First, let's look at what evolutionary psychologists think evolutionary psychology brings to the table. Here's the opening to Leda Cosmides and John Tooby's primer on evolutionary psychology
The goal of research in evolutionary psychology is to discover and understand the design of the human mind. Evolutionary psychology is an approach to psychology, in which knowledge and principles from evolutionary biology are put to use in research on the structure of the human mind. It is not an area of study, like vision, reasoning, or social behavior. It is a way of thinking about psychology that can be applied to any topic within it. [original emphasis]In other words, evolutionary psychology is a paradigm that will allow us to understand the mind. Moreover, it is a paradigm designed to replace the naive paradigm that has dominated the psychological sciences since the days of William James. The paradigm approaches problems of the mind with 5 principles, which Cosmides and Tooby claim come from biology:
Principle 1. The brain is a physical system. It functions as a computer. Its circuits are designed to generate behavior that is appropriate to your environmental circumstances.
Principle 2. Our neural circuits were designed by natural selection to solve problems that our ancestors faced during our species' evolutionary history.
Principle 3. Consciousness is just the tip of the iceberg; most of what goes on in your mind is hidden from you. As a result, your conscious experience can mislead you into thinking that our circuitry is simpler that it really is. Most problems that you experience as easy to solve are very difficult to solve -- they require very complicated neural circuitry.
Principle 4. Different neural circuits are specialized for solving different adaptive problems.
Principle 5. Our modern skulls house a stone age mind.
Even putting aside any disagreements I might have with any of these principles, there's something odd about listing them as the principles of evolutionary psychology. What's strange about it is that, at least since the beginning of the cognitive revolution, each of these principles has been accepted by many, if not all cognitive scientists. There's something slightly disingenuous about listing them as the principles that define a new paradigm, then. A more accurate description of this new paradigm would be that it accepts the same principles that other cognitive scientists do, but places more emphasis on some principles than the accepted paradigm has. In particularly, it places more emphasis on principles 2 and 5. In practice, this has also led to a different interpretation of 4, but we'll get to that later.
Our questions about the usefuleness of evolutionary psychology, and why, with its popular success, most experts haven't bought into it, can be rephrased as questions about the usefuleness of increased emphasis on principles 2 and 5, and why so few experts have accepted this increase. So, we should start with why evolutionary psychologists believe that emphasizing 2 and 5 is important, and after that, we might begin to understand why the rest of psychology doesn't share their belief.
The most common explanation for the usefuleness of an emphasis on 2 (given by Cosmides and Tooby, as well as other prominent evolutionary psychologists like Pinker and Buss) is that understanding the conditions under which human cognitive capacities evolved will help us to understand their functions, which will in turn help us to understand the capacities themselves. Cosmides and Tooby write:
Realizing that the function of the brain is information-processing has allowed cognitive scientists to resolve (at least one version of) the mind/body problem. For cognitive scientists, brain and mind are terms that refer to the same system, which can be described in two complementary ways -- either in terms of its physical properties (the brain), or in terms of its information-processing operation (the mind). The physical organization of the brain evolved because that physical organization brought about certain information-processing relationships -- ones that were adaptive.This is, in fact, an excellent example of how understanding the function of cognition helped us to understand it. This understanding of function is, in fact, the paradigm shift that brought about the cognitive revolution. It helped us to go beyond simple associationist conceptions of behavior, and has led to the bulk of what we have learned about human cognition over the last 50 years. But once again, I'm afraid that Cosmides and Tooby are being a bit sneaky. They imply that the foundational insight of the cognitive revolution was brought about through evolutionary considerations. In fact, if you read the papers that launched the cognitive revolution, you will find few if any references to evolution or adaptation. Consider, for instance, Alan Turing's famous paper from 1950 titled "Computing Machinery and Intelligence." In it, you will find some references to evolution, but not as a producer of the information-processing brain. Instead, evolution is used as an analogy. Here's what Turing writes:
So, even though evolution had little to do with the adoption of the information-processing paradigm, it may serve as an argument for the utility of evolutionary psychology by showing that knowledge of functions is important, if evolutionary psychology can show that it provides knowledge of functions that we wouldn't have gained without evolutionary considerations. This would justify the emphasis on principle 5, which in turn requires principle 2 to work (if the contemporary cognitive system is not the ancient one, then evolutionary considerations won't do us any good), so we have an argument for an increased emphasis on both.
We have thus divided our problem into two parts. The child-programme and the education process. These two remain very closely connected. We cannot expect to find a good child-machine at the first attempt. One must experiment with teaching one such machine and see how well it learns. One can then try another and see if it is better or worse. There is an obvious connection between this process and evolution, by the identifications
Structure of the child machine = Hereditary material
Changes of the child machine = Mutations
Natural selection = Judgment of the experimenter
One may hope, however, that this process will be more expeditious than evolution. The survival of the fittest is a slow method for measuring advantages. The experimenter, by the exercise of intelligence, should be able to speed it up. Equally important is the fact that he is not restricted to random mutations. If he can trace a cause for some weakness he can probably think of the kind of mutation which will improve it.
