as the Black Death, plague, AIDS, and so forth, might have differentially impacted particular individuals based on their behavioral traits. For example, individuals with high conscientiousness, high harm avoidance, and low novelty seeking and promiscuity, as well as the most closed and introverted individuals, might have had a lower chance of contamination compared to others. These epidemics were associated with a high mortality rate, which could have led to the natural selection of certain traits that consequently spread into the surviving population. Also, the depletion of environmental resources that caused individuals to migrate in search of richer environments might have positively selected novelty seeking and openness in migrating populations. Concomitantly, migration might have left the original population depleted of such traits (Chen, Burton, Greenberger, & Dmitrieva, 1999).
There is a general underestimation of the role of migration in shaping personality traits and transforming the allele frequency influencing personality traits in small populations. This led a few researchers to hypothesize that in small populations where there is a limited array of socio-ecological niches, individuals with personality traits not adapted to such socio-ecological niches would leave the population. They also hypothesized that if the population is sufficiently isolated, with no immigration, then the personality of the small isolated population would progressively change due to the constant gene outflow of emigrants. With continued isolation, the average personality of its inhabitants would shift toward the most adapted traits to survive in such isolated socio-ecological niches. Camperio Ciani and Ceccarini (2002) labeled this hypothesis the personality gene flow hypothesis.
In this chapter we introduce the assumptions and background for this hypothesis and describe how these researchers empirically tested it. This was done in a variety of Italian small island populations, all well isolated from the corresponding mainland.
In considering personality differences among populations, many authors have debated the role of environmental and genetic influences. The question of whether differences in personality really exist, and whether they are due to cultural and linguistic differences, or have a genetically selected adaptive value, is a controversial issue between social psychologists and behavioral geneticists. It is still unclear why selective pressures allow such a large variety of combinations of traits in populations. Why instead doesn’t natural selection drive toward a single most successful level or combination of personality traits for everyone? Selection generally favors the most successful traits and eliminates all others given enough time, thus transmitting the single most successful phenotype to the whole population at the expense (extinction) of all less successful ones (Fisher, 1930).
Variability in personality traits among individuals seems well recognized (McCrae & Costa, 1987), but whether this is true at the population level is far less clear (Church, 2010). Differences of opinion still exist as to how and why there is so much variability of personality within populations, and little convincing evidence has been found for reliable personality differences between populations that have different cultural and linguistic origins (Allik & McCrae, 2002, 2004; Eysenck & Yanai, 1985). Poortinga, Van de Vijver, and Van Hemert (2002) argued that a genetic explanation for group differences in personality traits is unlikely, since they found no persuasive reasons why certain traits should be differentially selected in various groups. Terracciano et al. (2005) studied perceptions of the typical personality of members of different cultures. They found that shared but unfounded national stereotypes showed little agreement with measured trait levels, even for people describing their own geographic area. Others have observed some resistance in recognizing genetically influenced population differences in behavioral traits. This has led most people to dismiss the existence of such differences (Crawford, 2007). However, such differences can be predicted from the perspective of evolutionary genetics.
In the framework of evolutionary psychology, there are good reasons to believe that individual variation within populations may be explained by genetic differences, which have an adaptive value to the local environment and social structure. It should therefore be expected that not only the physical environment but also various aspects of the socio-ecological environment could play a role in selecting for specific personality traits in individuals belonging to different populations. This means that different socio-ecological niches could in principle favor in different ways the most adapted individuals within the population (Bereczkei, 2000; Camperio Ciani, 2010; Turkheimer, 1998).
Behavioral genetic studies suggest that individual variability in personality traits has a substantial (30–60%) genetic component with specific genes closely associated with specific personality traits (Benjamin, Ebstein, & Belmaker, 2008; Benjamin et al., 1996; Ebstein et al., 1996; Jang, Livesley, Angleitner, Riemann, & Vernon, 2002; Plomin, Pedersen, Lichtenstein, & McLearn, 1994; Turkheimer, D’Onofrio, Maes, & Eaves, 2005). Thus, molecular, twin, and pedigree studies have suggested consistent heritability in personality traits (Buss, 1999; Plomin & Caspi, 1999; however, see Gangestad, 2010, for an opposing view).
An important prerequisite for a possible adaptive value to select for specific personality traits is that such inherited traits are constant and stable within individuals across a lifetime. It has now been shown that personality traits are much less influenced by the external environment and are much more stable than generally thought, and do not change much from late adolescence to adulthood. Allik and McCrae (2002, 2004) maintain that personality traits are relatively stable in adults and found no evidence that culture plays a major role in altering these traits. An extensive longitudinal study by Caspi et al. (2003) in New Zealand showed a high stability of personality traits in one thousand subjects observed from infancy to adulthood.
If personality is relatively stable and partly inherited, which selective forces could shape such high variability? Most researchers agree that personality traits vary due to the effect of a variety of alleles and do not converge toward a single successful personality profile even generation after generation (Buss, 1999; Gangestad, 2010; Loehlin & Rowe, 1992; Turkheimer, 1998). In a seminal study, Penke, Denissen, and Miller (2007) responded to the question of the persistence of such a variety of personality profiles by examining three alternative hypotheses: (a) neutral selection, (b) mutation-selection equilibrium, and (c) balanced selection. By evaluating the evolutionary dynamics and requirements of all of the hypotheses within a multi-genic behavioral genetics perspective, these authors suggested that only balanced selection might well explain the persistence of a large number of polymorphic alleles influencing personality traits (again, however, see Gangestad, 2010, for an opposing view).
