(Alternate, Twitter-sourced titles: "5 Ways to Prove Darwin Wasn't Crazy," "Shut the Eff Up and Science Already," "5 Ways Psychology Needs to Evolve.")

Evolutionary psychology, the study of human psychological adaptations, does not have a popular or scientific reputation for being rigorous, even though there are rigorous, thoughtful scientists in the field. The field is trying to take on an incredibly challenging task: understand what of human behavior is adaptive and why. We can better circumvent the conditions that lead to violence, war, and hatred if we know as much as we can about why we are the way we are. What motivates us, excites us, angers us, and how can evolutionary theory help us understand it all?

Because of this, there are consequences to a bad evolutionary psychology interpretation of the world. The biggest problem, to my mind, is that so often the conclusions of the bad sort of evolutionary psychology match the stereotypes and cultural expectations we already hold about the world: more feminine women are more beautiful, more masculine men more handsome; appearance is important to men while wealth is important to women; women are prone to flighty changes in political and partner preference depending on the phase of their menstrual cycles. Rather than clue people in to problems with research design or interpretation, this alignment with stereotype further confirms the study. Variation gets erased: in bad evolutionary psychology, there are only straight people, and everyone wants the same things in life. Our brains are iPhones, each app designed for its own special adaptive purpose.

I once had a fellow from this field talk my ear off for fifteen minutes about his “one bad apple spoils the barrel” hypothesis (it was so long ago at this point that I’m not too worried about the story identifying him). He contended that an early-maturing boy was a “bad apple” that would drive other surrounding boys to early puberty. Whenever I politely inquired as to what the mechanism would be that would drive the other boys to mature, or why this would even be adaptive, he would move on feverishly to the next part of his metaphor. “But you see,” he said almost breathlessly, “it’s like the boys are all in a barrel, and when apples are in a barrel one rotten apple can make the others go rotten too.”

No one should ever love their idea so much that it becomes detached from reality, as much an issue for those testing hypotheses as those reading about them in blog posts and magazines. And I think I’ve come up with five exhortations to help any reader trying to tell the good ev psych from the bad.

1. You’re not measuring what you think you’re measuring.

Something we scientists like to do is to operationalize variables. That means that, since we cannot often measure what we want to measure, we come up with some sort of proxy that makes the best of a bad job.

For instance, let’s say what you’d really like to know is whether a trait affects reproductive success in Urbana, Illinois. There are a lot of barriers to being able to tell whether this trait – karaoke ability, for instance – affects the total number of children had by individuals in this population. Humans live a long time, so the project would have to span someone’s entire reproductive years. Many humans also plan their families and so use contraception from time to time, and many perfectly fertile humans make the perfectly rational decision to not have any babies at all. And so even if you could do this study as long as you needed to, you can’t with confidence say that the childfree person is less fertile than the one with seven children.

So, you use some sort of proxy for fertility, something necessary for reproductive success. In women, you may look at their ovarian hormone levels, their endometrial thickness, the length of their cycle or frequency of ovulatory cycles. In men, you could look at testosterone as well as sperm count and quality. Sometimes you have the resources to recruit a number of people trying to conceive, and then you can see how long it takes them to conceive, or whether they do at all. These are all considered pretty good proxies of fecundity, and thus also by extension fertility.

In some studies of evolutionary psychology, a never-before-used variable is often created to serve as a proxy for what they really want to know. Not too long ago I took issue with a “maternal tendencies” variable. Because they couldn’t assess maternal behavior in these young, childless undergraduate women, they asked them how many children they wanted to have. The more children these eighteen and nineteen year olds wanted, the more maternal they were.

Yet desired family size at eighteen, and maternal tendencies as a future mother, are very, very different things. As I pointed out in my post on this, there is too much context-dependence embedded in when you ask women how many kids they want for it to tell you anything with much biological meaning.

So, make sure you’re measuring what you think you’re measuring. And validate the heck out of any new proxy you come up with.

2. Undergrads only teach us about undergrads.

Much of the psychological research coming out of the US and other western countries are performed on the easiest to access sample population: undergraduates eager for cash or extra credit. Many of the major conclusions we make about humankind come out of this very specific group of individuals. Often, the undergrads sampled are mostly white and middle class. Undergraduate sampling is an extreme version of the challenge much human behavior research faces: the use of, and then extension from, WEIRD people.

