The Evolutionary Psychology FAQ

Edward H. Hagen, Institute for Theoretical Biology, Berlin

What is evolutionary psychology?

In the three and a half centuries since William Harvey proved that the purpose of the heart is to pump blood, physiologists have revealed the functional organization of the body in blinding detail. Their discoveries demonstrate beyond question that the structure of the body serves survival and reproduction. Further, there is near unanimity among biologists that this functional structure is a product of natural selection. In our century, psychologists have developed powerful techniques that conclusively demonstrate that cognition, too, has structure. Evolutionary psychologists are betting that cognitive structure, like physiological structure, has been designed by natural selection to serve survival and reproduction.

Evolutionary psychology focuses on the evolved properties of nervous systems, especially those of humans. Because virtually all tissue in living organisms is functionally organized, and because this organization is the product of evolution by natural selection, a major presumption of evolutionary psychology is that the brain, too, is functionally organized, and best understood in evolutionary perspective. It is clear that the body is composed of a very large number of parts, and that each part is highly specialized to perform a specific function in service of the survival and reproduction of the organism. Using the body as a model for the brain, it is a fair guess that the brain, too, is composed of one or more functional parts, each of which is also specialized to facilitate the survival and reproduction of the organism (we'll get to genes in a bit). Thus, according to evolutionary psychology, neural tissue is no different from any other tissue: it is functionally organized to serve survival and reproduction. This is the foundational assumption of evolutionary psychology. Because vision, hearing, smell, pain, and motor control are indisputable functions of the nervous system that clearly have utility for survival and reproduction, this assumption has a high degree of face validity. Further, these examples suggests that the brain may best be conceived not as an organ with a single function, but rather as composed of a large, and potentially vast number of functional parts. Evolutionary biologists refer to the functional components of organisms as 'adaptations'. Evolutionary psychologists often refer to brain functions as psychological adaptations, although they are not qualitatively different from other adaptations.

The functional organization of the body has been elucidated primarily by the direct examination of morphology. A detailed analysis of the structure and composition of our organs and tissues has resulted in an excellent understanding of their purpose. Unfortunately, this has not been the case with the brain. The gross morphology of the brain appears to have little connection with its functional properties. Although we have a fair understanding of nerve cells--the primary constituents of neural tissue--the properties of the brain clearly come from higher order assemblages of such cells, not just the cells themselves. This is just as true of organs like the heart as it is of the brain. Because nerve cells can rapidly change state (e.g., their firing rate), because such state-changes involve little energy, and because they can be well insulated from their neighbors, it is possible for a nerve cell to be in one state, whereas some of its close neighbors may be in completely different states. This is in marked contrast to, say, muscle cells. If one muscle cell is involved in a contraction, then nearby cells almost certainly are as well. Neural tissue is quite different. Even the individual states of nerve cells in a network depend critically on the topology of the network itself. Further, assemblages that are actually distinct may have a complex three-dimensional distribution that can be very difficult to untangle. These properties of neural tissue make it exceedingly difficult to "see" the morphology of neural assemblages--with few exceptions, the network topology of virtually our entire brain is currently "invisible." It exists at a scale above the individual cell, but well below that which can be teased apart with any imaging technology currently available. Until recent decades, much of our immune system was similarly "invisible."

Evolutionary psychology offers one way around this technological limitation. If researchers had a sound basis for proposing brain functions a priori, they could then seek indirect evidence that brains in fact have these functional properties. Philosophers and scientists had long wondered why living things are made up of an amazing array of beautifully designed mechanisms, an organization which non-living things completely lack. Why is it that entities that reproduce manifest overwhelming evidence of design, but entities that don't reproduce are utterly devoid of the same? As Darwin and Wallace first perceived, the association of reproduction and design is not accidental. Evolution by natural selection is currently accepted as the only process whereby entities can acquire functional properties. Functional organization is the consequence of the reproductive feedback that characterizes natural selection. If a population of reproducing entities (hereafter organisms) varies in some trait, if the variations can be passed on to offspring, and if, as a consequence of possessing a particular variant, an organism produces more offspring on average than organisms that lack that variant over evolutionary time, then the population will come to consist solely of organisms possessing the reproductively efficacious variant trait. In this way, populations of organisms will tend to acquire traits that facilitate reproduction and lose traits that hinder reproduction.

We now know that what is passed on to offspring is a large DNA molecule that is further partitioned into numerous sections called genes. Because the structure of this DNA is intimately bound up with the structure of the organism, variations in the DNA are strongly associated with variations in the organism. Changes in DNA are referred to as mutations, and result from environmental hazards such as radiation, toxins, etc.

Reproduction is an enormously complex process. At any given moment in the human body, there are thousands of process that, should they fail to complete successfully, would result in death within minutes. For this reason, any given random change in the body is likely to hinder survival and reproduction, not facilitate it. There are far more ways for a mechanism to fail than there are ways to improve it. How many times has a change occurred to your car so that it got much higher than the EPA estimated miles-per-gallon rather than much lower? Thus, the vast majority of DNA mutations result in changes to the body (also called the phenotype) that hinder reproduction. Occasionally, however, a mutation occurs that results in a change to the phenotype that facilitates reproduction. Because this mutation can be passed on to offspring, and because this mutation tends to result in more offspring, the mutation becomes more frequent in the population. Over time, this process will result in organisms that have a sophisticated repertoire of mechanisms that facilitate reproduction

We now have the answer to the question posed above: what functions is the brain likely to perform? If brain tissue is organized like all other tissue, it will perform precisely those functions that facilitate reproduction. More accurately, because evolution by natural selection is an historical process, and because the future cannot be predicted, the brain and body will perform functions that facilitated reproduction (note the past tense). Whether they currently do so will depend on how closely the present resembles the past. If we can develop an accurate picture of a species' reproductive ecology--the set of physical transformations that had to occur over evolutionary time for individuals to reproduce--we can infer those properties the organism is likely to have in order to ensure that those transformations reliably took place. Evolutionary time, the time it takes for reproductively efficacious mutations to arise and spread in the population, is often taken to be roughly 1000-10,000 generations; for humans, that equals about 20,000-200,000 years.

Over the last 200,000 years, humans regularly encountered spiders and snakes, creatures whose toxins would have significantly impeded the reproduction of individuals unlucky enough to get injected with them. Over the last 100 years, humans have regularly encountered automobiles, encounters that also can seriously impede reproduction (e.g., by getting run over). Because 200,000 years is long enough for humans to evolve protective mechanisms, but 100 years isn't, we can predict that humans may well possess an innate aversion to spiders and snakes, but not to automobiles--even though far more people are currently killed by cars than by spiders or snakes. Once we have firmly established that avoiding spiders and snakes would have reliably facilitated the reproduction of ancestral humans, we can then design experiments to determine whether humans in fact possess an innate, cognitive ability to detect and avoid these animals (more on how to do this below). A major lesson of evolutionary psychology is that if you want to understand the brain, look deeply at the environment of our ancestors as focused through the lens of reproduction. If the presumptions of evolutionary psychology are correct, the structure of our brains should closely reflect our ancestral reproductive ecology. Thus, evolutionary psychology provides a method for perceiving the functional organization of the brain by studying the world--currently a far more tractable problem than disentangling neural assemblages.

Copyright 1999-2002 Edward H. Hagen