Some natural experiments confirm that cases of unbalanced selection might be at work and evidence for the adaptive value of personality differences are starting to emerge. Earlier studies showed definitively that large populations are not at all suitable to study the possible adaptive value of personality differences (Buss, 1999; Eysenck, 1982; Eysenck & Yanai, 1985). In large populations with complete sets of socio-ecological niches, individuals with different personality traits will actively search for their best fitting environments, balancing each one’s success possibility (Barrick & Mount, 1991; Hettema & Kenrick, 1992; Tett, Jackson, & Rothstein, 1991). Thus, selective differences in personality distributions through variable selection would be averaged out in sampling a large population (Gangestad, 2010; Nettle, 2002). These selective balanced advantages of a variety of personality traits to a variety of socio-ecological environments could explain why putative alleles influencing opposing personality traits (e.g., extraversion vs. introversion) are rarely eliminated through natural selection and high population variability is thus preserved (Roff, 2002).
In contrast, the prediction from balanced selection suggests that the selective equilibrium maintained in large populations would be disrupted in small populations where there is a substantial reduction in socio-ecological niches. In this case, certain phenotypes would better succeed in such environments and would be favored over others that cannot find their own socio-ecological niche in such a restricted population.
Accordingly, to adequately test the personality gene flow hypothesis, proponents of the hypothesis selected populations that are small and homogeneous and that met the following specific conditions: (a) populations that are sufficiently ancient to allow selection to act, (b) populations that are sufficiently small (selection works better in small populations), (c) those with a very limited array of socio-ecological niches (to alter the balanced effect of selection active in larger populations), and (d) those that are relatively isolated (to avoid the dilution effect of external immigration). Populations with all of these characteristics could be isolated communities such as those living in high mountains, nomads living in deserted areas, or best of all, populations living on small offshore islands.
In order to assess personality differences between populations, Camperio Ciani and Ceccarini (2002) pioneered this new approach, which involves comparing small island populations with a reference population that shares a common historical, linguistic, and cultural context, thus minimizing confounding of genetic and cultural differences. In small populations with similar socio-ecological conditions, a disequilibrium in balanced selection mechanisms may be created. Under such conditions, it was predicted that the personality traits of people in these small island populations would be selected toward a more convergent profile. Hence, Camperio Ciani and Ceccarini proposed the personality gene flow hypothesis, which, in terms of the Five Factor Model (FFM; Goldberg, 1990; McCrae & Costa, 1999), posits the following: If a small population living on a small island fulfills particular conditions—such as an ancient foundation, isolation, limited array of socio-ecological niches, and high emigration and low immigration—the personality of the islanders will progressively converge toward low levels of extraversion and openness and high levels of emotional stability and conscientiousness, traits that are all well adapted to the prevalent socio-ecological niche of a small isolated island. The mechanism that allows this selection to act is a selective gene outflow: A progressive unidirectional drain of alleles from specific polymorphic genes from a small isolated population to a larger one.
In the following sections, we report the results of a series of studies that focus on population personality profiles of 16 small island populations divided between four archipelagos off the west and south coast of Italy. The islands selected in each archipelago all meet the specific conditions just described. They are all composed of small islands, between 10 and 40 miles offshore, with only one or two old settlements, thus reducing the variety of socio-ecological niches. The islanders were all forced to resettle from mainland villages before the sixteenth century, and for at least 20 generations they have experienced a high degree of isolation from the respective mainland. Population growth was high in terms of fecundity on each island but permanence of residency was limited in the past by the small available surface area (i.e., reduction of environmental niches). Consequently, the populations experienced constantly high levels of emigration (Camperio Ciani, Capiluppi, Veronese, & Sartori, 2007; De Fabrizio, 2000; Roani Villani, 1993). All these islands show a significant reduction of socio-ecological niches because the natural environments offer scarce and inaccessible resources on land and at sea. The social environment is limited by the small size of the communities, which do not interact much with the outside world given the scarcity of transportation to the mainland. In the past few decades, however, this marked isolation has ended, and immigration has been facilitated by the development of connections with the mainland and attracted by tourism. The presence of recent immigrants allowed us to explore the effect of phenotypic flexibility due to the environmental requirements on personality traits.
To assess the personality traits of the islands’ inhabitants, an adjective-based FFM questionnaire was selected (Norman, 1963). According to the Five Factor Theory (FFT) of personality, five independent basic dimensions of personality can be identified. These are extraversion, agreeableness, conscientiousness, emotional stability and openness to experience (Goldberg, 1990; McCrae & Costa, 1999). The FFT of personality was selected because most psychologists today agree that the dimensions of the FFM of personality account for the co-variation of most personality traits. The FFM is also ideal to explore from the perspective of behavioral genetics; behavioral genetics studies have shown that traits from all five factors are heritable (Terracciano et al., 2005). According to FFT, personality traits, as products of the human genome, are universal. Further, cross-cultural research suggests that the structure and development of personality traits are very similar in nations as dissimilar as India, Argentina, and Burkina Faso (Goldberg, 1993; Terracciano et al., 2005). Crucial for the following studies is that, according to FFT, personality is stable over time (Costa & McCrae, 1997) and relatively untouched by life experiences (Buss, 1999; Loehlin, McCrae, Costa, & John, 1998). Church et al. (2011) partly challenged the validity of population personality comparisons by testing an important prerequisite, the cross-cultural measurement invariance of the Revised NEO Personality Inventory (Costa & McCrae, 1992) at the item and facet levels. Their results highlight the need for caution in making mean trait comparisons across populations.
It was hypothesized that, given enough time across generations, a heritable personality profile specifically adapted to life on a small island, and its unique socio-ecological environment, should emerge and be measurable. Indeed, along this line, anyone visiting an Italian small island can notice at first sight that the villages on the coast and in the countryside are very well maintained, preserved, and looked after, as compared to the corresponding mainland areas. The island inhabitants do not show any distinctive morphological traits compared with the corresponding mainland populations. However, they all apparently share certain distinctive behavioral and personality traits that make them particularly conservative, traditional, attached to their land, hardworking and apparently adapted to the costs of what looks like a rigid, isolated and frugal life in a harsh, albeit beautiful, environment.
