What is behavior? A dictionary definition reveals that behavior consists of our activities and actions, especially actions toward one another. As such definitions suggest, many behavioral terms have meaning only in social comparisons: We identify others as contentious, courteous, or conscientious only by their actions in social contexts. A long-standing question in science and in everyday affairs inquires about the causes of individual differences in behavior: Why are some people gregarious extroverts and others timid, shy introverts?

Behavior genetics is a hybrid area of science, at the intersection of human genetics and psychology. Its focus is on how genes and environments contribute to differences in behavior. It is a young discipline. A book that gave the field its name was published in 1960, and a decade later the Behavior Genetics Association was founded. For a time, most behavior genetics research was an effort to show that the term was...