WEIRD stands for Western, Educated, Industrialized, Rich and Democratic. This particular subset of humans, despite the experience of so many of us who work at universities, is actually not the majority worldwide. The lived experience of being WEIRD means a particular kind of access to resources in terms of money, vaccines, food, school, and government.

Have you ever had someone say they “speak for the moms” or some other subgroup and it made you feel uncomfortable? Have you ever been inadvertently put in the position of having to speak for a group of people, but felt that group was way too variable for your one experience to apply to everyone? This is why testing hypotheses and sampling populations from WEIRD places is such a bad idea, and not just from a cultural standpoint but a biological one. The daily lived experience of those resources, vaccines, schools, and other aspects of the WEIRD environment produce a person very different from one that grows up without a nearby hospital, running water or shoes. Even if your Institutional Review Board gave you permission to separate identical twins at birth and have them grow up in the city and the savanna, their height, weight, hormones, sensitivity to stressors, nearly every imaginable metric would show some interesting variation, much of it due to this lifetime of different experiences.

The two reasons oversampling from WEIRD people is bad is first that oversampling in general is bad, but second that being WEIRD puts you about as far removed from the conditions in which we evolved as you can get. WEIRD stressors are chronic and psychosocial (which makes them great if that’s your research interest, otherwise not so much). They have a lot of weird (ha ha) immune problems, possibly related to under-challenging their immune systems when young. They tend to survive the major childhood illnesses but then die of heart attacks, strokes or cancer. Many of them delay childbearing into well into their reproductive years and breastfeed for a short duration if at all, meaning they have eight to ten times as many menstrual cycles as the average forager. And they tend to have nuclear families, rather than breed cooperatively in large groups, sharing the parenting load among peers and across generations.

So the punchline here is: don’t speak for everyone until you’ve spoken to everyone. (My one exception: The Lorax gets to speak for the trees.)

3. It’s not true that everything happens for a reason.

One of my least favorite papers is on rape as an evolved sexual strategy among humans. The abstract begins,

“Psychological adaptation underlies all human behavior.”

I still remember the photocopied version I first read of this for a class in graduate school, because it was marked up by the professor who had read it first. Next to this line, the professor had written,


And I remember mulling over this, particularly because this professor was not exactly emotive, and so it was interesting to see him have a strong reaction to something. Sure, the whole paper was problematic, and the great thing about Thornhill and Thornhill (1992) is that because they published in Behavioral and Brain Sciences it came with a bunch of commentaries, most of them negative, some of them hilariously witty in their takedowns. Yes, we academics can be witty.

Years later, the first time I taught with this paper, I pulled out that same old photocopied version to make a pdf, and I saw that professor’s comment again. And it struck me how this was one of the fundamental problems with many disciplines that tend towards the adaptationist, including evolutionary psychology. We forget that natural selection and sexual selection are only two ways in which evolution – which is really just change over time – happens. There are also things like genetic drift and mutation, which can also have a direction and also produce change. While this may drive some adaptationists into an existential crisis, sometimes there is no reason at all for a given human behavior or trait. My decision to wear navy socks today, the route I walk from one campus building to another, making cupcakes instead of cookies for my daughter’s playdate, these are behaviors we can tell adaptive stories about.

But it may not be realistic or accurate to do so. And if you do want to tell an adaptive story about it, you have to make sure the argument is pretty airtight.

There are ways to be able to be more confident about whether a trait you’re interested in has been selected (or rather, not eliminated). You can see if it conforms to these three principles:

  1. The trait is variable. The number of fingers on a human hand is not significantly variable since most everybody has five. Hair length is variable.
  2. The trait is heritable. Hair length is not heritable since we cut it to suit our personal and cultural preferences. Freckles are variable and heritable.
  3. The trait produces variation in reproductive success. As far as I know so far, freckles do not affect how many kids you have. Voice pitch, however, is a good example of a trait that is variable, heritable, and has been shown to be correlated with the number of children a man has – in a non-WEIRD environment, no less.

It’s tempting to see selection’s hand in everything people do. But doing so makes the same mistake as those who say that they see design in evolution. It is possible to be enchanted by the amazing things biology can tell us, while accepting the added randomness of existence.

4. There is more than one way to skin a cat.

When an evolutionary psychologist makes a claim about the effectiveness of a human male reproductive strategy like breadwinning, or dominating behavior, or large muscle mass, I cringe. They are usually described – in the press releases, yes, but also in the articles themselves – as a prime determinant for reproductive success. The best strategy. The only strategy.

