So where do brain structures come from? Why do our brains work as they do? To answer this question, we need to look at both our genes and our environment—what psychologists have sometimes called "nature" and "nurture."

The Nature-Nurture Debate
For psychologists studying the individual development of personality a few decades ago, "nature vs. nurture" was a central debate. "Nature vs. nurture" suggests that biology (a person's genes) and society (the environment in which a person grows up) are competing developmental forces, and that one may be more important than the other. We now know that both are necessary for normal development. Both help to make us who we are.

To find out how genetics and environment affect personality, psychologists turn to a field of study called behavior genetics. Looking at behavior within a group of individuals, psychologists try to determine what percentage is related to genetic differences and what percentage is related to environmental differences.

For studying genetic differences, twin studies are useful. Identical twins share a home environment and have the same genetic makeup, while fraternal twins share a home environment but do not share the exact same genetic makeup. In studies comparing behavioral traits in identical and fraternal twins, researchers found that genetic differences can account for 40 to 50 percent of differences in personality traits, while environmental influences account for about 30 percent of differences in personality traits.

The Role of the Family
One surprising finding from behavior genetics concerns the relative lack of influence of family environments on personality development. Except for extremes like severe neglect or abuse, parental behavior has little impact on a child's developing personality. This means, for example, that it doesn't matter whether a child is reared in a family where affection is openly expressed or in a family where affection is not expressed at all; the child's personality will turn out the same in both cases.

Environmental influences outside the family environment, such as school and friends, are often more important to the development of personality. However, these apparent "outside influences" may have more to do with genes than it would seem at first. People seek out or even create environments to which they are genetically predisposed. For example, a combative person is more likely than a peace-loving person to find an environment in which arguments are likely to take place, or to create such an environment by starting fights.

Genes do not directly influence personality traits. Instead, genes direct the development of the nervous and endocrine systems, the body chemistry that rules our behavior. Looking at personality traits, we should therefore expect to find links to differences in body chemistry—and in fact, recent research is discovering these links.

Read more about this topic in "The Chemistry of Personality."