Original ArticleGenetic analysis of human extrapair mating: heritability, between-sex correlation, and receptor genes for vasopressin and oxytocin
Introduction
In most socially monogamous species (e.g. many birds and some mammals), both male and female members of a pair commonly seek copulations with other individuals (Barash and Lipton, 2001, Griffith et al., 2002, Reichard, 1995). Males have a low minimal investment to reproduce (i.e. one copulation), so males mating outside the pair can increase their reproductive output; any genes predisposing males to seek extrapair mates would be adaptive (in the absence of strong countervailing selective pressures). However, females' reproductive potential is constrained by their biological capacity to reproduce, so females do not necessarily increase their reproductive potential by extrapair mating—in addition, females may also incur direct costs from extrapair copulations, such as disease transmission and withdrawal of paternal investment into offspring of uncertain paternity (Albrecht et al., 2006, Arnqvist and Kirkpatrick, 2005). As such, it is not clear why females in socially monogamous species have evolved such that they mate outside the pair (Forstmeier, Nakagawa, Griffith, & Kempenaers, 2014).
There have been proposed a number of adaptive explanations for female extrapair mating, along with challenges to the traditional theoretical and empirical basis for the expectation of sex-differentiation in adaptation for extrapair mating (Gowaty, 2013, Gowaty et al., 2012). The dominant explanation of female extrapair mating has been that it can be adaptive if females are able to obtain extrapair mates of higher genetic quality than their social mates, thereby increasing the genetic quality of their offspring and increasing their number of grandoffspring (Jennions and Petrie, 2000, Neff and Pitcher, 2005). However, reviews of the empirical evidence in socially monogamous birds suggest that the genetic benefits to offspring of extrapair matings are generally very weak or nonexistent, and are likely to be outweighed by direct costs (Akçay and Roughgarden, 2007, Arnqvist and Kirkpatrick, 2005). While there was debate as to the correct interpretation of these results (Eliassen and Kokko, 2008, Griffith, 2007), several more recent studies directly testing for such indirect benefits in birds suggest that offspring of extrapair matings actually have lower lifetime fitness and genetic value than offspring of within-pair matings (Hsu et al., 2014, Reid and Sardell, 2012, Sardell et al., 2012; though see Gerlach, McGlothlin, Parker, & Ketterson, 2012), which poses a major challenge to this as a general adaptive explanation of female extrapair mating. As such, alternative explanations need to be considered.
One such alternative (nonadaptive) explanation is the between-sex genetic correlation hypothesis, which is that genetic variants predisposing males to male extrapair mating (and hence putatively selected for) might also predispose females to extrapair mating (Arnqvist and Kirkpatrick, 2005, Forstmeier et al., 2011, Forstmeier et al., 2014). That is, female extrapair mating behaviour is maintained as a byproduct of selection for this behaviour in males. A recent finding of genetic correlations between measures of male and female extrapair mating behaviour in zebra finches (Forstmeier et al., 2011) is consistent with this hypothesis. While this finding does not in itself invalidate adaptive hypotheses in this or other species, it does warrant the consideration of between-sex genetic correlation as a plausible alternative to adaptive explanations of female extrapair mating.
These findings have important implications for evolutionary research into human mating; socially monogamous partnerships are the most common form of marriage even among forager societies in which other arrangements (e.g. polygyny, polyandry, promiscuity) are also common (Marlowe, 2003). As in other species, extrapair copulation is common in humans across cultures (Greiling and Buss, 2000, Marlowe, 2000), and nonpaternity rates are non-zero in all societies that have been studied (Anderson, 2006) and are quite high (9% and 17%) in the two small-scale natural-fertility (i.e. similar to ancestral) populations in which this has been carefully investigated (Neel and Weiss, 1975, Scelza, 2011)—this rate is comparable to an estimated average rate of extrapair paternity among bird species (11%; Griffith et al., 2002).
The dominant evolutionary theories of human mating strategies (e.g. sexual strategies theory Buss & Schmitt, 1993; strategic pluralism Gangestad & Simpson, 2000, dual mating strategies Fisher, 1992) regard both men and women as having evolved distinct psychological mechanisms adapted for both long-term and short-term (including extrapair) mating strategies. Pillsworth and Haselton (2006) specifically propose that women are endowed with suites of adaptations that function to form a social partnership with a man she judges to be a reliable investing partner while surreptitiously seeking good genes (for her offspring) from another man through extrapair sexual encounters. While there is indirect evidence from a variety of sources consistent with this hypothesis (reviewed in Gangestad, 2006, Pillsworth and Haselton, 2006), there is no direct evidence to this effect (e.g. there is no evidence that offspring of extrapair matings are fitter than offspring of within-pair matings). Given this and the aforementioned recent findings in socially monogamous birds, which suggest that extrapair offspring are less fit than within-pair offspring (Hsu et al., 2014, Reid and Sardell, 2012, Sardell et al., 2012) and that there is substantial cross-sex correlation in extrapair mating behaviours (Forstmeier et al., 2011), it is worthwhile investigating the plausibility of the between-sex genetic correlation as an alternative explanation for female extrapair mating in humans. Previously, this alternative explanation has barely been considered.