Specifically, given that personality traits are at least in part inherited, we expected to find measurable differences in FFM questionnaire scores between islander and mainlander samples (Camperio Ciani et al., 2007; Camperio Ciani & Ceccarini, 2002). Accordingly, we tried to answer the following questions: (1) Are there differences in personality traits between islanders and mainlanders? (2) Can these differences be ascribed to stable traits of genetic origin, or are they due to individual flexible adaptation to the environment? (3) What mechanisms could account for the origin of these differences? (4) Is there any measurable evidence at the molecular level that these differences in personality have corresponding differences in allele frequency, and do these differences correspond to the specific traits that differ between islanders and mainlanders? and (5) Can it be confirmed that in islands where the prerequisites of isolation and long-lasting island ancestry are not present, the population will not show the typical islander personality?
The project aimed to sample all small Italian islands. The researchers started by sampling Giglio Island in Tuscany, in the western sea off the center of Italy. This population was compared with the corresponding nearest mainland regions of Castiglione della Pescaia and Argentario (Camperio Ciani & Ceccarini, 2002). Next, they sampled Ponza and Ventotene islands in the Lazio region, further south in the western sea, which were compared with Gaeta and Formia cities in the mainland. Capri, Ischia, and Procida islands were not included because they have been constantly visited for at least the past 2000 years with continuous flows of new residents entering and leaving the islands. In these three islands it was not possible to differentiate long-lasting residents from recent immigrants; hence, the immigration rate violated one of the prerequisites of the hypothesis. The project continued with all of the seven inhabited Aeolian islands, located between the south western coast of Italy and the island of Sicily, including Lipari, Salina, Vulcano, Panarea, Stromboli, Alicudi, and Filicudi, which were all compared with the closest corresponding mainland region of Milazzo in Sicily (Camperio Ciani et al., 2007). A few years later a new sampling campaign included all the inhabited Egadi Islands, off the western most coast of Sicily Island, including Marettimo, Levanzo, and Favignana, which were compared with the corresponding mainland region of Trapani (Camperio Ciani & Capiluppi, 2011). To explore if island isolation and consequent gene flow might have influenced the personality of islanders at a molecular level, Camperio Ciani, Edelmann, and Ebstein (2013) returned to Giglio Island and collected blood samples of a hundred of the long-lasting resident islanders. They compared the samples with those collected in a sample of Tuscan mainlanders, focusing on the allele frequencies of three polymorphic genes influencing personality traits: DRD4 exon 3 VNTR, SLC6A4 5-HTTLPR indel, and the SLC6A3 DAT1 30UTR repeat region (Camperio Ciani et al., 2013). More recently, Pozzan (2014) sampled Elba Island, which was compared with the Piombino mainland, as well as the Tremiti archipelago, off the southeastern coast of Italy, including the only inhabited islands of San Domino and San Nicola, which were compared with the corresponding closest mainland region of Termoli. This last study included islands that did not fulfill all the prerequisites of a small island population needed to develop a specific personality profile. This last study was planned to test the robustness of the personality gene flow hypothesis and the precision of its prerequisites (Pozzan, 2014).
Most Italian small islands were inhabited since prehistory or the Bronze Age. In those early times the Mediterranean was highly populated and for centuries small islands were used as food depots or bases for coastal stops. During the classic Roman period many small islands became private and beautiful roman villas were built (Della Monaca, Roselli, & Tosi, 1996). These so-called villas were actually large villages populated by workers, slaves, and traders. During the Middle Ages the small islands were involved in the conflicts between Italian states and the southern Arab and then Turkish nations. The Tunisian pirate Adir Kadir undertook in 1516 the last significant raid, which depleted the whole population of all the Tyrrhenian small islands, starting from the Egadi, and proceeding to the Aeolian archipelago, the Pontinian, and the Tuscan archipelago (except Elba Island), and deported all their populations to Tunis. Very few islanders survived and apparently no one returned to the original islands. After that last episode, no other large-scale invasions occurred (Gallitto, 2008; Roani Villani, 1993). Since that massive invasion no one wanted to live on these unsafe and deserted small islands. However, all of these islands were forcefully repopulated at the beginning of the 16th century by the various Italian landlords: the Bourbon in the south, the Catholic Pope in the Pontian archipelago, and the Great Duke of Tuscany in the north. Whole mainland villages were chosen and forced to repopulate the abandoned islands. The new villagers were provided with food, seeds, and support until they settled safely and could fortify their villages, which were never again invaded. This history allows us to estimate that the populations of all these islands have been isolated for about 20–25 generations (Gallitto, 2008; Roani Villani, 1993). Catholic parish records that are still preserved in the islands since those times show through surname distributions that very few people immigrated into the islands after repopulation and that many people in every generation emigrated to the mainland. This is revealed by the correspondence of those who were born on the islands and those who were buried. No people with new names were buried and many born in the island do not have any record of burial. Emigration was distinguishable from death at sea. If someone was known to have died during fishing, his death was recorded in the registry even if not buried (De Fabrizio, 2000; Roani Villani, 1993). These registries enable estimates of population growth, fecundity, possible emigration and immigration, and the level of isolation of the population overall.
Each island was visited for various weeks at a time. With the help of large numbers of research assistants from various universities in Italy, islanders were interviewed and administered personality questionnaires. A snowball survey method was used to cover most of the population of each island sampled (Cicchitelli, Herzel, & Montanari, 1992). Interviews and assessments were conducted in the streets or in participants’ place of commercial activity. This method targeted the largest possible number of inhabitants of all ages and genders.
All participants first provided socio-demographic information (e.g., origin, job, age, emigration, immigration), and then completed a 50-adjective measure of the FFM personality traits. Individuals with difficulties in understanding some of the adjectives were read a standard definition of the adjective by the interviewer. Items were rated on a 7-point scale. There are 10 adjectives for each of the five personality dimensions, five with positive and five with negative polarity. Individual scores were derived by summing all the item ratings for each factor, after reverse-scoring the negative polarity adjectives. Then all individual scores were averaged to obtain values of the five dimensions for each sample population. The 50 adjectives belong to a pool of adjectives widely used for personality assessment with Italian subjects (Di Blas & Perugini, 2002; Perugini & Leone, 1994) and the instrument has been previously validated for consistency and reliability on the FFM in a series of Italian samples (Piconi, 1998).
