part0020

Biologically speaking, incest occurs when someone has sex with a genetic relative (see Bixler, 1983). In this chapter, I focus on sexual contacts between a child and an older relative. Incest is a theoretical puzzle because incest offenders, as a group, are not readily explained by the factors described in chapter 4 . In that chapter, I argued that sexual offenses against children can be explained by antisociality and pedophilia. However, incest offenders typically score lower on measures of antisociality than do other sex offenders against children (Rice & Harris, 2002; Seto & Barbaree, 1999) and typically show less sexual arousal to stimuli depicting children in the phallometric laboratory (Frenzel & Lang, 1989; Freund, Watson, & Dickey, 1991; Lang, Black, Frenzel, & Checkley, 1988; W. L. Marshall, Barbaree, & Christophe, 1986; Quinsey, Chaplin, & Carrigan, 1979; Rice & Harris, 2002). Other studies have not found a significant difference in sexual responding between incest offenders and other sex offenders against children, but no studies have found that incest offenders show greater sexual arousal to children (Abel, Becker, Murphy, & Flanagan, 1981; Barsetti, Earls, Lalumière, & Bélanger, 1998; Chaplin, Rice, & Harris, 1995; Langevin & Watson, 1991; Malcolm, Andrews, & Quinsey, 1993; Murphy, Haynes, Stalgaitis, & Flanagan, 1986). Taken together, these studies indicate that as a group incest offenders are less likely to be pedophiles than other sex offenders against children.

These group differences in antisociality and pedophilia are reflected in the fact that incest offenders are less likely to reoffend, both sexually or nonsexually, than other sex offenders against children (see Appendix 6.1 ). Some incest offenders are highly antisocial, and some incest offenders are pedophiles, but these factors are not sufficient to account for the majority of incest offenders, particularly those who offend only against related children. Incest offenders who also offend against unrelated children tend to score higher on measures of antisociality and pedophilia (e.g., Porter et al., 2000; Rice & Harris, 2002; Seto, Lalumière, & Kuban, 1999).

Incest is a Darwinian puzzle as well. It reduces one’s inclusive fitness to the extent that it psychologically or physically harms a related child, for example, by causing the child to avoid sexual relationships or by impairing his or her reproductive capacity through physical injury or a sexually transmitted disease. The risks of inbreeding in terms of physical anomalies or greater mortality of offspring are well documented in experimental research on nonhuman animals (for a review, see Bixler, 1992). 1 There is also evidence that inbreeding can result in greater morbidity and mortality in human offspring (Adams & Neel, 1967; Seemanova, 1971; but not Bittles, Mason, Greene, & Appaji Rao, 1991). The evidence for incest avoidance in nonhuman species is reviewed by Silverman and Bevc (2004) and Thornhill (1993).

In this chapter, I begin by reviewing evidence regarding the ubiquity of incest prohibitions across time and across cultures. I then discuss the leading proximate explanation for incest avoidance, originally proposed by Westermarck (1891/1921), and analyze other proximate explanations for why incest still occurs. The main purpose of this chapter is to link theoretically driven research in evolutionary psychology and anthropology, inspired by the challenge of incest to inclusive fitness theory, with applied research on the characteristics of incest offenders and their offenses. I conclude with theoretically derived predictions that can provide directions for future research on incest.

Although the specifically prohibited relationships vary across time and place, ethnographic and historical evidence indicates incest taboos are ubiquitous, especially with regard to close relationships such as parent and child (Maisch, 1972; Van den Berghe, 1979). The few exceptions that have been found restrict incestuous relationships to special circumstances, such as royal marriages between brothers and sisters in ancient Egypt to maintain a family dynasty (Bixler, 1982). Incest taboos are present in Persian, Egyptian, Greek, and Roman myths, and there are religious prohibitions regarding incestuous relationships (see the discussion that follows). There are also historical records regarding incest (e.g., L. Gordon & O’Keefe, 1984).

In the Old Testament of the Judeo-Christian Bible, Leviticus 18:6 (thought by many Biblical scholars to have been written around 400 or 500 BCE) begins with a general prohibition of sex with relatives and then specifically lists prohibited familial relationships from a heterosexual male perspective: Sexual relationships with mothers, stepmothers, sisters, stepsisters, aunts, cousins, nieces, and female in-laws (except for a wife’s sister, if the wife has died) are forbidden; curiously, sex with one’s own daughter is not specifically prohibited. The inclusion of nongenetic relationships suggests these prohibitions were intended in part to prevent family conflict and instability. In the Quran, the holy text of Islam, incest is also specifically forbidden:

Prohibited for you (in marriage ) are your mothers, your daughters, your sisters, the sisters of your fathers, the sisters of your mothers, the daughters of your brother, the daughters of your sister, your nursing mothers, the girls who nursed from the same woman as you, the mothers of your wives, the daughters of your wives with whom you have consummated the marriage—if the marriage has not been consummated, you may marry the daughter. Also prohibited for you are the women who were married to your genetic sons. Also, you shall not be married to two sisters at the same time—but do not break up existing marriages. GOD is Forgiver, Most Merciful. (Khalifa, 2005, p. 81).

One of the oldest legal prohibitions regarding incest is contained in the Code of Hammurabi (c. 1780 BCE), which also represents one of the earliest known records of law. The Code of Hammurabi consists of 282 laws proclaimed by the eponymous Babylonian king. These laws were inscribed on stone pillars and placed in public areas for reference; an 8-foot high example inscribed in basalt is currently located in the Louvre Museum in Paris. Two Hammurabi laws specifically refer to incest (The Code of Hammurabi, n.d.): Law 154 states, “If a man be guilty of incest with his daughter, he shall be driven from the place,” and Law 157 states, “If any one be guilty of incest with his mother after his father, both shall be burned.” Incest prohibitions continue to be represented in current laws, including those of Canada and the United States (see Criminal Code of Canada, 1985; see also American Prosecutors Research Institute, 2003).

Despite these cultural, religious, and legal prohibitions, incest occurs. Of the juvenile victims of sexual offenses who were reported to law enforcement in the United States, 34% were related to the perpetrator (Snyder, 2000). These aggregate crime data do not further distinguish victims according to their specific relationships to the perpetrators. As is true for sexual offenses against children by unrelated offenders, girls were more likely to be victimized than boys, and the large majority of incest offenders were male. Finkelhor, Hotaling, Lewis, and Smith (1990) found that 2% of the 416 women in their retrospective survey sample reported being sexually abused by a father, grandfather, or brother, and 14% reported being sexually abused by an uncle. Only 1% of the sample (i.e., 4 women) reported being sexually abused by a female relative. In the next section, I address explanations for the occurrence of incest.

Westermarck (1891/1921) suggested that individuals raised together as children would not be sexually attracted to each other in later life. 2 He defined incest avoidance as

A propinquity-based mechanism (i.e., a mechanism based on living closely together) would have been effective in preventing incest in ancestral environments because siblings would almost always have been raised together. Westermarck’s (1891/1921) theory might also apply to parent–child incest, because one can presume that parents in ancestral environments were involved in the early care of their children. If Westermarck’s theory is correct, then incest would be more likely to occur if something were to interrupt early propinquity. Two related children who were not raised together could become sexually interested in each other, and two unrelated children who were raised together when very young would develop the same sexual indifference toward each other as siblings normally do.

There is a good evidence to support Westermarck’s (1891/1921) theory about incest avoidance. First, there are anthropological studies of childrearing practices that have served as natural quasi experiments regarding the impact of early rearing on mate choice. Shepher (1983) studied children who grew up together in small groups on Israeli communes (kibbutzim ). All of the children were within 1 or 2 years of age of each other, and their daily activities included eating, washing, playing, and sleeping together. Although marriages within the communal group were encouraged by their parents, Shepher found that only 14 of 2,769 marriages recorded across 211 kibbutzim involved couples who were raised in the same child-rearing group. Moreover, of the 14 couples who were raised together, 9 couples did not live together during their first 6 years of life, and the remaining 5 couples were together for 2 or fewer years of their first 6 years of life, suggesting there is a critical window (before the age of 6) for propinquity to trigger incest avoidance.

