Note: The primary target of the post is not professional, academic evolutionary psychology. Rather, I am primarily cautioning amateurs (such as LW regulars) about some of the caveats involved in (armchair) evpsych and noting the rigor required for good theories. While the post does also serve as a warning to be cautious about sloppy research (or sloppy science journalism) that doesn't seem to be taking these issues into account, I do believe that most of the researchers doing serious evpsych research are quite aware of these issues.

Evolutionary theories get mentioned a lot on this site, and I frequently feel that they are given far more weight than would be warranted. In particular, evolutionary theories about sex differences seem to get mentioned and appealed to as if they had an iron-cast certainty. People also don't hesitate to make up their own evolutionary psychological explanations. To counterbalance this, I present a list of evolutionary psychology-related problems, divided into four rough categories.

Problems in hypothesis generation

Rationalization bias. We know that human minds are very prone to first deciding on a desired outcome, then coming up with a plausible-sounding story of why it must be so. In general, our minds have difficulty noticing faulty reasoning if it leads to the right conclusion. It's easy and tempting to come up with an ad-hoc evolutionary explanation for any behavior, regardless of whether or not it actually has any biological roots.

Over-attributing meaning. Humans also have a strong tendency to attribute meaning to random chance. We might easily come up with explanations that are unnecessarily complex, and try to make everything into an evolved adaptation. For instance, humans tend to avoid thinking about unpleasant thoughts about themselves. A contrived evpsych explanation might be that this is evolved self-deception: by not acknowledging our own faults, it makes it easier for us to deceive others about them. But mental unpleasantness tends to be correlated with harmful experiences: we avoid situations where we'd be afraid, and fear is correlated with danger. It could just as well be that the mechanism for avoiding mental unpleasantness evolved from the mechanism for avoiding physical unpleasantness, and we avoid thinking unpleasant thoughts of ourselves for the same reason why we avoid poking our fingers at hot stoves. (Example courtesy of Anna Salamon.)

Alternative ways of reaching the goal. Eliezer previously gave us the example of the scientists who thought insects would under the right circumstances limit their breeding, but the insects ended up eating their competitors' offspring instead. We can only cover a limited part of the space of all possible routes evolution could take. While ”but another hypothesis might explain it better” is admittedly a problem all scientific disciplines face, it is especially acute here, since we have very little knowledge of what life in the EEA was actually like.

Problems in background assumptions

Did a genetic path to the adaptation exist? Evolution works by the rule of immediate advantage: for mutation X to reach fixation, it has to provide an immediate advantage. It's well and good to propose that under specific circumstances, organisms that developed a specific behavior would have gained a fitness advantage. But that, by itself, tells us nothing about how many mutations reaching such a behavior would have required. Nor does it tell us anything about whether all of those intermediate stages actually conferred the organism a fitness benefit, making it possible for the final form of the adaptation to actually be reached.

Was there enough genetic variance of the right kind? For an adaptation to evolve, there had to be enough genetic variance for evolution to feed on at the right time. Again, postulating that an adaptation could have been useful tells us next to nothing about whether or not the variation needed to make it real existed.

Problems in verification

Memetic pressures shaping cultures. When trying to show the existence of biological sex differences, evolutionary psychologists sometimes appeal to cross-cultural studies that show sex differences across a wide variety of cultures. But while this is certainly evidence towards the differences being biological in origin, it's rather weak evidence. Pretty much all cultures in the world tend to be more or less patriarchal in nature. This could be caused by biological causes, but it's equally plausible that it was caused by a memetic selection pressure acting on non-psychological sex differences. Women have less strength than men and are the ones who bear children, which could easily have affected their social position even without drastic psychological differences.

Is something an adaptation? We consider the possibility that certain specific aspects of the faculty of language are “spandrels” — by-products of preexisting constraints rather than end products of a history of natural selection (39). This possibility, which opens the door to other empirical lines of inquiry, is perfectly compatible with our firm support of the adaptationist program. Indeed, it follows directly from the foundational notion that adaptation is an “onerous concept” to be invoked only when alternative explanations fail (40). The question is not whether FLN [the Faculty of Language in a Narrow sense] in toto is adaptive. By allowing us to communicate an endless variety of thoughts, recursion is clearly an adaptive computation. The question is whether particular components of the functioning of FLN are adaptations for language, specifically acted upon by natural selection—or, even more broadly, whether FLN evolved for reasons other than communication.

An analogy may make this distinction clear. The trunk and branches of trees are near-optimal solutions for providing an individual tree’s leaves with access to sunlight. For shrubs and small trees, a wide variety of forms (spreading, spherical, multistalked, etc.) provide good solutions to this problem. For a towering rainforest canopy tree, however, most of these forms are rendered impossible by the various constraints imposed by the properties of cellulose and the problems of sucking water and nutrients up to the leaves high in the air. Some aspects of such trees are clearly adaptations channeled by these constraints; others (e.g., the popping of xylem tubes on hot days, the propensity to be toppled in hurricanes) are presumably unavoidable by-products of such constraints. (Hauser, Chomsky & Fitch 2002)

What is something an adaptation for? For instance, it might seem intuitively obvious that language evolved as a way to communicate. But language also has plenty of other uses, including functions like problem-solving, enhancing social intelligence by rehearsing the thoughts of others, memory aids, focusing attention, and so on. There is evidence that even animals without human language can, for instance, do things such as discriminate various phonemes, suggesting that many key components of language may have evolved as general cognitive capabilities. Human language may then primarily be a result of many non-language related adaptations happening to combine in the appropriate way. It's an empirical question which, if any, of these functions has been the primary force driving the evolution of language. (For a debate on this, see Hauser, Chomsky & Fitch 2002; Pinker & Jackendoff 2005; Fitch, Hauser & Chomsky 2005; Jackendoff & Pinker 2005.)

