As most of us are aware, humans are not monogamous. We’re sometimes socially monogamous, but in practice almost all of us have more than one sexual partner during a lifetime.
As with most behavioral patterns, it would be naive to suggest that there is a single cause for human infidelity, but we can certainly point to some factors as having a lot of weight. Similarly, we must be aware that while some evolutionary developments are the same for both sexes, there are a large number of inherent differences between males and females.
Male polygamy is — at least on the surface — easier to explain than female polygamy. Males produce sperm. Sperm is cheap. The energy expenditure of inseminating a female is minimal. In theory, a male can father as many offspring as the number of females he can copulate with (or more in species with multiple offspring per insemination). The genetic winner in any male population is the one with the most surviving (and reproducing) offspring. So it makes sense for males to be polygamous.
But this is only one piece of the puzzle. The great equalizer is parental investment. Some species, like many fish and insects, have virtually no parental involvement after birth. The young are cast into the world in huge numbers with the hope that a small percentage will survive. Other species, like humans and birds, produce very few offspring at a time. In order for any of them to survive, they need help. Parents guard them, feed them, train them, and often develop emotional bonds with them.
In general, as the amount of required parental involvement goes up, the value of extra-pair copulation (cheating) goes down. The reason for this is simple economics. If it requires X resources for an offspring to survive, and the female can only provide X-Y resources, it falls to the male to provide Y. The value of polygamy is then limited to Y*M, where M refers to the total number of mates and offspring a male can support with his available resources.
Female mammals are uniquely able to support their young in ways that males cannot. They produce milk. (We don’t call them mamm-ary glands for nothing.) In this capacity, females are in a position to at least provide a minimal degree of sustenance by themselves, and be less “needy” when it comes to male parental investment.
Based on this observation alone, we would predict that mammals ought to be highly polygamous. And this prediction bears out. Of the approximately 4000 species of mammals, only a handful are truly monogamous.
When we examine human society, we often fall victim to what I’ll call the “myth of convenience.” That is, we equate parental hardship with evolutionary prescription. In reality, evolution doesn’t particularly care about parents. Note that in many species, parents die during or shortly after childbirth. The important thing for the success of a species is the survival and reproduction of offspring — not the relative happiness of any individual. Viewed from this perspective, it becomes easy to see that the success of males in philandering is directly in line with the survival of offspring. Females, having the greater investment in their offspring, will do everything they can to help them survive — even if the father abandons them. Mammals are uniquely equipped to handle paternal abandonment, and thus uniquely equipped to support male infidelity or polygamy.
Is it a coincidence that humans are obsessed with breasts?* I think that it cannot be. While it’s true that by definition, breasts are “secondary sexual organs,” the social impact of the function of breasts cannot be overlooked in explaining our cultural infatuation with them. And while it’s also true that cultural obsession with breasts seems at least partially linked to the degree of modesty (that is, the more we insist on them being covered, the more we want them exposed), we cannot avoid the observation that if they weren’t important at all, we wouldn’t insist on them being covered in the first place. All sexual propaganda aside, there really is something unique and special about breasts, and unfortunately for women, it doesn’t seem to be to their advantage, at least in the pursuit of monogamy.
As with all discussions of evolutionary descriptions, we must be careful not to jump directly to prescriptions. Again, evolution didn’t give us our drives so that we’d be as personally self-actualized as possible. It gave them to us so that we’d reproduce in a way that benefits the species in a general sense. We have big brains, and we can decide to act differently than our biological drives demand. But if we make that decision, we must recognize that we are standing against millions — billions — of years of programming, and that such decisions will be hard work. On the other hand, we can also choose to intelligently follow our biological dictates while living up to our post-industrial obligations to our peers. Either option represents a struggle, since societal norms are sometimes as strong or stronger than biological imperatives. But in any case, knowing more about our nature is better than knowing less, or worse — indulging in fantasies about humans’ “higher natures,” or imperatives to be “better than other animals.”
* Without even a close second, my most popular blog entry ever is this one: Let’s Talk About Breasts.