Incest Avoidance and Dating
Humans, like many other animals, possess information-processing programs that guide mate choice. Inherent in mate choice programs are procedures that weigh the potential benefits versus costs of selecting a particular individual as a sexual partner. The question is, which attributes would have been causally linked to the probability of reproductive success and thus part of the calculation that assesses the costs versus benefits of a given sexual partner? Furthermore, how are different attributes traded off against one another? After all, one rarely encounters a mate that is a “10” in every desirable category. That is, one is unlikely to encounter (never mind attract and retain) a mate that is a 10 in every relevant category. Instead, individuals vary in their value as a sexual partner, and to the extent that certain trade-offs had greater positive feedback with respect to selection, then we should see psychologies sensitive to making such trade-offs.
The field of evolutionary psychology has identified many attributes that contribute to a man’s and woman’s value as a sexual partner, including attractiveness, intelligence, and resource status (e.g., Buss 1994). There has been less discussion, however, of how these attributes are traded off against the other factors that might influence mating and dating. One model of mate choice discussed by Lieberman and colleagues (e.g., Tybur et al. 2013; Lieberman and Patrick 2018; Lieberman et al. 2018) suggests there are four major factors that interact to shape perceptions of sexual desirability: (i) the mate value of a potential sexual partner, (ii) one’s own mate value, (iii) the perceived availability of mates in the social environment, and (iv) the estimated genetic relatedness between oneself and a potential sexual partner.
In unpacking the factors highlighted by this model, let’s first clear up some terminology and distinguish two key terms: sexual value and mate value. Sexual value is a term for sexual desirability, but it refers specifically to the desirability of another individual as a sexual partner for oneself. In this sense, sexual value is a subjective term. Mate value, by contrast, is more objective; it is an abstract assessment that refers to the sexual desirability of another individual in general, not for oneself.
Several observations support the notion that sexual value and mate value are related, but distinct, psychological computations. First, individuals from around the globe show surprising consistency in their ratings of physical attractiveness (Symons 1979). Second, people are capable of assessing the mate value of both men and women. Specifically, men and women can rate both men and women in terms of their physical attractiveness, evaluating members of the same and opposite sex on a single scale ranging from extremely attractive to extremely unattractive. But ratings of physical attractiveness do not equal sexual attractiveness to the rater: A man might know his best friend is physically attractive, for instance, yet find the prospect of having sex with him quite disgusting. Third, we can assess the mate value of our genetic relatives (both same and opposite sex), but typically object to sex with our relatives even when assessing their mate value as high. A brother might know his sister is very good looking (and vice versa) but the fact that they have categorized each other as close genetic relatives means that an otherwise attractive sexual partner is turned into a sexual partner of last resort. Thus, mate value is not sensitive to genetic relatedness, but sexual value is.
In general, then, two factors that get traded off against one another to generate a person’s sexual value are mate value and degree of genetic relatedness. Much as mate value is computed based on cues that in ancestral environments correlated with fertility and the ability to rear offspring, genetic relatedness is computed based on cues that correlated with a person being a close genetic relative in ancestral social environments. Extensive research has examined how humans assess relatedness (for review, see Bressan and Kramer 2015). Such research suggests that different cues help to identify different types of close relatives. For instance, men use cues of partner sexual fidelity to identify potential offspring (Daly and Wilson 1988; Apicella and Marlowe 2004; Billingsley et al. 2018), whereas women likely use cues of birth. Children use cues of shared parental investment to identify potential siblings (Lieberman et al. 2007; Lieberman and Billingsley 2016), and likely use information regarding nursing frequency to determine motherness. It is not yet known how children identify their father, but one hypothesis is that children track which man invests most in them and their mother to discern a likely father from other male relatives and unrelated men in the social environment.
Whatever the cues may be, a model of kin detection in humans posits a specific type of program that computes a kinship estimate based on the cues associated with a particular individual. This estimate of genetic relatedness is then one input for the system that calculates a person’s sexual value. For instance, a woman who is evaluating a male might find him to have high mate value, that is, he is healthy and is intelligent, he has resources, and appears willing to invest those resources in a mate. If this male is not a relative, this positive mate value could lead to a high and positive sexual value. However, if he is a close genetic relative, like a brother, the elevated kinship estimate counteracts the high mate value, promoting incest avoidance by pushing sexual value very low.
