Human population geneticists can tell us about patterns of human migration, they can tell us how we are related to other species (e.g., Neanderthals and Denisovans). They can tell us the diseases that have afflicted our ancestors; which ancient versions of those diseases are found in some contemporary populations, and much, much more. Their work is a bio-detective synthesis of human phylogeny. A window into evolutionary processes.

Evolutionary psychologists want to tell us about patterns in human psychology; our ways of “thinking” and therefore behaving with respect to certain stimuli—choosing whom to mate with, engaging in xenophobia, extending other-regard to kin, and much, much more. This work is inference-making and extrapolating. It too is supposed to be a window into evolutionary processes.

In both of these domains, researchers aim to understand the forces that shape us, contemporary humans, and those that shaped our ancient ancestors. Underlying all of this is a kind of bio-historical document, our DNA, and what is said to be “written” on it.

To my mind, evolutionary psychologists have not shown that there are specific psychological programs that are written in our bio-historical document. In my recent paper “Is Evolutionary Psychology Possible?”, published in the journal Biological Theory, I argue that it is not possible to give true evolutionary explanations of contemporary human behavior.1 The focus of my argument is that there is a matching problem at the core of evolutionary psychology that is irresolvable and thus renders the project impossible to execute.

The human mind was fashioned by evolutionary forces in ways that allowed our ancestors to be successful. We know that they did achieve success because we are the beneficiaries of their having succeeded. It is success gained through the business of living: they avoided becoming food for others, they procured sufficient food for themselves, they reproduced and they cared for their young. Such feats were made possible because they adapted to environmental contingencies. Outrunning prey and caring for kin were responses to particular events on the ground that were filtered through their psychology.

The view from evolutionary psychology is that such psychological programs did not just aid our ancestors and then disappear. On the contrary, they became a part of the bio-historical document, our genetic foundation. So, we are equipped with psychological adaptations that were genetically inherited from our ancestors. These ancient adaptations have not evolved away from when they were first laid down.

The mandate of evolutionary psychology is to give true evolutionary explanations for contemporary human behavior. Evolutionary psychologists believe that many of our behaviors in the present are caused by psychological mechanisms that operate today as they did in the past. Each mechanism was selected for its specific fitness-enhancing effects, and each of them is responsive only to the kinds of inputs for which it is an adaptation. To secure their claims, evolutionary psychologists need to show that particular kinds of behavior are underwritten by particular mechanisms.

Evolutionary psychological inferences can succeed only if it is possible to determine that particular kinds of behavior are caused by particular psychological structures. These structures must have the evolved function of producing behaviors of just these kinds. If present-day human behaviors are caused by psychological structures, and that was also true of our stone-age ancestors, and if there is a high degree of concordance between the structures populating the modern mind and those that populated the minds of our prehistoric ancestors, this would still fall short of securing evolutionary psychological inferences. This is because similarities between prehistoric and modern psychological structures may be due to ontogenetic processes—similar experiences producing similar functional differentiation in the brain.

For a present-day psychological trait to be related to an ancestral psychological trait in the way that evolutionary psychology requires, the present-day trait must be of the same kind as the ancestral one. It must also have the same function as the ancestral one and must be descended from that ancestral trait as part of a reproductive lineage extending back to prehistory. Also, importantly, the present-day trait and the ancestral trait must be of the same kind and have the same function because the former is descended from the latter. This is key because it might be that a present-day trait and an ancestral trait are of the same kind and have the same function without one being descended from the other. The architecture of the modern mind might resemble that of early humans without this architecture having being selected for and genetically transmitted through the generations. Evolutionary psychological claims, therefore, fail unless practitioners can show that mental structures underpinning present-day behaviors are structures that evolved in prehistory for the performance of adaptive tasks that it is still their function to perform. This is the matching problem.

Ancestral and present-day psychological structures have to match in the way that is needed for evolutionary psychological inferences to succeed. For this, three conditions must be met. First, determine that the function of some contemporary mechanism is the one that an ancestral mechanism was selected for performing. Next, determine that the contemporary mechanism has the same function as the ancestral one because of its being descended from the ancestral mechanism. Finally, determine which ancestral mechanisms are related to which contemporary ones in this way.