There is evidence from studies of identical and nonidentical twins that sociosexuality (i.e. orientation towards short- or long-term mating strategy) is heritable in both men and women. Bailey, Kirk, Zhu, Dunne, and Martin (2000) estimated that genetic factors account for 26% and 43% of the variance in men and women, respectively, although it should be noted that the male genetic variance did not reach statistical significance. Furthermore, there was a significant between-sex correlation, consistent with the between-sex genetic correlation hypothesis. However, the sociosexuality score was made up of a variety of measures, most of which did not pertain to extrapair mating per se (i.e. copulating with others while in a pair–bond relationship). There has been one twin study specifically on extrapair mating, but only in women (Cherkas, Oelsner, Mak, Valdes, & Spector, 2004); in that study, 41% of the variance in female infidelity was estimated to be accounted for by genetic factors. It remains unknown as to what extent genetic factors influence men's extrapair mating behaviour and whether they are the same genetic factors as influence on women's extrapair mating behaviour. This knowledge is crucial in weighing the relative merits of adaptionist and genetic-constraint explanations of female extrapair mating in humans.
Here we conduct two studies investigating potential genetic influences on male and female extrapair mating, and whether the same genetic factors influence the behaviour in both sexes. Study 1 uses the classical twin design to estimate the proportion of variation in extrapair mating that can be attributed to genetic differences in general, while study 2 tests variation in two specific genes (oxytocin and vasopressin receptor genes) for association with extrapair mating.
Section snippets
Study 1
In study 1 we used data from 7,378 twins and siblings who are in long-term relationships to estimate within-sex heritability and test for a between-sex correlation in recent extrapair copulation in order to assess the plausibility of the between-sex genetic correlation explanation of female extrapair mating in humans.
Participants
The full Finnish community-based twin-sibling sample consisted of 13,092 individuals aged from 18 to 49 (M = 29.2, SD = 7.3) from 7,737 families (see Johansson et al., 2013); for analysis we used the subset of individuals who had been in a relationship for at least the last year (see Measures for details), which consisted of 7,378 individuals aged from 18 to 49 (M = 29.8, SD = 6.4). Families with only one participating member who was in a relationship were retained because those data help stabilise the
Preliminary testing
Of the individuals who had been in a relationship for at least the last year, 9.8% of men and 6.4% of women reported two or more sexual partners in the last year, indicating extrapair mating. Age effects on extrapair mating were not significant.
Corresponding DZ twin and sibling correlations were equated (i.e. male DZ/sibling pairs; female DZ/sibling pairs; opposite-sex DZ/sibling pairs) without loss of model-fit (χ23 = 3.28, p = .35), consistent with the equal genetic similarity of DZ and sibling
Study 2
In study 2 we tested whether some of the genetic variation in extrapair mating identified in study 1 might be due to specific genes that have previously been associated with pair-bonding behaviour. Arginine vasopressin and oxytocin are hormones found in most mammals. A substantial body of work on monogamous and non-monogamous species of voles implicates these hormones and their receptor genes—arginine vasopressin receptor 1A gene (AVPR1A) and oxytocin receptor gene (OXTR)—in the striking
DNA extraction and genotyping
From saliva samples, 12 SNPs were genotyped in the OXTR gene, and 7 SNPs in the AVPR1A gene. In addition, two microsatellites in the promoter region of AVPR1A (RS1, which is a (GATA)14 tetranucleotide repeat; and RS3, a complex (CT)4-TT-(CT)8-(GT)24 repeat, both upstream from the transcription start site) were genotyped. Saliva samples were collected using the Oragene™ DNA (DNA Genotek, Inc.) self-collection kits that were posted to the participants and returned by mail. The participants were
Descriptive statistics
Genotype data were available for a subset of individuals (n = 2483–2527, the exact sample size varying between different loci due to individual occurrences of genotyping error) from the second data collection of the GSA sample (Johansson et al., 2013). The allele frequencies and genotype distributions for the SNPs can be seen in Table 3 (data for men and women presented together). On average, SNP data were available for 946 men (range = 933–953) and 1564 women (range = 1550–1579).
A standard test for
Discussion
There are several novel findings from these two studies. First, we found significant genetic influences accounting for around half the variation in extrapair mating in both sexes, confirming biological underpinnings to the behaviour. Second, we found a near-zero cross-sex correlation in extrapair mating—that is, 697 brother–sister pairs showed no similarity in likelihood of having extrapair mates. A near-zero cross-sex correlation means that extrapair mating in females is unlikely to be
Acknowledgments
We thank Stuart Macgregor for assistance with the gene-based tests. BPZ is supported by a Discovery Early Career Research Award from the Australian Research Council. PJ was funded by a research grant (no. 138291) from the Academy of Finland.
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