In the same manner, bats use echolocation to find and capture prey (feeding), to navigate, to find mates, and to engage in aerial dogfights with competitors. We can study bats to obtain plenty of information about how the bat sonar physically and cognitively works and how bats use it. Yet its evolutionary history and the functions that the sonar's early stages were the most useful for are questions that we are mostly incapable of answering. For the most part, such knowledge wouldn't even tell us anything we couldn't more reliably discover via other means. Our inability to verify theories about the adaptive origin of various traits weakens the faith we can place on such theories.

Problems in modern-day meaningfulness

Evolution did not stop after the Pleistoscene. This was covered in more detail in my review of The 10,000 Year Explosion. We know that new adaptations such as the one for lactose tolerance have shown up in the last 8,000 years. We also know that hundreds of "gene sweeps" of specific alleles increasing their frequency in the population are still going on today. While the full functions of these alleles are still not known, it is known that most involve changes in metabolism and digestion, defenses against infectious disease, reproduction, DNA repair, or in the central nervous system. And so on; see the link for more.

The modern environment may alter our biology. To name one example, hormones have a strong impact on human psychology. Yet especially women are likely to have very different hormonal activity than they used to have. We have less children, and have them at a later age we probably did in the EEA. The Pill basically works by screwing up the normal hormonal balance. Some extra hormones are fed to livestock and find their way to our bodies via our food. Even ignoring that possibility, our modern-day diet is very much unlike the one we used to have. We also get far less exercise, and so on. Our environment is likely making our brains different from the way they used to be.

Evolution may have exploited gene-environment relationships that no longer exist. This one is huge. For instance, we know that daylight has a role in regulating our sleep patterns. Now that artificial lightning exists, we routinely stay up for far longer than we would if we had to only go by the sun. More generally, the environment has a massive role in influencing how our brains develop. Children raised by animals do not, as a rule, ever reach a level where they could fully adjust to human society. As our whole society works in a completely different way than it used to, it's nearly certain to have broken numerous relationships that regulated the adaptations in the EEA.

”Human universals” mainly apply on a cultural level.  Even behaviors that were very widespread may or may not apply to any particular individual. Lists of ”human universals” will tell us that members in every tribe found so far will interpret facial expressions, love their children, tell stories, feel pain, experience emotions, and so on. But there are also individuals who do not know how to read facial expressions, do not care for their children, are not interested in stories, do not experience pain or emotions, and so on. Sexuality is one of the drives that would have had the strongest selection pressures operating on it, but we regardless have people who have no interest in sex, are mainly interested in sex with things that you cannot reproduce with (same-sex partners, children, cars...), or prefer to just masturbate.

Conclusion. Evpsych can certainly point us towards interesting novel hypotheses about human behavior. When such hypotheses turn out to be true, then there's indeed a strong possibility that they evolved as adaptations. But it's important to note that while science can provide us strong evidence about the existence of some behavior, it is incapable of providing strong evidence about the evolutionary origins of that behavior. Behavior, as a rule, does not leave convenient fossils behind.

There are basically two kinds of ev-psych explanations: one proposing an evolutionary origin for a present-day trait (an explanation) and one proposing a previously unknown trait based on evolutionary considerations (a prediction). Of these, explanations seem to only have limited value. To make a typical evolutionary psychological claim about the origins of something is to assume, among other things, that the thing in question is an adaptation, that its suggested origin was the primary driver of selection pressure for the adaptation's evolution, that a genetic path existed to the adaptation and there was enough genetic variation to make it possible. These are all claims that are almost impossible to verify or falsify. In most cases, it is better to merely talk about what empirical research has revealed about the thing in question, without giving too much weight to its (unverifiable) evolutionary origins.

Evpsych is more useful for predictions. And it does occasionally produce results you'd never have thought of to test otherwise. Still, even if there seemed to be a very strong case for selection pressures to have existed towards something becoming an adaptation, this tells us next to nothing about whether it actually ended up evolving. Even if we can ascertain that this kind of an effect seems to be prevalent in the world, evolutionary psychology alone cannot tell us the degree to which the effect is amenable to environmental conditions. That sort of information can only be found by ordinary empirical research, and ordinary empirical research doesn't need evolutionary psychology for anything else than suggesting interesting hypotheses.

Evpsych should primarily be used for helping build coherent explanatory frameworks for human behavior and for coming up with new predictions. But someone arguing in favor of some behavior being universal or biologically determined in the modern day shouldn't appeal to evpsych for support, for evpsych can at most weakly suggest such things.

Acknowledgements. Part of the content in this article was adapted from the materials of the Cognitive Science 121 course at University of Helsinki, written by Otto Lappi and Anna-Mari Rusanen.