Standardized T-scores (mean = 50; standard deviation = 10) were used to compare personality traits between populations, following Benjamin et al. (1996), Ebstein et al. (1996), Terracciano et al. (2005), and Allik et al. (2009). In all these studies, the T-scores were computed by standardizing the raw scores with reference to the distribution of scores of the respective mainlander comparison sample. Since studies have shown a correlation of personality traits with gender, age, and education level (Costa, Terracciano, & McCrae, 2001; Feingold, 1994; Goldberg, Sweeney, Merenda, & Hughes, 1998), preliminary stepwise regression analyses were conducted. The significant results reported refer to analyses of covariance (ANCOVA) with sex, age and education level as covariates.
Following a first exploratory study by Camperio Ciani and Ceccarini (2002) on Giglio Island, the first systematic study compared three Italian populations living on three small archipelagos in the Tyrrhenian Sea (n = 993) with their corresponding neighboring mainlanders (n = 598). That is, the islanders and mainlanders shared the same geographical origin, culture, and language (Camperio Ciani et al., 2007). The researchers found that islanders on the 10 islands surveyed all showed, on average, lower levels of extraversion and openness and higher levels of emotional stability and conscientiousness, compared to the mainlanders, while agreeableness was undistinguishable between islanders and mainlanders (Camperio Ciani et al., 2007).
To further understand these results, the population for each archipelago/mainland population was divided into four categories of individuals: (a) original islanders, (b) non-original islanders (individuals without a complete island ancestry), (c) mainlanders, and (d) immigrants to the islands. The researchers compared original islanders, that is, those with a long-lasting island ancestry, with immigrants, that is, individuals who live in the island but were not born there and do not have island ancestors. They found, on average, higher extraversion and openness by immigrants than original islanders. That is, once on the island, the immigrants did not acquire the personality traits of the ancient original islanders, even if they shared the same insular environment for, on average, more than 20 years. This finding seemed to support a genetic interpretation of the mean differences. Personality appeared, in this case, resilient to external influence; even after a long time in the respective islands, immigrants maintained differences from the original islanders on specific traits.
The researchers further analyzed original and non-original islanders who had or had not emigrated from the islands (emigrants from the islands, n = 209) and found that they were significantly more extraverted and open to experience than original and non-original islanders who had never left their island (n = 741). These findings induced the researchers to infer that personality differences could reflect selection for traits that result in better adaptation to the island environment. Measurable differences were indeed present in comparing original islanders with either mainlanders or immigrants. However, it had still to be shown whether these differences were due to the influence of the external environment during development (Newcomb, Koenig, Flacks, & Warwick, 1967) or were due, in part, to inborn genetic differences between islanders and mainlanders.
The particular personality characteristics of the people living on the Italian small islands (i.e., lower extraversion and openness) induced Camperio Ciani et al. (2007) to hypothesize that the genetic mechanism underlying differences between islanders and mainlanders might be the gene outflow produced by the emigration of extraverted and open individuals, generation after generation (i.e., drainage of alleles influencing extraversion and openness from the islands) (Bodmer & Cavalli-Sforza, 1976; Camperio Ciani, Stanyon, Scheffrahn, & Sampurno, 1989).
Camperio Ciani et al. hypothesized a particular form of gene flow, not just a casual arrival of foreign individuals into a closed population as is generally understood by gene flow (Mallet, 2001). Rather, they proposed a specific selective gene outflow (allele drainage), not due to mortality but determined by emigration before reproduction. Individuals that shared personality traits that made them less adequate to island life would leave the islands more readily, and therefore promote their emigration before reproduction. The speed of the process would depend on the population size at every generation and on the fraction of individuals of the population emigrating at every generation. The authors concluded that if the population remains relatively small, and the emigration rate is high, this process would explain why the remaining islander population would become progressively less open and extraverted.
Heine, Lehman, Peng, and Greenholtz (2002) have pointed out that reference group effects—the tendency for respondents in different countries to rate their traits in comparison to different cultural norms or reference groups—could confound mean comparisons between populations. Camperio Ciani et al. (2007) examined this possibility in the present case and concluded that this would reduce differences in personality between islanders and mainlanders. Individuals could indeed self-report their own personality profile in comparison to their own community. As an example, a slightly more extraverted islander would overrate their extraversion as compared to an overall introverted reference group, which is opposite, however, to what has been found. Hence, if significant trait differences are observed, in this case lower extraversion in islanders, these differences exist notwithstanding the reference group effect (Camperio Ciani et al., 2007).
To address some possible criticisms that emerged from the first study, a second study was successively undertaken to confirm, with another set of islands, the same process and similar findings. There was an alternative to the genetic explanation of the personality differences found in the previous study—the influence of early experience. According to the FFM, the first years of life might contribute to the shaping of the personality profile (McCrae & Costa, 1999). Given that most immigrants sampled in the first study lived their childhood on the mainland, the environment could have influenced their personalities through early experiences during infancy (Forgas & Van Heck, 1992). To address this alternative early experiences hypothesis, a second study explored the immigrant subjects in greater depth.
In the second population survey the researchers compared the personality traits of inhabitants from a novel Italian archipelago (the three Egadi Islands; n = 622) with those of the closest mainland population (Trapani area; n = 106) (Camperio Ciani & Capiluppi, 2011). The Egadi Islands (Favignana, Levanzo, Marettimo, and a few other islets) constitute the southernmost Italian archipelago, 15–30 sea miles off the western shore of Sicily where the town of Trapani is situated. The total actual population living in the Egadi archipelago is at present around 2000, of which about 200 are resident in Levanzo Island, 400 in Marettimo Island, and the rest in Favignana Island. In reality, only a fraction of those listed as residents are actually living on the islands; the remaining are listed as residents but have, in fact, emigrated to the mainland and return to the island only occasionally. Even if the archipelago is now a tourist destination, during the past 400 years these islands have experienced limited immigration, a fact confirmed by surname analysis and by studies of birth, marriage, and death registries (Gallitto, 2008).
