Evolutionary psychology is the study of how evolution has shaped human psychology. Despite not having a particularly creative name, this discipline is critically important in understanding why humans act, think and feel the way we do. Unfortunately for those of interested in why human behaviour can be so bizarre, evolutionary psychology is a field that's riddled with psuedoscience.
The problem
The big issue stems from how data is gathered. After all, for evolution to influence behaviour that characteristic has to be heritable (i.e., have a genetic foundation). However, establishing a definitive link between a particular behaviour and a gene would require human testing on a scale that would make the Nazis queasy.
Instead, the researchers reason that since humans are almost genetically identical then behaviours with a genetic basis should also be very similar between people. So rather than conducting horrific genetic experiments they just look for "universal" behaviours instead. Everyone's a winner.
Or at least they would be if the evolutionary psychologists were better at looking for these behaviours. Reviews of research reveals that upwards of 90% of psychology studies are conducted on Westerners 1 (and ~85% of those may be performed on undergrads only 2). It's rather hard to make conclusions about behaviours present in all humans when you're only looking at a really narrow sub-section of humanity.
And sure enough, many of these behaviours begin to disappear when you start to study people who aren't Western university students. Turns out these "universal" behaviours are actually limited to people growing up in a very specific culture 3. Even optical illusions affect people from different cultures differently. Our psychology can be that different 4.
All of this makes it rather hard to trust claims that evolutionary psychologists looking at Westerners have discovered evolved, universal behaviours.
The rebuttal
Of course, many researchers are quick to leap to the defence of evolutionary psychology. They point out that the study which found so much research relies on Westerners surveyed psychology as a whole. Kurzban examined only evolutionary psychology research and found that the actual numbers may be far lower; indicating that only 65% of EP research used Westerners. Although there's still room for improvement, clearly things aren't as hopeless for evolutionary psychology as they may first seem.
In fact, last time I wrote about the issue of sampling in evolutionary psychology several commenters pointed me towards Kurzban's work in an effort to dispute what I was saying. However I was not convinced. Kurzban examined a single year of publications in a single EP journal. The aforementioned research into the whole of psychology studied six journals for a five year period 1. In other words, I suspected Kurzban's conclusions may not be representative of the whole of EP literature. A larger, more long term survey was needed.
The survey
So I set out to do that survey. I wanted to examine research published between 2003 - 2007 as that was the period examined in the aforementioned survey of psychology as a whole. Thus the two data sets would be comparable. I also looked at research published in 2013; for more up-to-date data.
Unfortunately many evolutionary psychology journals are relatively new, or moved around during this period and archives were lost in the move. In the end though I was able to find 3 journals that met my criteria: Evolutionary Psychology, Human Nature and Evolution and Human Behaviour.
So I went through all the research conducted on humans published in those journals during this period. Whilst I was doing this I also looked for whether the research was conducted on university students. Because that would just exacerbate the unrepresentative nature of these samples.
The results
The results of this survey are presented in that table. As it shows, on average 75% of evolutionary psychology research uses only Western (aka WEIRD) samples. I'm not sure there's really much more I can add to that. Clearly there is a significant issue with the sampling practices of evolutionary psychology research.
The vast majority of evolutionary psychology research is potentially inapplicable to non-Westerners. This effectively undermines so many EP ideas it isn't even funny. So much of our knowledge about the evolution of human behaviour may have to be discarded.
On the plus side, a statistical comparison with the results for the entirety of psychology (which as previously mentioned, uses Western samples in ~95% of cases 1) reveals that evolutionary psychology uses Westerners significantly less often. Although I think that's small comfort in this case. Oh; and it turns out EP research utilises university students about 40% of the time, which is half the rate (~80% 2) of psychology as a whole. Again, not sure this is really a cause of celebration.
In short, evolutionary psychology's reliability is contingent upon the fact its results can be applied to all of humanity. Yet the fact that 75% of research only studies Westerners means that this may not be the case. For all we know 3/4 of EP is basically psuedoscience.
References
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Arnett, J. (2008). The neglected 95%: why American psychology needs to become less American. American Psychologist, 63(7), p.602.
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Sears, D. (1986). College sophomores in the laboratory: Influences of a narrow data base on social psychology's view of human nature. Journal of personality and social psychology, 51(3), p.515.
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Henrich, J., Heine, S. and Norenzayan, A. (2010). Beyond WEIRD: Towards a broad-based behavioral science. Behavioral and Brain Sciences, 33(2-3), pp.111-135.
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Henrich, J., Heine, S. and Norenzayan, A. (2010). The weirdest people in the world?. Behavioral and brain sciences, 33(2-3), pp.61-83.