Are there examples of such knowledge gained from evolutionary psychology? Tooby and Cosmides would suggest that their social exchange theory has provided just such knowledge about one type of reasoning. In case you don't know, social exchange theory posits that exchange is ubiquitous in social interactions, and we have an adaptive interest in determining whether those with whom we are exchanging are cheating. We have therefore evolved a cheater-detection module. This module explains our performance on social exchange versions of the Wason selection task, they argue. Thus, the evolutionary perspective has provided us with knowledge of function, and knowledge of function has helped us to understand particular behaviors, in this case, particular types of reasoning. Is this theory correct, and if so, could we have arrived at it without considering our evolutionary history? The answer to the first question is almost certainly no (see here and here). This makes the second question unnecessary, but just in case, we should try to answer it anyway. Clearly, the evolutionary perspective aided Tooby and Cosmides in the generation of hypotheses and experiments designed to test them. There are other perspectives that might have yielded the same hypotheses, however. Tooby and Cosmides are fond of comparing their theory to theories in economics. This is because their evolutionary story is largely a reworded economic one. An economic theory, therefore, might have yielded similar hypotheses.
But back to the fact that Tooby and Cosmides' theory is probably wrong. What if, instead of adopting a theory of the function of a particular cognitive capacity beforehand, based on evolutionary considerations, the two researchers had instead explored the contemporary cognitive data, and developed a theory of function out of this? If they had, they would have learned quite early on that there are several factors unrelated to social exchange that improve performance in the Wason task. There theory would therefore have been more viable. Now we start to see why cognitive psychologists, by and large, are not evolutionary psychologists, but more on that will have to wait. We should still look at the different understanding of principle 4 that we get from evolutionary psychology.
Principle 4 is essentially the principle of modularity. It says that there brain areas are specialized to perform particular functions. Under the evolutionary view, this means something else on top of this. It means that particular modules were developed through evolution to perform their specific functions. Examples of widely acknowledged modules include the visual system (which contains various sub-modules designed to perform particular visual tasks, ranging from color vision and edge-detection to object recognition), the motor system, and other pre-cortical systems. We may even have a model designed specifically to recognize faces, though the jury is still out on that one. The cheater-detection module is an example of a frontal brain area (or system) that may have evolved to perform a specific function, in this case detecting cheaters. There is even some (sketchy) neuroscientific evidence indicating that particular cortical regions are active during cheater-detection tasks but not in similarly-structured but unrelated tasks. However, while the modularity of certain pre-frontal regions, mostly associated with lower level perceptual, motor, or regulatory tasks, is well established, frontal lobe modularity is not. Furthermore, the frontal lobe modularity that has been theorized in neuroscience need not be evolved. Many of them may develop ontogenetically. In fact, at this point, it's not clear that we are going to discover many brain region-higher-order cognitive capacity associations that are generalizable across individuals. It may be that much of the organization of the frontal lobe is due largely to experience, rather than evolution. If this is the case, then the evolutionary psychological understanding of modularity may be misguided for just those cognitive capacities that it is designed to explain.
So, we now have two reasons for doubting the utility of evolutionary reasoning for understanding cognition. The first is that experience and the scientific method tell us that, at least in most cases, we will develop better hypotheses by considering data from modern human subjects, rather than their evolutionary history. This is not to say that evolution should not be used to constrain the range of possible hypotheses, but it does mean that evolutionary considerations are not likely to provide much more than theoretical boundaries. Furthermore, as our neuroscientific knowledge of higher-order cognitive capacities becomes more sophisticated, it may replace evolutionary considerations in their role as boundary providers. The second is that the evolutionary paradigm requires a strong, massive, and hard-wired modularity in the brain areas responsible for higher-order cognitive functions, and this sort of modularity may not be the way cortical regions work, and what modularity does exist may be ontogenetically designed, rather than phylogenetically. In fact, to the extent that data supports domain-general views of various higher-order cognitive capacities, the evolutionary psychological view of modularity is demonstrably false.
To sum up, most cognitive psychologists seem to feel that evolutionary considerations provide little insight into the mind. The best way to gain an understanding of cognition is to run experiments on modern subjects, and use the resulting data to form hypotheses. At most, evolutionary stories can tell us why human cognition works the way it does, but we don't need them to tell us how it works. To the extent that evolutionary stories have provided how exlanations, rather than why explanations, they have been either wrong or merely repeated what we already knew.