The results confirmed the previously found pattern, with even larger differences between the islanders and mainlanders than in the first study. Islanders were again more introverted and closed as compared to mainlanders from the Trapani region. Moreover, whereas immigrants altogether did not differ from mainlanders on any traits, except emotional stability, they were significantly more extraverted and open to experience than the original islanders. The study also confirmed that emigrants were more extraverted and open than islanders who never emigrated, but, like the other islanders, emigrants were significantly less open and extraverted than mainlanders.
However, the aim of this study was also to analyze in more detail the effect of the relative permanence of immigrants in the island. Islanders were subdivided according to a “gradient of insularity” going from original islanders to recent immigrants. In this way, participants were distinguished in different classes starting from the individuals who had all four grandparents coming from the island and who themselves had never emigrated. These individuals could be considered the most insular, in contrast to the most recently arrived, namely the immigrants who arrived within the most recent five years. Classified in-between were islanders with non-complete insular ancestry (i.e., with a variable number of grandparents born on the island) and native immigrants (i.e., individuals born and living on the island but from immigrated parents).
All these classes of individuals were compared for extraversion, openness, and conscientiousness. In this way, we explored the effects predicted by the genetic hypothesis, while testing the differences predicted by individuals’ flexible response to the environment hypothesis.
Summarizing the results, it was found that time spent on the island after immigration, either short or long, resulted in minimal effect on the personality of the immigrants, especially on extraversion. Even those individuals born on the island from immigrants, who shared all of their lives with islanders, did not show, on average, personality differences from mainlanders, except for higher conscientiousness. However, they were significantly more extraverted and open than islanders. In combination, these results suggest only a marginal effect of early experience and environmental response. Importantly, in progressing from mainlander (no grandparents from the island), through the intermediate groups, to original islanders (four grandparents from the island), profiles became progressively less extraverted and open, on average. These results were unequivocal for extraversion, which showed no influence by the external environment, and to a lesser extent for openness, while the findings for emotional stability and conscientiousness suggested gene-environment interaction. Many researchers highlight the role of gene-environment interactions in the study of personality at an individual level (Penke et al., 2007; Turkheimer, 2000). However, this was the first demonstration at a population level. Once more, agreeableness did not show any differences among all the subclasses of individuals compared (Camperio Ciani & Capiluppi, 2011).
The results of the two studies show that islanders coming from four distinct archipelagos isolated from each other share a consistent, distinctive personality profile. Islanders differ from their respective mainlander comparison groups. They are generally more conscientious and more emotionally stable, while also less extraverted and open to experience. These differences were significant and observed in all the archipelago/mainland population pairs that we investigated. Furthermore, these results were observed after adjustment for differences due to sex, age, and education level. All islanders and their corresponding mainland participants belong to the same population in terms of latitude, vicinity, and similar cultural and historical background. This sharply differs from other studies, which compared populations that were geographically, ethnically, or linguistically far apart (e.g., Eysenck, 1982).
These results are not consistent with an environmental perspective on these personality traits. In particular, the early experiences hypothesis (Forgas & Van Heck, 1992; McCrae & Costa, 1999) is not supported by these findings as far as extraversion and openness are concerned. All comparisons in this second study lent support to the genetic hypothesis. Although our research design could not exclude the possible role of parental education on native immigrants, many studies have found minimal evidence for the role of parental education on personality development (Dunn & Plomin, 1991; Turkheimer, 2000). The results show that the typical islander’s level of extraversion and openness is an inherent characteristic, which evolved in the population due to a genetic selection process.
As a consequence of these results, Camperio Ciani (2010) proposed the personality gene flow hypothesis as a general explanation for such a notable personality divergence in small isolated environments. The hypothesis suggests that, in small isolated communities, living in low-density mountain areas, small islands, or other harsh isolated environments, given enough time in generations, the effect of a reduced and random immigration flow, in combination with a strong and nonrandom emigration flow, can rapidly produce significant average personality differences within the whole population. The difference between outflow and inflow rates and the number of generations that the gene flow process has been active constitute critical factors. Whereas immigration to a small island is a rare and random process—due to external forces unrelated to personality (e.g., a priest being sent from the mainland church or a public officer being sent by the government)—emigration from the island instead is systematic (Calleri, 2006; De Fabrizio, 2000). Under these conditions, Camperio Ciani (2010) suggested that more extraverted and open-to-experience individuals would tend to leave the confined environment of the island because it offers few perspectives and opportunities, thus decreasing the frequency of alleles influencing extraversion. This hypothesis would be consistent with an interactionist perspective (Barrick & Mount, 1991; Hettema & Kenrick, 1992; Tett et al., 1991) that proposes that people with particular personalities actively look for the best fitting environment and are ready to displace themselves to find it. If this displacement happens before reproduction, these individuals’ alleles would be removed from the original population.
Recently a number of other studies have shown that personality is a major determinant in the decision to emigrate elsewhere (Boneva & Frieze, 2001; Tabor & Milfont, 2011). In a study of university students, high achievers who value work over family connections were those most likely to have migration intentions (Boneva & Frieze, 2001). There are other studies suggesting such a process. In a number of studies of Finnish, U.S., and New Zealand citizens who migrated within or between states, it has been observed that extraversion and openness to experience strongly predict migration propensity (Jokela, 2009; Jokela, Elovainio, Kivimäki, & Keltikangas-Järvinen, 2008; Tabor, 2014). According to Johnson, Penke, and Spinath (2011), the human capacity to move toward environments that appear to be “comfortable” and away from environments that are “uncomfortable” also implies the presence of gene-environment interactions. The greater propensity of extraverted and open individuals to emigrate is also supported by the results of molecular genetic studies, affirming a correlation between extraversion and novelty seeking (Benjamin et al., 1996) and by the results of population studies showing that novelty seekers tend to emigrate more (Chen et al., 1999). If these traits were influenced by specific alleles, then it could be that these alleles were progressively removed from the island population gene pool.
