To fully understand personality, we have to continually seek deeper explanations. We explain our social reputations by consistencies in our behavior. We explain behavioral traits by consistent patterns in thoughts and feelings. Thoughts and feelings arise from the unconscious, from patterns of activity in brain structures that we cannot directly experience.
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
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
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."