At some point a scientific theory may become so well established, having withstood test after test, that it can then be used for useful predictions. Electromagnetism and general relativity comes to mind. These theories are being used every day for prediction, in scientific research as well as for more practical purposes.
Biology has mostly been devoid of theories of this caliber, and this is mainly due to the complexity of life. And by complexity I here mean that it is very complicated to understand, because there are so many more parameters to account for compared to electromagnetism etc. The theory of evolution, mathematically the most advanced in biology, has also been tested relentlessly, and, while a theory in flux, has come up on top every time. Additionally, some parts of the theory can be used to predict outcomes with high precision, namely in the case of population genetics, which predicts allele frequencies in population under the four forces of evolution: natural selection, mutation, genetic drift, and gene flow.
But the part of the theory which is historical, i.e. where it attempts to deduce what happened in the past, isn't quite as rigid. This is not a special feature of the theory of evolution, really, but just a general problem inferring about past events. Physics is on comparably shaky ground when it is used to tell us about the history of the Universe, for example. Phylogenetics, the discipline of reconstructing evolutionary relationships among species (or larger groups of species, or of genes), based on the idea of common ancestry of species, have had many successes "predicting" past events. Hypotheses can actually be tested, given the fossil record and the extensive genetic data we now posses.
Evolutionary psychology, on the other hand, is quite another story. It is a discipline in psychology (and not in biology) that attempts to make sense of the human psyche in the light of evolution. Specifically, human traits, such as behavior and instincts, are given explanations that make sense in the light of natural selection. EP relies heavily, to the point of exclusion, on natural selection (including sexual selection) to understand why we are as we are. In fact, the Tooby and Cosmides 2005 paper on the foundations of evolutionary psychology begins by recognizing that it is the "theory of evolution by natural selection [that] has revolutionary implications for understanding the design of the human mind and brain," not the theory of evolution. At this point we notice that this means only one of the four forces of evolution is invoked. Notably, genetic drift is completely ignored, as it is an effect of random sampling, which does not offer sexy explanations of human traits. "The fact that humans are instinctively afraid of snakes evolved via genetic drift, and as such is a trait that doesn't really mean anything in our evolutionary past or present." That doesn't really make any sense in the case of ohidiophobia, but even if it did it wouldn't be something that would be worth the effort for an evolutionary psychologist to report to the rest of the scientific community, nor, and this becomes important*, to the general public.
The reason I started out by talking about the theory of evolution as an established theory was to drive home the point that we now do know, as much as it is possible to know anything about the past if you weren't there in person, that natural selection has been a major component in shaping life as it evolved, and that undoubtedly includes the human psyche. However, that does not mean that we can know for certain what the adaptive value of a given trait was when it evolved (here I really mean "went to fixation because of" when I say "evolved"). Take the female affinity to the color pink. Women like pink more than men do. This is something that can be easily confirmed by experiments with real live humans. Once the fact is substantiated in this way, evolutionary psychologists proceed to invoke the natural selection to explain it. "Women like pink because our female ancestors had to be more sensitive to that color when they were gathering fruit, and had to pick the best ones." This might be a decent hypothesis, but it is one that cannot be tested. If it cannot be tested, it is a dead end, scientifically speaking. If we could rule out all other hypotheses, then we would have a strong case, but we humans are very good at coming up with stories to explain the world around us. "Women like pink because the were the ones raising babies and thus needed to be highly attuned to the color of baby cheeks (which are pink on occasion)."
Moreover, just because a trait has a specific function at the present doesn't mean that it initially evolved by selection acting on that trait. The trait could have had a different function, which then changed as the environment (or rather, the fitness landscape) changed. One example is feathers, which without a doubt is functional in flying, but most likely first evolved as insulation. Such co-option, or exaptation, is very common in evolution. Yet, selection is all evolutionary psychologists have to work with, and they apply it heavily to tell some very intriguing stories* about the evolution of the human mind.
*Which is the subject of another post coming soon.
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