We’re often told that how or why we have sex is the result of an evolved strategy. Such assertions seek to inform us of the real reason why we have sex the way we do—and most people equate “evolved” with “natural.” This is a problem, because sex does not serve only one purpose, and there is no one “right” way to have sex.

Don’t get me wrong: Our bodies and behavior are shaped by our evolutionary histories, and evolutionary processes do influence why and how we have sex. Sex evolved millions of years ago and has worked out pretty well for many organisms on the planet. But sex is complicated, very complicated, even for the simplest of worms. So why should it be an easier to understand in humans?

Unfortunately, instead of embracing the complexity of sex, many recent “evolutionary” hypotheses get too specific about attraction and sex. One tells us that men naturally find women with tattoos more sexually attractive because they are sending signals of heightened receptivity. Another that men naturally prefer a specific waist-to-hip ratio (the “hourglass" figure) in their female sex partners because it signals better reproductive qualities. There is even one that tells us that women prefer men with the most symmetrical faces because they have “better genes” and that these men give off a scent that indicates the quality of their symmetry and really turns on women at the peak of their fertility cycle.

All of these hypotheses make two assumptions:

  1. People really have sex because sex is “for” reproduction.
  2. Sexual attraction is primarily driven by evolved tendencies.

These assumptions imply that if one is not a heterosexual, has no interest in sex, does not behave like the hypotheses predict, or is primarily interested in sexual activity that does not involve the possibility of a sperm and egg meeting up, he or she is doing something out of the evolutionary norm, and maybe even “unnatural.” They also underplay the enormous complexity in human social lives and individual experience and represent a serious misunderstanding of how evolution works, what sex is, and why humans have it.

Behavioral and physiological processes (like attraction and sexual activity) are not “evolved for” a purpose, they are “evolved as” part of a system. Our hands did not evolve for the fine-tuned motor skills we have. They evolved increasingly better capacities at fine-tuned motor skills as part of our changing bodies, brains, and ecologies over time—as part of a system. Human hands are perfectly designed to play online games and to text and tweet, but they did not “evolve” for any of those things. Hands also help build, cook, massage, tie knots, send signals, play the piano—our anatomy and behavior are shaped by evolutionary history, but that history does not necessarily predetermine how we use them. Sex can involve genital-genital contact and even the exchange of fluids, and possibly end up in reproduction, but sexual activity is not limited to, or always driven by, those aspects.

In fact, all evolutionary processes need to do is ensure that individuals within a species want to, at least occasionally, have sex with each other and that some of that sex results in reproduction. Of course, given the way natural selection and other processes of evolution work, some behavioral and physiological processes will be favored over time and become more common in subsequent populations. This “favoring” might occur through being particularly attractive to sexual partners or it might be related to improved abilities to get food, avoid predators, and/or cooperate better or fight more effectively with other members of the species. The reasons for why and how we have sex are not always, or even necessarily, tied to making babies or even to the evolutionary quality of reproductive partners.

Sex in many species (humans, primates, whales, and dolphins, for example) can be as much about building bonds and friendships, and negotiating conflicts and providing social support, as it is about reproduction. In humans it’s even more social and more frequent than in other animals. Most human sex, including heterosexual genital-genital contact, does not result in reproduction—in fact, much of it occurs outside any possibility of reproduction.

Sure, there might be benefits to having sex with a partner who has a specific physical or genetic characteristic, but it also might be really fun and/or socially beneficial to have sex with someone who does not. In most cases, the specific details of each bout of sexual activity do not matter in the bigger evolutionary picture. In a strict biological evolutionary context all that matters is whether or not you and/or your close relatives get copies of your genetic material into the next generation. The “how” is not so important.

The point is that in complex social animals (and humans are amongst the most social and the most complex) no single evolutionary explanation is going to come close to providing an encompassing explanation for why they do what they do, or feel what they feel.

As such, how can we get a better handle on why we have sex (or not) the way we do? Sex and sexuality in humans, across the lifespan, is messy and changes depending on a multitude of variables. Looking for one or even two core explanations of why you feel and act the way you do will leave you with a very incomplete answer. For example, being attracted to someone does not necessarily create a desire to have sex with them. Alternately, being attracted to someone is not always the first reason, or even a necessary reason, for people to have sex. Also, humans can have a huge range of sexual desires, urges, and feelings on which they may or may not act. Evolutionary history tells us a lot about the general parameters of sexual activity; a bit about why sex feels the way it does; and maybe even a little about why we get turned on (or off) by something. But thinking simply in a model that relates sex to the likelihood of reproduction tells us very little about how individuals experience feelings, desires, and sexuality.

Biological sex is the result of entangled biologies, bodies, minds, experiences, and expectations. Why would we expect any less complexity in how and why we actually engage in sexual activity? Each of our sex lives exists in the context of a particular society with rules and expectations, in a particular cultural history wherein we are told what is correct and incorrect about sex, and in the context of our evolving biology.

Trying to pigeonhole sex into one or two “real” explanations is fruitless and damaging. A better approach is to recognize the multiple influences on our actions, to recognize that there are many fruitful ways to successfully be human, and to try to understand how sex and sexuality emerge and play out for individuals, in societies, and across our species.

Evolutionary hypotheses, societal expectations, and personal experience all matter in explaining why we do what we do. Choosing one as more important than the others is not going to make sex any less complex, or easier to deal with. As the science writer Carl Zimmer tells us about the vinegar worm, even “in the simplest animal imaginable, sex can be wonderfully difficult to decipher.”

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