Or sometimes you see two strategies proposed, which follows sexual selection theory a bit more closely, but then puts those individuals in two categories rather than along a spectrum: for instance, big strong males who show off their gene quality versus more nurturing males who demonstrate their parenting quality. The way many of these studies are designed end up eliciting responses that lead to stark categories.

As it turns out, reproductive strategies – most behavioral strategies, in fact – are widely variable, and you see a pretty stable constellation of them in any given population. Rather than try to promote the idea that one particular strategy is the only one any successful person would think of using, we should be identifying, appreciating, and understanding this variation.

Unsurprisingly, most of these one-size-fits-all assessments of human behavior conform to how we already think men and women should behave in our culture.

What saddens me the most about this particular problem is the way it makes people with non-straight identities invisible, or worse, implicitly pathological. The straighties are doing their adaptive darndest to make babies, but those homosexuals aren’t following the Darwinian directorate to seek opposite sex partners and spread their seed! No matter that many people who identify in one of the many non-straight identities have children of their own, and many of those children are in fact genetically related to them. In fact, to some extent it makes sense to parse out sexual identity and sexual activities from reproductive success.

Finally, for something to be an evolutionarily stable strategy it has to fit a few conditions:

  1. You need clear evidence it is an adaption, which means it has to conform to the conditions from the previous section: being heritable, variable, and producing differential reproductive success.
  2. You would also need evidence that what you’re seeing isn’t simply a correlated response from another, linked behavior being selected.
  3. You would need to demonstrate that the behavior is at least equivalent to, if not resistant to, alternative strategies, in terms of its rate of success.

I laid out how hebephilia fails these tests in a post last year. The problem with demonstrating natural selection, and in particular evolutionarily stable strategies, is that the burden of evidence is incredibly high. Which means most stories that buttress bad evolutionary psychology work will ultimately lead to that study’s collapse, when we see the stories are made of thin air.

If you design your study really well, finding ways to anticipate and control for cultural bias, and still find a correlation, I’m quite happy for you! But chances are good you don’t have enough to contend what you’re seeing is an evolutionarily stable strategy. So hold the storytelling. Just for a little while.

5. Just because it works today, doesn’t mean it worked back in the day.

To illustrate my final point, I turn to a recent post from Scicurious on the supposed significance of wearing high heels. It’s a classic Sci takedown, and it’s worth a thorough read.

Sci details a paper that demonstrates a positive relationship between wearing high heels and perceived attractiveness. The researchers had women walk with and without high heels, then used point light displays to demonstrate walkers’ gait without revealing their appearance. The methods certainly seem carefully constructed to avoid some kinds of bias.

Where the researchers lost Sci – and me – was where they contended that walking in heels is sexier, and represents a “super-stimulus” (think red lipstick to emphasize feminine lips, breast implants to enhance boobs). Part of the reason they make this assertion is that they claim high heels have a long history of being used to emphasize women’s assets. And of course, this is where they’re very wrong, since high heels have a long history of being worn by men, and since in this study they had no way to parse out watchers’ expectations of what constitutes a sexy walk based on their cultural conditioning.

In any case, many of the things we do today are things we did not do in the environment of evolutionary adaptedness – itself a mythical construct where everyone lived happily in the savanna eating wild game and mongongo nuts (totally Paleo diet, man!!!).

A far more useful way to interpret modern behavior is not the specific behavior itself, but perhaps the temperament or aims of the actor. So, making tumblrs of animated gifs about roller derby is not an adaptive behavior, but the motivations that underlie it could be, depending on the circumstance. A sense of humor is likely heritable, variable and can lead to reproductive success. And those gifs are hilarious.

But I want to see someone test it first. And of course, it would be great if we could get to the point where we can do better than presume that many of these behaviors (or again, the motivations behind them) have a genetic underpinning.


The bad parts of evolutionary psychology confirm what we think we already know about the world. And confirming stereotypes and calling it science tends to keep women and GLBT folk as perpetual second class citizens in this world, rather than the amazing, vibrant contributors to society they are and can be.

Evolutionary theory has been developed and tested for quite a long time, and there is a strong, reliable set of conditions we have developed to help us determine adaptive significance for a given trait. All the field of evolutionary psychology really needs is to be put to the test.