How low? Well, this depends on the other two factors mentioned above: the availability of potential mates and one’s own mate value. Plus, it depends on one’s own gender, specifically, on how the sexual value system was calibrated during development. The availability of potential mates should moderate the estimated sexual value assigned to a given person. For instance, a person of moderate mate value might be assigned a higher sexual value when there are few potential partners available than when the environment is rich with possible sexual partners. Aside from anecdotal evidence of imprisoned heterosexual men who might engage in homosexual activities in the absence of available females, psychological research suggests mate availability is an important factor in our sexual psychology. As an example, Daniel Conroy-Beam et al. (2016) found that for people in a relationship with a partner of relatively lower mate value, the perceived availability of better partners was linked to lower relationship satisfaction. The context of what’s available and perceptions of an individual’s mate value are somehow interacting to generate sexual motivations and partner preferences.
As Conroy-Beam’s study also suggests, one’s own mate value also influences assessments of sexual value. Only individuals who perceived themselves to have higher mate value than their partner experienced the reduction in relationship satisfaction. In another study (Morgan and Kinsley 2014), researchers manipulated how men felt about their own mate value and then had them look at both attractive and unattractive faces. They found that all men in the study looked at the attractive faces and did so regardless of whether they were led to believe they were of high or low mate value. But men who were led to believe they were of high mate value looked less at the unattractive faces as compared to the men led to believe they were of low mate value. These results suggest that the mind takes into account one’s own condition when making decisions regarding whom to pursue as a sexual partner.
Despite the fact that both men and women likely weigh similar kinds of information when assessing a person’s sexual attractiveness, there are good reasons to expect that male and female brains show different sensitivities to these pieces of information. Take relatedness as an example. Although there are costs of inbreeding that affect both men and women, women are expected to be much more sensitive to these costs because women have a much higher minimum level of investment in a given offspring as compared to men (Trivers 1972). That is, relative to men, women should find a given event of inbreeding more objectionable because women experience larger opportunity costs when it comes to biologically risky sexual encounters. What this means is that the conditions under which a female might consider having sex with a brother need to be far more dire (e.g., on a deserted island with no available mates at all for decades) as compared to the conditions under which a male might consider having sex with a sister (e.g., on a deserted island with no available mates for a month).
In terms of the psychological programming, female minds might be organized to weigh relatedness quite heavily and negatively and not allow for other inputs to change this easily (e.g., number of mates available and one’s own mate value). By contrast, in male minds, sexual value might be more labile, and changed more easily given various contexts. Certainly more work is required to determine the veracity of these claims and how the model proposed above fares in comparison to other proposed models of sexual attraction.
What does all of this have to do with dating? By and large, men and women possess sophisticated programming, calibrated and tuned over the course of one’s lifetime, to help guide mating decisions. Our evolved systems are well equipped to prevent us from sexually desiring our genetic relatives and those perceived to be of low mate value. However, under novel cultural conditions, our system might not operate in the manner we typically expect. For instance, it is possible that the natural mechanism for detecting kin and thus for avoiding incest, can be disrupted. One example would be siblings raised on the Kibbutz. If most siblings were reared apart in separate children’s houses, then siblings would not have categorized one another as kin to the same extent as they would have had they lived under the same roof more regularly. This holds true for siblings separated at birth. Another modern phenomenon is sperm-banks. As sperm-banks become more widely used, women who have never met might be giving birth to paternal half siblings. Should they meet, no natural aversion will be present that arose due to the operation of a kin detection system. Like two strangers, should they meet, half siblings will be left to evaluate each other based on the desired characteristics in a potential mate. Although the chances of such an encounter in the US are extremely low, in some places it can be a bit higher. For instance, in Iceland, a country with a population of just under 400,000, there is now an app to help people identify potential relatives. Before they date, Icelanders can now find out if they are cousins.
In conclusion, Darwinian insights into mating psychology can help reveal the factors that generate a sense of sexual attraction and the circumstances in which each factor might weigh heavily or not at all.
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