These first studies were based only on personality differences as revealed by personality questionnaires. Molecular evidence was not available to support the presence of gene flow that might have altered allele frequencies in the small island populations. Nor was there evidence showing that islanders, besides responding differently to personality questionnaires, also showed biomolecular differences. This lack of information called for a new study based on genetic tests to support the arguments that assume an association between ancestry and alleles.
The personality gene flow hypothesis predicted that a genetic investigation would unveil significantly different allelic frequency distributions between isolated small populations and corresponding mainland populations. However, this prediction would be limited to those polymorphic genes that can be shown to selectively influence traits of extraversion and openness to experience. Thus, our third study was intended to search for the “smoking gun” biomolecular evidence in support of the personality gene flow hypothesis.
Camperio Ciani et al. (2013) analyzed blood samples of 117 adult Giglio islanders (in the Tuscan archipelago). Only ancient-origin islanders of adult age of either sex were sampled. This included adult individuals who were born on the island, whose four grandparents were all born on the island, and who had a typical Giglio Island surname, indicating that they belonged to the ancient population. Samples were collected from 117 of 500 individuals inhabiting the island, which is approximately one-fourth of the total population of ancient residents. These individuals had been previously typed for personality traits by Camperio Ciani and Ceccarini (2002) and Camperio Ciani et al. (2007). This sample was compared with a control sample composed of 96 adult Tuscany mainlanders, including participants of both sexes. Tuscany is the Italian region geographically adjacent to Giglio Island.
Previous studies had already shown that specific genes are directly associated with specific traits (Benjamin et al., 2008; Benjamin et al., 1996). This sort of evidence was growing, but still debated, as in the case of DRD4 (Ebstein, Neamanov, Klotz, Gritsenko, & Belmaker, 1997). Personality traits, which constitute a complex phenotype, are likely to be influenced by a number of interacting genes (Jang, McCrae, Angleitner, Riemann, & Livesley, 1998). However, there are critics of a strong genetic influence on personality. For example, a series of meta-analyses yielded weak or no robust associations between a number of candidate genes and major personality dimensions once controls were instituted (Munafo et al., 2003; Munafo et al., 2009; Munafo, Yalcin, Willis-Owen, & Flint, 2008). Notably, the DRD4 polymorphism showed the most robust association. The researchers hypothesized that the DRD4 exon 3 repeat would show variation in allele frequencies between islanders and mainlanders, since this polymorphism was associated with human migration and expected to influence personality traits such as extraversion, openness, and novelty seeking (Chen et al., 1999; Matthews & Butler, 2011). On the contrary, two other polymorphic genes, the serotonin transporter SLC6A4 5-HTTLPR indel and the dopamine transporter SLC6A3 DAT1 30UTR repeat region, were not expected to differ in the population samples. There is no known evidence that these two polymorphic genes influence extraversion, openness, or emigration, although these genes may impact other personality traits (Canli & Lesch, 2007; Faraone & Khan, 2006; Manor et al., 2001).
Indeed, significant differences between islanders and mainlanders were observed in the frequency of the DRD4 exon 3 alleles. The DRD4.2 repeat was more common in mainlanders, as expected, whereas the DRD4.7 allele was over-represented among islanders who never emigrated. A highly significant difference between the islanders and the mainlanders was also observed for the DRD4 exon 3 VNTR. The short form of DRD4.2 was significantly more common in the mainlanders than in the islanders when the DRD4.2 repeat was compared with the combined DRD4.not 2 alleles. The long 7R allele was over-represented in the islanders. Again, a highly significant difference in allele frequencies was observed when the long DRD4.7 repeat was compared with the combined classification of the DRD4.not 7 alleles.
Notably, whereas the DRD4 polymorphism showed remarkable allelic frequency differences in the islanders compared with the mainlanders; no differences were found for the two other polymorphisms examined (DAT1 30UTR VNTR [SLC6A3] and SLC6A4 5-HTTLPR), in either the allele or genotype frequencies between the islanders and the mainlanders. As expected, this finding further confirmed the validity of the selective predictions made by the personality gene flow hypothesis.
Thus, the present results confirmed the hypothesis that strong gene flow due to emigration should have selectively altered the DRD4 allele distribution but not the allelic distribution for the two polymorphisms that are not directly associated with emigration. However, the predictions regarding the direction in which the alleles should be diminished and which allele should increase were only partly confirmed. The data showed that allele 2R, which is one of the alleles that was previously suggested to be positively associated with emigration and possibly risk-taking, was indeed less frequent in the island population. However, the 7R allele was much more highly represented in the island population, which is contrary to the prediction that 7R is an emigration-prone allele (Chen et al., 1999; Matthews & Butler, 2011). Nevertheless, this finding is in agreement with the general observation that the less common DRD4 7R allele appears to be maintained in most populations as a balanced polymorphism (Chang, Kidd, Livak, Pakstis, & Kidd, 1996; Ding et al., 2002), suggesting that this allele presents adaptive value in some socio-ecological environments. These findings show that not everything is clear yet on how DRD4 alleles influence extraversion and openness. Overall, however, these results confirm the presence of a substantially different allele frequency among islanders, selectively for those polymorphisms that influence migratory patterns.
A final study aimed at cross-validating the predictions of the personality gene flow hypothesis was recently conducted by Pozzan (2014). Pozzan explored the question of what happens in those islands that have not been inhabited for enough generations or are larger than a few settlements. According to the personality gene flow hypothesis, the inhabitants of these islands should not share the personality profile of the other small islands. Pozzan explored these predictions in two different natural contexts: the Tremiti archipelago in the southern Adriatic Sea and Elba Island in the northern Tyrrhenian Sea.