Wolf (1995) studied the major and minor forms of arranged marriage in northern Taiwan. In the major form, the bride lived with the husband’s family after the marriage. In the minor form, a sim pua (little bride) was adopted into the family at a very young age and would later become a bride for one of the sons. Wolf found that minor arranged marriages had greater rates of extramarital affairs and divorce and lower fertility than major arranged marriages. There were no differences in these outcomes between the major and minor forms if the bride-to-be was adopted after the age of 3, further supporting the idea of a critical period during which a propinquity-based sexual indifference develops (but setting it at an earlier age, before the age of 3 rather than the age of 6).

McCabe (1983) found a similar negative effect on sexual outcomes in arranged marriages between cousins in a southern Lebanese village. In this society, brothers tended to live closely together, and their children were raised like siblings—sharing meals, sleeping, playing, and attending school together. Marriages between first cousins were relatively common in Lebanon at that time. Marriages between cousins who were raised together were significantly more likely to end in divorce and produced significantly fewer children than marriages between cousins who were raised apart or marriages between unrelated individuals.

These anthropological studies were of nonsiblings who were raised together. In a survey of college students, Bevc and Silverman (1993) examined the impact of early separation and therefore a disruption in early propinquity on sexual behavior between siblings in an anonymous survey of college students. Individuals who attempted or had intercourse with a sibling were more likely to have been separated for more than 1 year during their first 6 years of life than individuals who did not attempt to or have sexual intercourse with a sibling. This difference was not observed when comparing individuals who had engaged in nonreproductive sexual behaviors such as kissing, hugging, or fondling with those who did not engage in these behaviors. This finding was replicated by Bevc and Silverman (2000) in a study of a mixed sample of students and community volunteers. However, they also found, contrary to their predictions, that participants who had sexual contact with a sibling were more likely to have seen their sibling nude and had more physical contact with their sibling as a child. This result suggests that it might be simply propinquity rather than how that time is spent together (e.g., seeing one another nude or having a great deal of physical contact) that activates incest avoidance. It also suggests that incest avoidance is most relevant to sexual intercourse and is perhaps less powerful for nonreproductive activities such as kissing or fondling.

There is additional evidence to support the idea that it is propinquity rather than knowledge of one’s genetic relatedness or similarity in appearance (or other cues of relatedness) that is the critical element in the development of incest avoidance. Lieberman, Tooby, and Cosmides (2003) examined judgments of moral wrongfulness as a measure of incest avoidance, arguing that such judgments are less susceptible to socially desirable responding than directly asking questions about sexual contacts or sexual attraction to siblings. They found that judgments of moral wrongfulness for sibling incest were positively related to whether the respondent lived with an opposite-sex sibling as a child. Judgments of wrongfulness were not significantly correlated with cohabitation with a same-sex sibling. In another study by the same research group, the number of years a male respondent lived with a sister before the age of 11 was positively correlated with his ratings of disgust at imagining various sexual acts with a sister (Lieberman, Tooby, & Cosmides, in press). Male respondents who did not live with a sister in their first 10 years of life were less disgusted at the idea of sex with a sister than those who did live with a sister for any amount of time before the age of 11.

Lieberman, Tooby, and Cosmides (2007) suggested there were two important cues for human kin detection: observing a younger sibling being cared for by one’s mother (what they called maternal perinatal association ), and duration of coresidence as children. These authors found that maternal perinatal association was a stronger determinant of self-reported altruistic behavior, personal aversion to the idea of incest, and judgments of moral wrongfulness regarding sibling incest than duration of coresidence for older siblings. In contrast, coresidence duration is the dominant cue for younger siblings, who could not by definition have observed their older sibling’s perinatal care, and for older siblings when maternal perinatal association cues are absent.

