This is the 2007 version. Click here for the 2017 chapter 08 table of contents.

Evidence for Evolutionary Influences on Human Behavior

Speculations about evolution and behavior are, in a sense, too easy. Anybody can conjure up explanations for why a currently-existing pattern is adaptive. It is all post hoc or after-the-fact (the behavior exists, therefore it must be adaptive), and nobody can disprove it. Are there better ways to test the claims of evolutionary psychologists?

Authors from the early to mid 20th Century, such as Carl Jung, Tinbergen, Lorenz, Eibl-Eibesfelt and popularizers like Morris, relied on universality as their main argument. If a pattern (such as the eyebrow flash, or attraction to made-up faces) appeared in culture after culture, in widely separated places on earth, then it might come from the human biological blueprint. That is still a compelling argument in the case of truly ubiquitous (universal) human behavioral patterns. Presumably they are ubiquitous for a reason.

Other standards of proof now exist and are more likely to result in journal publications. For example:

1. Straightforward evidence of genetic control

Now that DNA can be mapped and altered on a location-by-location basis, behavioral genetics (the field that relates behavior to genes) is becoming more precise and definitive. If knocking out or introducing a gene produces a reliable consequence in behavior, then this is clearly a point at which evolutionary change could be made through mutation or recombination.

What are epigenetic influences, and how do they add a whole new level of complexity?

Another avenue of change is through environmental influences that modify gene expression while leaving the underlying DNA untouched, an effect called epigenetics. Epigenetic factors include dietary influences on obesity, activity level, and health. Epigenetic factors may be passed on to children, so this is a whole new level of complexity to consider for modern scientists studying genetic influences.

What forms of scientific evidence are used by evolutionary psychologists?

2. Systematic observational evidence

In his classic study of the Yanomami Indians of the Amazon, Chagnon reported that warriors (despite the risk of injury and death) reproduced at a higher rate than men who chose not to fight. Warriors were preferred as mates, married more often, and produced more children. This finding is controversial, but it is an example of an evolutionary argument relevant to behavior and backed by replicable evidence that could be studied directly or in computer simulations.

Eibl-Eibesfelt's video archives are another important form of observational evidence. His recordings show behavioral patterns of daily village life, in a variety of native cultures all around the world. This is the best way to bolster arguments about universality or cultural variations of human behavior: simply collect data from a variety of relatively unspoiled native cultures (such as they exist…Eibl-Eibesfeldt went about his task with urgency because such cultures were disappearing rapidly).

3. Replicable experiments

Classic findings about non-verbal social communication lend themselves to laboratory study by psychologists, often in college settings using subject pools drawn from modern student populations. For example, students can be asked to rate faces for attractiveness. This type of research can be used to document widespread human preferences for such things as facial symmetry, "average" features, large pupil size, and clear healthy skin, although researchers must always take care not to overgeneralize from student populations.

4. Simulations

The effects of different strategies of competition or cooperation can be modeled on computers or simulated in laboratory games pitting one participant against another. The effect of genetic costs or benefits on the population numbers of a species can be simulated on a computer and the effects of varying assumptions about the ecosystem can be tested.

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