The Tremiti archipelago is composed of several islands but only two, San Domino and San Nicola, are inhabited. These two island populations were compared with a sample population from the corresponding mainland region of Termoli in the southeastern Puglia region. The Tremiti archipelago is populated by a very small population of 455 residents and it is about 30 miles from the mainland, which makes it ideal to test the evolution of personality of its inhabitants. However, these islands were completely deserted in the 19th century when the local monastery was abandoned and became subsequently the site for prisoners and exiled people. The island was populated again when the convicts left. The new settlers arrived after World War I, and the islands have been inhabited since then. The local population was therefore composed only of first, second, or maximum third-generation islanders (Mancini, 1979). In this case, the personality gene flow hypothesis clearly predicts that the islanders of Tremiti archipelago should not show any difference in personality profile from the corresponding mainland. Otherwise, the results would support the effects of environmental flexibility during development and would reject the genetic hypothesis.
Elba Island is the third largest island of Italy, after Sicily and Sardinia. It has many villages, both rural and costal, counting at present around 30,000 inhabitants. This island, in contrast to the other small islands, has been continuously inhabited since prehistory. For the Etruscans and ancient Romans, it was renowned for its iron, copper, and tin mines (Zecchini, 2001). Elba was the only island that was not raided by the pirate Adir Kadir, since it was very well protected and fortified. Nevertheless, the population of Elba Island, although continuously inhabited, fluctuated in the past. During the Middle Ages, the population was as low as a few hundred people, while the population flourished during Napoleon’s first exile (Zecchini, 2001). The Elba population was compared with the Piombino region in the corresponding mainland. In this case, the predictions of the gene flow hypothesis would be less definitive. On the one hand, the island experienced a relatively larger immigration of people attracted to Elba’s natural resources. In addition, the relatively large population size and variety of socio-ecological niches suggested similarity to the mainland. On the other hand, Elba is relatively isolated and has a long history of continuous habitation, indeed, across far more generations than all other islands. In combination, these features, plus constant emigration from the island (Zecchini, 2001), suggested a possible effect, albeit weaker, of gene flow from the island.
Pozzan (2014) compared random samples of the populations of Elba Island (n = 189) with Piombino (n = 454), the mainland village facing the archipelago, and random samples of the Tremiti Islands (n = 95) with the corresponding mainland, Termoli (n = 112). Participants were interviewed between 2010 and 2013 by a number of previously trained university volunteers. Again a snowball survey method was used to cover most of the populations. Individuals were interviewed using identical questionnaires as in the previous studies (Perugini & Leone, 1994; Piconi, 1998), in the streets or in the location of their commercial activity (Cicchitelli et al., 1992).
The results were enlightening. As expected, the Tremiti archipelago and Termoli mainland inhabitants showed no personality differences. These results confirm once more that without enough generational time, average personality does not change. No matter how isolated, challenging, and harsh the environment might be, the personality profile of its recent inhabitants, even if born there, does not show measurable differences as compared to mainlanders. Also, emigrants and immigrants were as a consequence indistinguishable from mainlanders.
In contrast, the Elba participants showed a very similar profile to participants in the other small islands. Elba inhabitants were significantly less extraverted and open than mainlanders, and this was especially true for original islanders for whom all grandparents were from the island. Emigrants were more extraverted, but not significantly more open, than islanders who did not emigrate. The Elba Island population results suggest that given enough generation time, even from a relatively larger island, continuous emigration can produce measurable differences in personality. Elba certainly had plenty of generation time for the gene outflow to manifest its effects on personality traits of the population. Overall, the results of the four studies reported here confirm the predictions of the personality gene flow hypothesis, provided that the population fulfills the prerequisites of ancient foundation, isolation, high emigration, and low immigration. Table 2.1 summarizes the mean trait differences found in the studies reported here.
As an alternative to the personality gene flow hypothesis, Chen et al. (1999) suggested that selective mortality of individuals who emigrate to new environments favors extraverted and open-to-experience individuals (Chen et al., 1999). These authors investigated a large sample of populations all over the world which had migrated several thousands of miles during human history over hundreds of generations. These authors suggested that people scoring high in novelty seeking, which correlates both with extraversion and openness, survived better in migrating societies where they could explore and exploit environments better, thus accumulating new resources essential to improved survival. They hypothesized that the differences between migrating and sedentary populations are the product of a slow Darwinian natural selection process of differential mortality in the two differing population types (Chen et al., 1999).
| Islanders vs. Mainlanders | Immigrants vs. Islanders | Emigrants vs. Nonemigrants | ||||
|---|---|---|---|---|---|---|
| Extraversion | Openness | Conscientiousness | Any trait | Extraversion | Openness | |
Tuscan Pontian, Aeolian Archipelago (Camperio Ciani & Ceccarini, 2002; Camperio Ciani et al., 2007) |
−1.6 |
−2.2 |
+1.4 |
— |
+2.3 |
+2.5 |
Egadi archipelago (Camperio Ciani & Capiluppi, 2011) |
−9.65 |
−6.22 |
+1.67 |
+2.9 (*) |
+3.7 |
+4.0 |
Elba island (Pozzan, 2014) |
−2.44 |
−2.53 |
— |
— |
+2.68 |
+0.12 |
Tremiti archipelago (Pozzan, 2014) |
−0.39 |
+0.28 |
−1.38 |
— |
−0.89 |
−0.80 |
Note. T-score differences estimated by an ANCOVA model with age, gender, and education as covariates are reported. Statistically significant effects are shown in bold. The sample sizes are combined for the groups being compared. In every archipelago studied, islanders are systematically less extraverted and open than mainlanders; immigrants show no differences from mainlanders and are distinct from islanders except that the immigrants of Egadi are significantly more emotionally stable(*); and emigrants are systematically more extraverted and open than islanders that do not emigrate. The results for the Tremiti archipelago do not conform to the others because it has been inhabited only recently. Elba resembles the other small islands except for the openness of emigrants, because Elba has a very ancient population. |
||||||
The selective mortality hypothesis produces two testable predictions in small islands: (a) evidence across a large number of generations of a constant high mortality in individuals with a specific personality and (b) absence of differences in personality between surviving individuals who decide to remain on the island and those who decide to emigrate. The gene flow hypothesis produces opposite predictions in this regard: (a) it does not suppose a higher than normal (for those times) mortality for islanders and (b) it predicts significant personality differences between individuals who emigrate and those who remain on the islands (Camperio Ciani, 2010).