Finally, with regard to evidence of a Westermarckian effect in father– daughter incest, Parker and Parker (1986) found that fathers or stepfathers who committed incest were less involved in early child-care activities than nonoffending controls. Williams and Finkelhor (1995) replicated this finding in a study of Navy fathers; moreover, the absences of Navy fathers from home during the early life of their daughters were not entirely voluntary (e.g., as a result of a tour of duty or overseas assignment), thereby countering the alternative explanation that a common factor predisposes men to both commit incest and to be absent from home.

Before discussing other explanations for why incest might occur, an important consideration in thinking about incest is the distinction between genetic relatedness and sociolegal relatedness. From a biological perspective, having sexual contact with a sociolegally related child is not incest, though it may be treated as such socially and legally. From inclusive fitness theory, one could predict differences between genetically related and genetically unrelated incest offenders for two reasons: First, inclusive fitness theory suggests there has been selection pressure over time for psychological mechanisms to treat genetic kin preferentially in terms of affection, care, and investment of resources and to avoid activities that might harm them (often referred to as discriminative solicitude ; see Daly & Wilson, 1998; Hamilton, 1964). In other words, individuals who did not vary their solicitude toward others as a function of their genetic relatedness would be less likely to pass on their genes to the next generation.

Consistent with the notion of discriminative solicitude toward kin, the perceived severity of sexual and nonsexual crime is linearly and positively related to the purported genetic relationship between perpetrator and victim; crimes against closer genetic kin are perceived to be more serious than crimes against more distant kin or unrelated persons (Quinsey, Lalumière, Querée, & McNaughton, 1999). M. Wilson and Daly (1987) reported data showing that genetically related children are much less likely to be physically abused than stepchildren. Looking at official Canadian homicide data, an unambiguous measure of maltreatment that is less vulnerable to reporting biases than cases of physical abuse or neglect, Daly and Wilson (1994) found that stepchildren were 60 times more likely to be killed than genetically related children. Similarly, having a stepfather greatly increases a girl’s risk of being sexually abused (Finkelhor et al., 1990; L. Gordon & O’Keefe, 1984).

Most stepfathers do not physically or sexually abuse their stepchildren; in fact, most stepfathers show a great deal of solicitude toward their new relatives, but they do pose a greater risk than do genetic fathers. In humans and many other species, discriminative solicitude is a function of genetic relatedness. This does not require a direct mechanism to identify kin, however; reviewing the evidence in nonhuman primates, Rendall (2004) concluded that familiarity is the leading candidate as a proximate mechanism for kin recognition, consistent with Westermarck’s (1891/1921) idea. Also, Lieberman et al. (2003) found that duration of cohabitation predicted judged wrongfulness even after statistically controlling for presumed degree of genetic relatedness of the sibling (full siblings, half siblings, or stepsiblings); however, degree of relatedness did not predict wrongfulness after controlling for duration of cohabitation.

From inclusive fitness theory, one would predict that different explanations are required to explain incest committed by genetic relatives compared with sexual offenses against children committed by sociolegal relatives (e.g., stepparents, adoptive parents, or stepsiblings). Sociolegal relatives do not have the same early propinquity (because most remarriages do not occur while the child in question is an infant), and they know they are not genetically related to the child. In contrast, genetic relatives usually have early propinquity and believe they are genetically related to the child (though they may be wrong, as discussed later in this chapter). Most studies of incest offenders have not made this distinction. It is possible, for example, that sociolegal relatives are significantly more antisocial and more likely to be pedophilic than genetic relatives. This would suggest that sociolegal offenders can still be accounted for by the theory described in chapter 5 and that the Darwinian puzzle of incest applies only to genetic relatives.

A few studies have distinguished incest offenders according to genetic relatedness and compared them on measures of antisociality. Rice and Harris (2002) compared 52 genetic fathers and 30 sociolegal fathers (stepfathers or adoptive fathers). The sociolegal fathers did not differ from the genetic fathers in their sexual offense behaviors or criminal history, but the sociolegal fathers had higher psychopathy scores. Greenberg, Firestone, Nunes, Bradford, and Curry (2005) compared 84 genetic fathers and 59 stepfathers who had committed incest against a daughter or stepdaughter, respectively. The genetic and sociolegal fathers were not significantly different in their criminal histories or scores on psychopathy. Unlike Rice and Harris, Greenberg et al. excluded men who had also offended against an unrelated child, thereby excluding men who would be expected to be higher in antisociality and to be more likely to be pedophilic. More comparisons of this kind are needed to clarify the role of antisociality in explaining sexual offenses by sociolegally related men.

Other studies have compared the phallometric responses of offenders distinguished according to genetic relatedness. Quinsey et al. (1979) found that men who sexually offended against only their daughters or stepdaughters (these two groups of men were not analyzed separately) showed less sexual arousal to children than men who offended against unrelated children, and men who offended against other female relatives showed a pattern of responses that placed them between those who offended only against daughters or stepdaughters and those who offended against unrelated children. Langevin and Watson (1991) found no difference in the relative sexual arousal to children exhibited by genetic fathers or stepfathers who committed incest.

In a larger sample of incest offenders with female victims, Seto et al. (1999) distinguished between genetic fathers, other family relatives (such as grandfathers, uncles, and brothers), and stepfathers to test the hypothesis that pedophilia can overwhelm incest avoidance. We distinguished between genetic fathers and other family relatives because genetic fathers have a higher average degree of genetic relatedness. Contrary to our prediction, the 70 genetic fathers responded less to children than the 73 other family relatives but did not significantly differ from the 87 stepfathers. Other family relatives and stepfathers did not differ from 254 men who offended against unrelated children.

Seto et al.’s (1999) findings have been replicated and extended by Rice and Harris (2002) and Greenberg et al. (2005); both of these studies found that stepfathers showed more sexual arousal to children than genetic fathers, though genetic fathers did differ from the men in the comparison groups. Blanchard et al. (2006) replicated these studies in a different sample of incest offenders who were assessed in the same laboratory as that used in Seto et al. (1999). Blanchard et al. selected only incest offenders with a single female victim who denied any sexual interest in children under age 12 and claimed that they were primarily sexually attracted to adult women. There was a linear increase from men who had offended only against adults, genetic fathers, and other relatives in sexual arousal to children. There was no significant difference between genetic fathers and stepfathers and no significant difference between other relatives and men who offended against unrelated children. Rice and Harris (2002) also analyzed data from offenders with only a single child victim and found that men who offended against a related child did not significantly differ from men who offended against an unrelated child in their sexual arousal to children.

These results suggest that genetic fathers and sociolegal fathers are not significantly different from each other in antisociality. The evidence for pedophilia is mixed; some studies have found no significant difference between genetic fathers and sociolegal fathers, whereas other studies have shown that sociolegal fathers show relatively greater sexual arousal to children than genetic fathers (Blanchard et al., 2006; Greenberg et al., 2005; Rice & Harris, 2002; Seto et al., 1999). Men who offend against other family relatives (not their daughters or stepdaughters) are more likely to be pedophiles and may be more likely to be antisocial. For this group of genetic relatives, pedophilic sexual interests or antisocial tendencies may indeed overwhelm incest avoidance.

Genetic fathers are usually combined with stepfathers and other family relatives (such as grandfathers, uncles, or cousins) and then compared with offenders against unrelated children in group comparison studies. To the extent that the incest group contained a high proportion of genetic fathers, the possibility that a study would find a significant difference between incest offenders and unrelated sex offenders against children would be increased. Differences in sample composition could explain, at least in part, the heterogeneity of previous findings.

