part0022

Haig’s (1999) evolutionary hypotheses about incest suggest that paternity uncertainty could play an important role in the understanding of incest. If these evolutionary hypotheses are correct, paternity uncertainty must have been sufficiently high in the human ancestral past to have affected Darwinian fitness. This approach to thinking about incest would be moot if heterosexual couples always had been entirely monogamous and fathers were certain about their paternity, just as mothers are always certain about their maternity. The reality is that heterosexual couples are not entirely monogamous, and fathers cannot be certain about their genetic relationship to their putative children. National survey data suggest that 10% to 15% of married women have had sex with someone other than their husband compared with 20% to 25% of married men (Laumann, Gagnon, Michael, & Michaels, 1994; Wiederman, 1997).

What is the extent of false paternity? That is, what proportion of men who believe they are the genetic father of a child are wrong? Estimates of nonpaternity are noisy, depending on the population being sampled and the reasons for the genetic testing. For example, higher rates of nonpaternity would be expected when testing is conducted because of suspicion or a dispute about paternity (e.g., in cases involving custody or child support claims) than in studies of families who are seen for medical genetic screening. On the other hand, many medical geneticists will prescreen for genetic counseling by advising mothers that genetic testing will reveal nonpaternity, thereby biasing the estimates that are obtained; Lisker, Carnevale, Villa, Armendares, and Wertz (1998) found that 88% of medical geneticists they surveyed engaged in prescreening of this kind. Self-selection by mothers who are aware that the genetic testing might reveal this information about nonpaternity could produce underestimates of false paternity.

The literature on genetic evidence of nonpaternity has been recently reviewed by Anderson (2006). He identified 22 nonpaternity rates from studies that analyzed genetic samples collected as part of medical screening or counseling (Mdn = 2%) and 30 nonpaternity rates from studies of genetic data collected as part of a paternity dispute of some kind (e.g., custody cases and to prove relatedness for immigration) that had a much higher median rate of 30%. One would expect higher rates of nonpaternity among incest offenders who are higher in antisociality than the general population because they have more unstable marital relationships (see Lalumière, Harris, Quinsey, & Rice, 2005). Nonpaternity rates of 30% or higher suggest that paternity uncertainty could indeed account for a substantial proportion of putatively genetic incest cases.

Other potential adaptations have also been viewed as related to the ancestral risk of infidelity and nonpaternity. Men who are not sexually jealous are at risk of investing in offspring who are not their genetic kin; such men are less likely to pass on their genes than those who are vigilant about the risk of their mate’s infidelity (for a review, see Buss, 1994, 1999). Evolutionary theorists have suggested that the tendency to report that children resemble their fathers more than their mothers may be an attempt to reassure the fathers of their paternity (Christenfeld & Hill, 1995; Daly & Wilson, 1982; Regalski & Gaulin, 1993; but see Brédart & French, 1999). Bressan and Dal Martello (2002) reviewed a series of studies showing that people do little better than chance at matching parent and child; the biggest influence on resemblance ratings was the respondent’s belief about the genetic relationship between parent and child. Pagel (1997) suggested that there might be selection for generic-looking babies; such babies would not trigger suspicions about paternity because of an obvious lack of resemblance between a baby and his or her putative father.

Several studies have also found that perceived resemblance to a child significantly influenced men’s hypothetical decisions about important matters such as adoption, spending time or money on a child, and paying child support (Platek, Burch, Panyavin, Wasserman, & Gallup, 2002; Platek et al., 2003) but not women’s decisions about these matters (Platek et al., 2003). Moreover, another study found that perceived resemblance to a child was significantly and positively related to the self-reported quality of a relationship with that child and inversely related to the severity of injuries experienced by the mother in a sample of physically abusive men (Burch & Gallup, 2000). Burch and Gallup’s (2000) results suggest that another possible cue of paternity uncertainty to study in future research on incest offenders is the occurrence of violence against the victim’s mother, especially violence that occurs as a result of sexual jealousy or the suspicion of infidelity.

1 This is not true for all species, however. Bixler (1992) noted that these studies found negative effects, even though the research animals were at low risk of predation or disease, did not have to compete for food and water, and lived under controlled conditions.

2 An alternative view proposed by Freud (1913/1952) is that incest is not universally avoided. Instead, a son or daughter is expected to be sexually interested in the opposite-sex parent, and his or her psychological adjustment depends on how this conflict is resolved (the Oedipal and Electra complexes). In this view, incest would occur if incest taboos did not exist to reinforce social norms. I do not discuss this view further because of the great difficulty in formulating and testing scientific hypotheses from it and the subsequent absence of an empirical literature on the Freudian view of incest.

3 This observation is not consistent with a conditioning explanation of pedophilia, because one would expect incest offenders to have much greater sexual arousal to children given the higher number of sexual contacts they have had with children compared with offenders against unrelated children. W. L. Marshall et al. (1986) have suggested that the conditioning would be specific to the related child, so stimuli depicting other children would not be sexually arousing to an incest offender. This does not explain, however, why men who offend against a particular unrelated child are able to generalize to other children.

4 This factor does not confound a paternity uncertainty explanation with a Westermarckian explanation. Being away around the time of the girl’s conception would increase paternity uncertainty but would not contribute to the early propinquity that Westermarck (1891/1921) thought contributed to incest avoidance (which would involve being away after the child was born).