However, this gain is exactly counterbalanced by the evolutionary cost they pay by having their own extra-pair children rejected by other men (Bressan, 2002). By being able to reject adulterine children, fathers who put their own stamp on their offspring do gain an evolutionary benefit. It may be argued that this is indeed why babies do not resemble their biological fathers. ![]() They invest more in children who resemble them more (e.g., Alvergne et al., 2009 Apicella & Marlowe, 2004). Yet, men do use children’s resemblance to themselves as a cue of genetic relatedness (Alexander, 1974). When judges are not informed about genetic relatedness, nearly 20% of 1-year-olds receive a parental resemblance rating of zero on a 0–10 scale, as opposed to less than 5% of 8-year-olds (Bressan & Grassi, 2004 Bressan & Dal Martello, 2002) and the probability of selecting a newborn’s correct parent (out of three potential ones) is, at best, 1.2 times higher than chance (McLain et al., 2000). Third, parental resemblance in very young children is quite poor. Second, parental resemblance develops over time, rather than emerging at birth as it should do if it had evolved for the purpose of rejecting adulterine offspring: the same children are matched far more accurately to their true parents at 16 than at 1 year of age (Bressan & Dal Pos, 2012). Children are matched to one parent as (un)reliably as to the other (Brédart & French, 1999) infants’ parental resemblance to one parent relative to the other is a Gaussian curve, with most individuals resembling mother and father about equally (Bressan & Grassi, 2004). First, paternal resemblance is no stronger than maternal resemblance. Men do not appear to mark their offspring either, as suggested by different lines of evidence. As it happens, direct offspring recognition has never been demonstrated (e.g., Kempenaers, 1996), suggesting that birds and beetles do not mark their offspring-although it seems that, in principle, they could label them with specific plumage or elytra cues. The strategy that might look like the best-labeling one’s progeny with some distinctive, recognizable, heritable paternal badge-seems to be avoided. To estimate their certainty of paternity, animals appear to use behavioral rules of thumb-for example, the number of copulations with their mate (Davies & Quinn, 1992) or by their mate with other males (Møller, 1988), or the frequency of their own encounters with sneaking males (Hunt & Simmons, 2002). This suggests that men can detect the likelihood of extra-pair paternity and that this ability is highly imperfect. ![]() A survey of published estimates shows that in men with high paternity confidence the median non-paternity rate is below 2%, whereas in men with low paternity confidence it jumps to about 30% (Anderson, 2006). In our species, too, paternal investment is strongly related to paternity confidence (Apicella & Marlowe, 2004 Fox & Bruce, 2001), and paternity confidence is strongly related to the probability of paternity. There is ample evidence that paternal effort is reduced in response to lowered likelihood of paternity in several species of birds, such as dunnocks and swallows (Davies & Quinn, 1992 Møller, 1988), and in some insects, for example in beetles (Hunt & Simmons, 2002). What they can do is adjust parental care to the probability of being genetically related to them (Trivers, 1972). Males can never be certain they have fathered their mates’ offspring. An exploratory analysis showed that these large, robust effects disappeared in men who had felt rejected by their fathers while growing up-suggesting that such men are not expecting to invest in their own children either. Light-eyed men liked light-eyed women better (particularly as long-term companions), and feared light-eyed rivals more, than did dark-eyed men. ![]() Here I test these ideas using the data of over 1000 men who rated the facial attractiveness of potential partners, and the threat of potential rivals, whose eye color had been manipulated. Yet because choosiness is costly and paternity concerns are entirely driven by the prospect of paternal investment, any such inclinations would be adaptive only in men who expect to invest in their children. This notion implies that men with light (blue or green) eyes should (1) prefer light-eyed women, especially in a long-term context, and (2) feel more threatened by light-eyed than by dark-eyed rivals. Pairing with a woman who lacks the same allele must increase paternity confidence in these men, because any children with dark eyes would be extremely unlikely to have been fathered by them. Men with light eyes lack the dominant gene allele that codes for dark-brown eyes.
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