What then can account for incest committed by genetic fathers or brothers? There are several observations that need to be included in a satisfactory explanation of father–daughter or brother–sister incest. First, the perpetrators are usually male and the victims tend to be female. Second, there is evidence that victims of incest are younger, on average, than victims of unrelated perpetrators (Greenberg et al., 2005; Seto et al., 1999; but see Maisch, 1972). Finally, incest offenses have more sexual contacts per victim than offenders against unrelated children, and the duration of their offending is longer. 3 It is likely that there is more than one explanation for incest, depending on factors such as antisociality, pedophilia, early propinquity, the actual genetic relationship between the perpetrator and the child, the age and attractiveness of the child, the age and attractiveness of the father or brother, and family dysfunction. These explanations are discussed in the following sections.

From Westermarck’s (1891/1921) theory, one can predict that fathers and stepfathers would be more likely to commit incest with their daughters if they spent a significant amount of time apart during the first 3 to 6 years of the girl’s life (e.g., men who are required to travel a great deal for their work, men serving a prison sentence, or families in which the child temporarily lives with another relative or in state care; Parker & Parker, 1986; Williams & Finkelhor, 1995). Similarly, one can predict that stepfathers would be more likely to commit incest if they did not begin cohabitating with the girl until after she was 6 years of age. Because the genetic fathers are often not pedophilic, one would expect the girls to be more likely to have entered puberty before the offenses occur (Langevin, Handy, Hook, Day, & Russon, 1985; Maisch, 1972). For example, Maisch (1972) reviewed data on 70 cases of incest and found that the age of onset of the sexual contact was highly correlated with the youth’s age of onset of puberty.

The most parsimonious explanation for father–daughter and brother–sister incest is that it is a function of sexual opportunity. In Seto et al. (1999), we suggested that some men who prefer sexually mature females will have sexual contacts with their daughters or stepdaughters because they do not have satisfactory access to their preferred group of sexual partners. Reasons for this might include dissatisfaction with their current relationship; an inability to attract an adult female partner because of low status, lack of resources, or physical unattractiveness; or a particularly high sex drive. Men adjust their sexual behavior as a function of their access to potential adult partners (e.g., Landolt, Lalumière, & Quinsey, 1995). Although they are not preferred by most heterosexual men, both prepubescent and pubescent girls still elicit some sexual arousal (Freund, McKnight, Langevin, & Cibiri, 1972). Men who do not have access to their most preferred partners might seek out other partners who are still sexually attractive to them, following what has been described as a sexual preference gradient . In other words, heterosexual men who do not have access to adult women would seek out prepubescent or pubescent girls before they would seek out same-sex partners. Similarly, some heterosexual males without any access to females will engage in same-sex contacts in all-male environments such as boarding schools or prisons.

Consistent with this notion of a heterosexual preference gradient from adult to pubescent to prepubescent females, 11% of the fathers who did not commit incest in the survey by Williams and Finkelhor (1995) acknowledged they had experienced some sexual arousal to their daughters. Given the highly sensitive nature of this question, this estimate of the potential sexual response of fathers and stepfathers to their children is likely to be conservative. Nonpedophilic men are capable of some sexual response to girls, and some men will initiate sexual contact with a girl given enough opportunity and a lack of success in pursuing sexual contact with their peers. By definition, fathers or brothers (genetic or sociolegal) with a daughter or sister, respectively, living in the same residence have much more opportunity than men who do not have a young girl living with them.

The opportunity explanation could be tested by examining the conventional sexual histories of incest offenders. If it is correct that some men commit incest because they lack other sexual opportunities, one could predict that fathers or brothers who commit incest would have a later age of sexual onset and fewer adult sexual partners than other relatives, statistically controlling for pedophilic sexual interests (because pedophiles would also tend to have fewer adult sexual partners) and for antisociality (because antisociality is positively correlated with number of sexual partners and negatively correlated with age of sexual onset). I would also predict that fathers or brothers who commit incest would be lower in social status, resources, and physical attractiveness than other kinds of incest offenders. The opportunity explanation applies to men who seek sexual opportunities but do not have the status, resources, or physical attractiveness to be successful in attracting adult women.