Using Catholic parish records of birth and death dating back to the sixteenth century, historians could not find any evidence of notably high mortality in these islands (De Fabrizio, 2000; Gallitto, 2008; Roani Villani, 1993). Life was hard and resources limited, but these islands, once pirate invasion was under control, were rather safe and seldom affected by the frequent conflicts afflicting the corresponding mainland regions. Most mortality occurred in infants, but this is similarly the case throughout rural Italy, and it is difficult to imagine that this infant mortality pattern is due to personality differences. On the other hand, by defining an emigrant as an individual who has an island birth registration on parish records, but no parish death record, De Fabrizio (2000) estimated an average emigration rate per generation of approximately 30% of the population constantly over the past 400 years. Hence, the island demographic pattern seems more consistent with the personality gene flow hypothesis than the mortality hypothesis.
Van Oers (2007) suggested that the personality of the founding populations of the small islands might have been different from the mainlanders from the beginning, as per the founder effect or genetic drift (Fisher, 1930). However, Camperio Ciani et al. (2007) observed systematically the same pattern of personality differences in three different archipelagos, and Camperio Ciani and Capiluppi (2011) again observed almost exactly the same pattern of differences in another archipelago with a different historical and cultural background. Even Pozzan (2014) did not find random differences in personality but rather systematic differences all involving extraversion and openness and always in the predicted direction. This contrasts with the random nature of founder effects or genetic drift. In all cases, immigrants resembled islanders and not mainlanders. In addition, emigrants were systematically more open and extraverted than non-emigrants and agreeableness never showed any significant differences (see Table 2.1). This evidence came from four independent archipelagos (plus Elba Island), composed of 16 inhabited islands in total, all showing a convergent personality pattern. This makes it very unlikely that the differences are the random result of multiple, and all similar, founder effects. These effects are all in the same direction, making the hypothesis of genetic drift for the personality distribution in small islands very improbable.
Even the results of the biomolecular study are inconsistent with founder effects. In the case of Giglio Island, where the complete demography is known from its founding, Camperio Ciani et al. (2013) calculated that a founder effect based on stochastic selection of the first 70 or so reproducing founders was extremely unlikely to lead to the present island allele distribution of DRD4, while maintaining constant the other two polymorphic gene allele frequencies. Moreover, if the founding sample was larger than 70, support for the founder effect in these calculations would be even weaker (Camperio Ciani et al., 2013).
Rebollo and Broomsma (2007) suggested inbreeding depression as an alternative explanation for low extraversion and openness to experience in islanders. Inbreeding depression occurs whenever individuals mate and reproduce with relatives. This condition is more likely in small closed populations and promotes the emergence of a variety of recessive genetic impairments. While it is certain that inbreeding depression occurred in these island populations, as Penke et al. (2007) noted, it seems counterintuitive that inbreeding depression should exclusively affect openness and extraversion traits and not the others. Consistent with the personality gene flow hypothesis, however, extraversion and openness can be directly associated with migration tendencies and active niche selection. Indeed, in the small island studies, the most extraverted native islanders were found among those who emigrated. In addition, the inbreeding depression hypothesis, contrary to the personality gene flow hypothesis, does not predict systematic phenotypic differences within the inbred population. Thus, the finding of differences between emigrants and non-emigrants also favors the latter hypothesis.
Summing up, the findings in the small Italian island populations are not consistent with the predictions of the selective mortality hypothesis, genetic drift, founder effects, or inbreeding depression. This suggests adaptive differences consistent with the personality gene flow hypothesis.
There are a multitude of small islands with similar situations as the Italian islands described here that await study. As an example, the island of Corvo in the Azores has less than 300 inhabitants, who live hundreds of miles from the rest of the archipelago and thousands of miles from mainland Portugal. Pitcairn, the famous island in the Pacific where the mutinous crew of the Bounty settled in the 18th century, provides another example. Pitcairn, like the Italian small islands, have experienced constant dramatic emigration from the island to Nuova Caledonia, where now there is a town named New Pitcairn, populated by its emigrants. In addition to their scientific interest, studies on small islands serve to avoid dramatic population management errors. A few years ago the Italian government closed a maximum security detention camp in a small island near Elba, called Pianosa. In addition to the camp detainees and guards, the island was inhabited for centuries by a small community of residents. In closing the detention camp the Italian government also removed all the inhabitants and deported them to the mainland, allegedly to increase natural conservation of the island. This was done against the will of most of the inhabitants. Once the island was abandoned, the environment rapidly deteriorated and subsequent efforts to enroll mainland volunteers and environmental workers to protect the island dramatically failed as nobody wanted to go there (Boggi, personal communication). Not satisfied, the Italian government is planning to remove another whole island community from Gorgona Island, again with the rationale of closing a state prison and with the excuse to improve conservation. This might be another blunder. As it was shown here, people who live in isolated areas are the descendants of a long process of genetic selection and adaptation to isolation. The inhabitants of small islands, more than others, are adapted to survive in the harsh and at times hostile environment. Those people who survive in isolated areas are resilient, and care for their environment in good and bad seasons; they preserve it. These particular inhabitants are part of the richness and diversity that small isolated communities offer and should be supported and defended as part of our biodiversity.
The present findings might offer new perspectives on an evolutionary puzzle: the existence of large individual differences in personality traits. The various alleles that produce personality differences seem very efficient at preserving themselves. Only in particular environments with a great reduction of alternative socio-ecological niches—such as the isolated communities studied here—will the frequency of certain alleles from the gene pool be diminished, through emigration (gene outflow), leaving only the most adapted individuals (alleles). The findings of these studies highlight the genetic component of personality differences at the population level.
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