The Genetic Temptation

A gene for social skills, we are told, has now entered the fast growing Pantheon of genes "for" various human behaviors and dispositions--taking its place alongside genes for risk-taking, happiness, aggression, and sexual orientation, among others. One gets the impression that we are well on the way to figuring ourselves out, and that we are far simpler than anyone imagined. After all, adeptness at cocktail party conversation--or addiction to the cocktails--may be no less genetically determined than hair color.

Defining the role that genes play in behavior, however, is not so easy. Many of the results of human studies are highly preliminary and do not actually identify a gene. In fact, they cannot, for genetic experimentation on humans is impossible. Even in animals whose genes are more easily studied and much better understood, such as Drosophila , the tiny fruit fly, there is no simple one-to-one correspondence between gene and behavioral trait. Instead, a wide variety of genes influence each characteristic.

Fruit fly genes are surprisingly similar to ours--as they are to those of most other creatures. Even many aspects of their behavior resemble our own. For example, when training a fruit fly to prefer one odor to another in a simple learning task, it will remember better if trained at repeated intervals over a long time rather than in one intense session. Fruit flies are not geniuses, but "cramming" works no better for them than it does for us. Even more recently, scientists at the Neurosciences Institute in San Diego have shown that according to all essential definitional criteria, fruit flies sleep at night.

But those of us who study how genes affect behavior are humbled by the fruit fly's complexity. Moreover, fruit flies exhibit this complexity under conditions in which "nurture" is kept constant--a condition that is never true of humans. If the relationship between fruit fly genes and behavior is so complex, can it be any less so with humans?

The view that an individual gene could account for a complete biological trait is as old as genetics itself. Whether describing genes for the color of flowers in pea plants or the shape of the fruit fly's wings, geneticists at the beginning of the last century believed that each gene governed one unique characteristic. Soon, a few of the early geneticists, founders of the Eugenics movement, made the odious assertion that "licentiousness," "shiftlessness," and "criminality" could all be attributed to individual genes as well.

As time passed and scientific knowledge accumulated--much of it from the fruit fly--the true complexity of the relationship between genes and characteristics became clear. By the 1920s, most geneticists had abandoned the idea of a single gene for each characteristic and were loath to ascribe human behavior solely to genes. Those in the Eugenics movement remained hold-outs, convinced that the salvation of the human race would be found in limiting the birth-rate of those they deemed to be genetically "unfit."

We are now in the midst of an explosion of genetic information. New technologies are uncovering more genes every day and comprehensive lists have been drawn up for worms, mice, and humans. But these species' behavior, no less than that of the fruit fly, is the product of a vast array of genes, none of which acts in isolation. Genes influence each other and are in turn influenced by the surrounding world. The result is always a unique individual.

The reason for this is simple. Any creature that reproduces sexually is made up of a unique combination of its species' gene set, which it inherits from its parents. All humans, for example, have the same set of genes--the same genetic context--but not exactly the same versions of each gene. (The exceptions to this in any species are identical twins and clones.) These slight variations are part of what makes each of us different from other members of our species. The importance of genetic context has been shown in studies of behavioral mutants in fruit flies and mice. The same mutation may have a strong effect in one genetic context but not in another.

Beyond genetics, all creatures experience a non-identical sequence of life events. This is as true for genetically identical bacteria growing in a uniform environment as it is for a person living in Paris, Tokyo, Moscow or Buenos Aires. Granted, the differences between any two Muscovites' experiences are greater than the differences between the experiences of any two bacteria in a test tube culture. But that is just the point. Genetically identical bacteria represent an extreme case. We expect them to be uniform. So if contingency holds for them, then it certainly holds for us.

Genes enable life, but they clearly do not determine it. So the real issue is not whether we humans are servants of our genes, but rather our infatuation with the idea that our behavior could so easily be explained and predicted. Perhaps we should not be so eager to be relieved of the burden of reflection, evaluation, and choice--in short, of grappling with what lies at the true heart of our humanity.