The opportunity explanation might be particularly relevant for stepfathers and stepbrothers. Belsky, Steinberg, and Draper (1991) and B. J. Ellis (2004) have reviewed evidence that girls brought up in homes characterized by marital conflict or father absence began menarche earlier and were more sexually precocious and active than girls brought up in homes characterized by marital harmony or father presence. Stepfathers or stepbrothers who subsequently join a family after the genetic father leaves typically begin residing with girls who could already be showing signs of sexual maturity and who might be beginning to engage in sexual behavior as well.

Theories that focus on family dysfunction as a cause of incest might also represent opportunistic offending (e.g., Maddock & Larson, 1995). In these theories, incest occurs when the parental relationship has broken down and the mother is sexually and emotionally unavailable. For example, a father may turn to his eldest daughter to fulfill his sexual and emotional needs because his spouse is depressed and uninterested in having sex with him. In essence, his daughter is placed in a spousal role. There are some data consistent with this view. Lang, Langevin, van Santen, Billingsley, and Wright (1990) compared 92 incest offenders (86% were genetic fathers or stepfathers) with 42 nonoffending controls and found that the incest offenders reported less communication with their partner, felt more lonely, and were less satisfied with their partner. The incest offenders and nonoffending controls did not differ in the length of their marriages or their number of prior marriages. Lang et al. did not include a comparison group of nonincest offenders, however, and did not further distinguish between genetic and sociolegal fathers. Research is needed on the spousal and father–daughter relationships in families in which incest occurs.

Family dysfunction may also be a proximate explanation of families in which there are multiple incest offenders and multiple child victims (Faller, 1991). A surprising finding in Faller’s study was the involvement of female perpetrators, all of whom offended alongside men. Little is known about female incest offenders (Lawson, 1993; McCartry, 1986).

Haig (1999) has suggested that the costs of incest are higher for females than for males because of the sex difference in minimal parental investment (Geary, 2000; Trivers, 1972). A father who has sex with his daughter does not lose other opportunities to engage in reproductive behavior; however, his daughter may lose an opportunity to have a healthy child fathered by a nonrelative. A brother is in a similarly asymmetric position with regard to his sister. Adding complexity to this evolutionary analysis is the issue of paternity uncertainty. Paternity uncertainty refers to the fact that although mothers can be certain that they are genetically related to the children that they bear, fathers cannot, because their putative child may have been the result of the mother having sex with another man (see Appendix 6.2 ).

Haig’s (1999) evolutionary hypotheses regarding incest leads to a number of testable predictions. I discuss each of these predictions in the paragraphs that follow, with the relevant research. First, because of sex differences in minimal investment, the consequences of incest are higher for females than males, so girls should exhibit greater aversion to the possibility of incest, resist it more than boys, and experience more negative reactions following incest. Males should be more likely to commit incest, so fathers, brothers, uncles, and grandfathers should be more likely to commit incest than their female counterparts. Indeed, incest offenders are more likely to be male (on the basis of both self-report and criminal justice data), and women are more likely than men to report negative reactions to childhood sexual abuse, much of which is committed by relatives (Rind, Tromovitch, & Bauserman, 1998; Snyder, 2000). Girls experience more negative reactions if the incest occurs after they reach puberty and are potentially fertile compared with girls who are prepubertal, and girls experience more negative reactions when the incest includes vaginal penetration and insemination. Rind et al. (1998), in their meta-analysis of studies on victims of child sexual abuse, did not find that the occurrence of penetration was a significant moderator of outcome, but there were too few studies to distinguish between female and male respondents. Rice and Harris (2002) found that genetic father incest offenders (36%) were significantly less likely than sex offenders against unrelated children (47%) to vaginally penetrate their victims.

Second, from inclusive fitness theory, the occurrence of incest should be inversely related to the degree of genetic relatedness between the offender and the victim. Thus, men who are not genetically related to a child (stepfathers and other steprelatives) should be more likely to commit incest than men who are closely related (fathers and brothers); other family members who have an intermediate degree of genetic relatedness to a child (e.g., cousins, uncles, or grandfathers) would fall in between these two groups in the commission of incest. As mentioned earlier, stepfathers are relatively much more likely to commit incest than genetic fathers (Finkelhor et al., 1990). I am not aware of studies that further distinguish the likelihood of incest by genetic relatives according to their degree of relatedness to the victim.

