A pervasive assumption in evolutionary psychology is that how we act is affected by the genes we carry. Is there good concrete evidence of this? Are our outcomes predetermined by our biology? The most intriguing findings on this issue came from twin studies.
Evidence that Genes Affect Human Behavior
The study of identical twins reared apart is a natural experiment where two individuals with exactly the same genes grow up in different environments. If they turn out to be similar, then the similarity can be attributed to genotype.
Apart from these scientific findings, researchers were impressed by many obvious similarities between twins when they were reunited for the first time after being separated from birth. Many of the pairs dressed similarly or had the same haircut, or glasses. They described remarkable similarities in hobbies and interests. One pair reported that they were the only ones in their neighborhood to construct a circular bench around a tree in their backyard.
Striking as such stories are, they remain mere anecdotes and have no scientific value. The main problem is that there is confirmation bias. If a pair of twins is wearing the same baseball hat, we tend to interpret this as a wonderful example of genetic control over the minutiae of behavior. If a pair shows up wearing different hats, however, we ignore that difference but instead register some similarities such as both twins wearing a black shirt.
Identical twins separated at birth have some striking differences. If one twin is schizophrenic, there is no more than a coin-toss chance that the other is diagnosed with the same mental disorder. This is striking given that schizophrenia is believed to have a basis in brain biology. (The same is true of political affiliation).
We must also recognize that identical twins are a special case whose relevance to the behavior of ordinary people is disputable. The problem is that many characteristics are affected by multiple genes. If there are six genes involved, identical twins will be the same because they have all six genes. Yet, taken separately, each of those genes might not have a detectable effect on the trait of interest if studied in the general population.
This wrinkle (known as epistasis) may help explain why it is so difficult to establish a biochemical chain of causation between specific genes and complex human behaviors, although researchers have made heroic efforts to account for various traits, such as sensation seeking as a function of dopamine receptors, and have investigated various candidate genes to account for criminal violence.
Biochemistry and Behavior
Establishing that some behavioral traits are heritable is not the end of the scientific mission but really just the beginning. We need to know not just that genes affect behavior but also have to establish which genes are involved and how they affect the biochemistry of brain cells in ways that influence behavior.
One of the first of such projects involved work on receptors for dopamine that are implicated in sensation seeking.
Another study looked at the so-called “warrior genes” that were over-represented among violent criminals. Criminal defense attorneys were excited by this finding because it offered a new defense strategy for violent offenders, namely that they were not fully responsible for their actions because their genes made them do it.
That genetic defense has been a flop, however. Warrior genes affect violent behavior only in the small category of individuals who grow up in extremely abusive homes. Children who are raised by loving parents are very unlikely to engage in orgies of uncontrolled aggression.
So there is a striking contradiction between the seeming clarity of the early research via twin and adoption studies, that established clear and substantial effects of genetic inheritance on personality and behavior, and subsequent efforts to work out how these influences play out.
Adaptation Without Genes
Although it is hard to deny genetic influences on human behavior, anyone who tries to explain what a person does in terms of simple biochemical differences is likely to be disappointed. Personality psychologists recognize that gene effects are difficult to separate from environmental influences. Children growing up in the same home experience that environment very differently because they have distinct temperaments, are treated differently by parents and siblings, and pursue different interests with different companions.
For example, a child with a greater sense of curiosity is going to cultivate varied interests and activities that feed the thirst for knowledge, whereas less curious siblings extract far less intellectual stimulation from their home environment. Such differences between siblings in what they get out of the environment are about as important as genes in determining personality and intelligence (1).
So there is little doubt that how we act is affected by genes in fairly generalized ways. Some individuals are born with a propensity to be outgoing, to be happy, emotionally reactive, sociable, creative, or intelligent. Yet, we do not have a good understanding of any of the relevant biochemical mechanisms.
Moreover, there is no satisfactory explanation of the underlying biochemical mechanisms in most cases. There is an important distinction between personality predispositions and actual behavior. Personality may be genetically heritable to some degree but human behavior never is.
Honeybees have a complex sequence of hygienic behavior that consists of digging out infected larvae and chucking them out of the hive — a sequence that is understood in terms of Mendelian genetics with one gene for uncapping and another for removing the dead larvae (2). As far as humans are concerned, we may or may not have strong hygienic tendencies, but there is no gene for cleaning out the refrigerator.
1 Plomin, R. (1990). Nature and nurture. Pacific Grove, CA: Brooks/Cole.
2 Grier, J. W. (1984). Biology of animal behavior. St. Louis, MO: Times Mirror/Mosby.