Don't be mad once you see that he want it
If you liked it, you shoulda put a ring on it.
-- Beyonce, Single Ladies
As a computational neuroscientist I'm a sucker for mathematical models of human behavior, so when biomathematician Sergey Gavrilets published a recent paper in the prestigious PNAS journal purportedly using equations to show how human pair-bonding evolved out of pre-human promiscuity, I was naturally intrigued. Intrigued, and skeptical. Mathematical models are only as useful as the assumptions that go into them. But Gavrilet's equations make some plausible and fairly sophisticated assumptions, the most important of which are:
- Male and female behavior influence one another (they evolve simultaneously and reciprocally)
- Men divide their time between fighting and females (i.e., between contending against other males for dominance, and provisioning mates and offspring with food and guarding these mates)
- Male skills are variable (specifically, fighting effectiveness is correlated with dominance rank)
- Female fidelity is variable (i.e., some ladies are more faithful than others, and faithful ladies are more likely to seek out a devoted gentleman)
- Ladies enjoy mating with dominant males (i.e., females seek out the "superior genes" of alphas independent of their level of devotion)
Gavrilet takes these assumptions, wrangles them together in some reasonable though hairy-looking non-linear equations, and starts his model running in a population without monogamy. Over time, less dominant men become more willing to stick with faithful ladies, and faithful ladies become more interested in devoted (high-provisioning) men. In other words, marriage evolves out of infidelity. After he runs the numbers, Gavrilet's equations also produce some plausible auxiliary consequences: women never become completely faithful, high-ranking men never become highly devoted, and there's always a dynamic balance between the faithfulness of women and the devotion.
All in all, a reasonable and intriguing model. Sure, we all know human relationships and sexual strategies are more complicated than these fairly complicated equations, but you need to start somewhere. Since the model seems credible on the face of it, we must now ask: does the empirical sexual behavior of men and women match the predictions of Gavrilet's model?
Gavrilet's model suggests that women should be attracted to both alpha males and high-provisioning males of any rank, and more willing to enter monogamous relationships with high-provisioning than low-provisioning males. Non-alpha men should invest resources exclusively in faithful women. In our own research, we looked at the sexual interests and activities of millions of men and women around the world. Do everyday erotic and romantic preferences support Gavrilet's model?
The most popular erotic artifact for women is the romance novel (as popular among women as pornography is among men), which consistently describes the predominant female erotic fantasy: a dominant, powerful alpha male who must spend the narrative demonstrating that he hides a kind, emotionally complex, and loving heart devoted solely to the heroine. Women's erotic literature (such as Fifty Shades of Grey) also features "coconut" heroes: hard and tough on the outside, soft and gooey on the inside. In other words, female fantasy confirms Gavrilet's two predictions about female preference: that women are attracted to alphas, and women are attracted to men willing to devote (material and emotional) resources to women. But they are most attracted to men who exhibit both dominance and devotion!
What about men wanting to invest their resources in faithful women? Unfortunately, this is where there is a very large discrepancy between prediction and data. First of all, faithful women are not in any way part of male erotic fantasies. Male-targeted erotica focuses on spontaneous, uncommitted, anonymous sex, and includes a number of popular erotic genres that are based upon promiscuous women (e.g., bukkake porn, gangbang porn, cheating wife porn, etc.) One extremely popular genre of porn across all cultures is MILF porn, which places value on older women who are aggressive and seductive and who don't require anything in the way of resource investment.
Ah, but these are sexual preferences--and most men distinguish between what they're looking for in a sex partner and in a relationship partner. After all, doesn't Gavrilet say that men are most willing to invest resources in faithful women? However, even if we consider resource investment, the data does not support the prediction. We interviewed a number of adult actresses, adult webmasters, and adult content producers as part of our research. One question we always asked was, when male fans write to adult models and actresses, what do they say? One prominent adult actress even shared a sampling of the fan mail she received with us.
So what do men say when they write to a porn star? The single most common theme is very clear: men offer resources in exchange for exclusive access. "She received jewelry, electronics, gift cards, clothes, even cash," explained one webmaster, discussing the most popular pornographic model on his website. Porn star fan mail frequently suggests that the actress does not need to be in the adult industry; the fan mail author inevitably offers their own services for exiting the industry. There are countless examples of wealthy men offering resources to strippers and other women who would appear to be poor prospects for sexual fidelity (e.g., J. Howard Marshall and Anna Nicole Smith.) In other words, men are entirely willing to devote resources to promiscuous women.
Men certainly value female fidelity and devote substantial energy to mate-guarding. But the relationship between male provisioning and female faithfulness appears to be operationally different than represented in Gavrilet's model. In a large population of men, provisioning seems to be triggered by erotic interest, rather than perceptions of female fidelity. In fact, when we consider the single largest data set on sexual behavior — the Internet — the most common trigger of the male provisioning impulse appears to be female youth.
Gavrilet's model appears to be an admirable and highly useful step forward in the mathematics of human social evolution. The greatest weakness (other than its necessary simplicity) appears to be its assumptions and predictions regarding male provisioning behavior.