Third, paternity uncertainty should be related to the likelihood of incest such that fathers who have more uncertainty about the paternity of their putative daughter are more likely to initiate sexual contact with her (Haig, 1999). Biologically speaking, a putative father with a high degree of paternity uncertainty is more similar to a stepfather than to a genetic father. This prediction could be tested retrospectively. After controlling for pedophilia (e.g., by restricting analyses to offenders with a single female victim, like Blanchard et al., 2006, or by including only men who clearly show greater sexual arousal to adults than to children in the phallometric laboratory), one could compare offenders who offended against a daughter and nonincest offenders who also have a daughter but offended against an unrelated girl. Some possible points for comparison could be their doubts about the paternity of their daughter, absences from the home around the time of the girl’s conception, 4 and other potential cues of paternity uncertainty such as their spouse’s history of infidelity (both with him and with previous partners) and the presence of a child fathered by another man in the same residence. K. G. Anderson, Hillard, and Lancaster (2006) found that being unmarried and having an unplanned pregnancy were correlated with lower reports of paternity confidence. Additional cues that might be correlated with paternity uncertainty would be marital dissatisfaction; sexual jealousy; the mother’s attractiveness to other men; and the perceived resemblance of the child to the putative father in physical appearance, personality traits, interests, and other psychological characteristics. This prediction regarding the paternity uncertainty of incest offenders and other fathers could also be tested prospectively by collecting information about these proximal cues in a large sample of expectant fathers and then following the sample to see if the occurrence of incest can be predicted.

Finally, paternity uncertainty would be expected to create asymmetries in the costs of incest involving paternally related versus maternally related family members. Beyond the putative father, incest is predicted to be more likely to occur with paternally related children than with maternally related children. To illustrate, a brother would be more likely to commit incest with a paternally related half sister than with a maternally related half sister because he is 100% certain he is related to the latter girl but less than 100% certain he is related to the former girl. Other predictions based on matrilineal (through the mother) compared with patrilineal (through the father) descent have been supported for attention, caring, and other investment by aunts and uncles (Gaulin, McBurney, & Brakeman-Wartell, 1997) and by grandparents (Euler & Weitzel, 1996).

Incest taboos are ubiquitous and appear in both historical and cross-cultural accounts of laws and customs regarding permissible sexual relationships. Westermarck’s (1891/1921) proposed propinquity mechanism provides an elegant explanation for incest avoidance and has good empirical support from nonhuman animal research and quasi experiments in humans. Overall, however, etiological understanding of incest involving girls lags behind the knowledge being accumulated regarding the origins of sexual offending against children in general.

There has been a gap between theoretical research in evolutionary psychology and anthropology and the more applied research carried out by researchers concerned about incest offenders and their offenses. This is unfortunate because there are many novel hypotheses that could be tested to provide understanding of not only why incest occurs but why some children may be more vulnerable than others. Some of these hypotheses can be derived from considering inclusive fitness, parental investment, and paternity uncertainty. Such research could have implications for prevention programs, particularly in the identification of at-risk families and children.

On the basis of the literature reviewed in this chapter, I suggest that the following factors can contribute to the understanding of incest:

In chapters 7 and 8 , I switch from a focus on a theoretical understanding of sexual offending against children, pedophilia, and incest to more immediate and practical problems having to do with the assessment of risk to commit sexual offenses against children (chap. 7 ) and interventions that may reduce the likelihood of such offenses (chap. 8 ). The theories I have reviewed in this chapter and in chapters 4 and 5 are germane, because putative risk factors could be identified by an understanding of the onset and maintenance of sexual offending against children and because interventions are more likely to be effective when they are consistent with a theoretical understanding of the problem.