It’s not sufficient to assume that the required identities are obvious. They need to be demonstrated. Solving the matching problem requires knowing about the psychological architecture of our prehistoric ancestors. But it is difficult to see how this knowledge can possibly be acquired. We do not, and very probably cannot, know much about the prehistoric human mind. Some evolutionary psychologists dispute this. They argue that although we do not have access to these individuals’ minds, we can “read off” ancestral mechanisms from the adaptive challenges that they faced. For example, because predator-evasion was an adaptive challenge, natural selection must have installed a predator-evasion mechanism. This inferential strategy works only if all mental structures are adaptations, if adaptationist explanations are difficult to come by, and if adaptations are easily characterized. There is no reason to assume that all mental structures are adaptations, just as there is no reason to assume that all traits are adaptations. We also know that adaptationist hypotheses are easy to come by. And finally, there is the problem of how to characterize traits. Any adaptive problem characterized in a coarse-grained way (for example, “predator evasion”) can equally be characterized as an aggregate of finer-grained problems. And these can, in turn, be characterized as an aggregate for even finer-grained problems. This introduces indeterminacy and arbitrariness into how adaptive challenges are to be characterized, and therefore, what mental structures are hypothesized to be responses to those challenges. This difficulty raises an additional obstacle for resolving the matching problem. If there is no fact of the matter about how psychological mechanisms are to be individuated, then there is no fact of the matter about how they are to be matched.

I have shown that there are obstacles to demonstrating that present-day behaviors are outputs of the kinds of evolved psychological structures proposed by evolutionary psychology. Even if these obstacles could be surmounted, the problem remains of identifying these behaviors with particular kinds of behavior that are hypothesized to have existed in prehistory. Psychological structures can be individuated only by the behaviors that they produce, so it follows that their individuation depends upon the individuation of behaviors.

We ordinarily individuate behaviors by attributing intentions to agents performing them. Evolutionary psychologists cannot avail themselves of this method because they must offer subpersonal explanations of behavior framed in terms of underlying computational mechanisms. So, evolutionary psychologists need some other way of individuating behaviors to make inferences about the psychological architecture of both contemporary and ancestral humans. There are three ways to do this. One is to individuate behaviors by their effects. Another is to individuate them by their functions. And a third is to individuate them by their causes.

The first option is to individuate behaviors by their effects. Suppose that a behavior is the same kind as an ancestral behavior because both of them produce the same effects. And suppose that it could somehow be established that a present-day behavior has the same effects, and is therefore of the same kind, as an ancestral behavior. This would not provide what evolutionary psychologists need, because evolutionary psychologists are concerned with the conservation of the psychological causes of behavior, and sameness of effect does not imply sameness of cause.

The second option is to individuate behaviors by their functions. The function of a phenotypic trait is the effect of that trait on fitness in a critical mass of ancestral cases. A present-day behavior is functionally identical with an ancestral behavior just in case the two behaviors have the same function. This criterion fails because it is circular. Individuating behaviors by their functions is the same as giving evolutionary explanations of them. If one assumes that a contemporary behavior has a function, in the relevant sense of “function”, one has already assumed that this behavior has an evolutionary explanation. Simply put, individuating behaviors on the basis of their functions illegitimately supposes that a behavior was selected for and then uses this supposition as evidence that the behavior was selected for.

Finally, one might individuate behaviors by their causes. This is the option that evolutionary psychologists should pursue because they need to be able to infer underlying psychology from behavioral effects in order to give evolutionary explanations of present-day psychology. On this option, if two behaviors have the same psychological causes then they belong to the same behavioral kind. If a present-day behavior is the same kind as a behavior in prehistory, and if behaviors are individuated by their causes, then the contemporary behavior that one wishes to explain, and the behavior in prehistory by means of which one wishes to explain it, must have the same kind of causes. But this strategy fails to be explanatory because it is circular. It relies on the principle that psychological mechanisms are individuated by the behaviors that they bring about, while also individuating these behaviors by the mechanisms that supposedly cause them.

Some readers might think that I am holding evolutionary psychology to a much higher epistemic standard than is normal in evolutionary biological sciences. But this is not the case. Evolutionary psychological inferences commonly fail to satisfy reasonable epistemic criteria. When making evolutionary inferences about paradigmatically biological traits, biologists use experimental manipulations, comparative methods, the fossil record, and optimality models to determine that selection has taken place and that the items under consideration have retained their selected-for functions.

Evolutionary psychologists are impeded by the fact that these methods are unavailable to them. Experimental manipulations of the sort used when studying other organisms are, in our case, usually ethically unacceptable or technically unachievable. Comparative methods are not reliably informative, as there are no extant species that are closely related to Homo sapiens and the relevant behaviors are not generally highly conserved. The fossil record is also unproductive, as mental processes leave no unambiguous material evidence, and optimality modeling is problematic because the underdetermination of behavior by psychological structures makes it difficult or impossible to apply an optimality calculus to the hypothesized psychological structures. Furthermore, evolutionary psychological hypotheses turn on inferences about hypothetical structures for which there is a dearth of empirical support, and there is no evidence that the minds of our prehistoric ancestors possessed this sort of architecture.

References: