Although some people consider twins to be double trouble, the duos actually are a double benefit to genetic research, says Lindon Eaves, distinguished professor of human genetics and psychiatry at Virginia Commonwealth University in Richmond.
“You expect identical twins to be more similar than nonidentical twins,” Mr. Eaves says. “If you can measure the similarity on that kind of a scale, a comparison of degrees of similarity gives an estimate of the strength of genetic influence.”
The extent to which identical twins don’t have identical behavior reveals how the environment has shaped the individuals, Mr. Eaves says. He has studied 1,400 families of twins who were located about 15 years ago through the Virginia school systems.
The families were followed for 12 years, focusing on the role genes play in adolescent behavior and young adult life. Adolescent depression, anxiety disorders, attention deficit hyperactivity disorder, conduct disorder, substance abuse and oppositional defiant disorder are among the problems considered in the study.
The results of Dr. Kendler’s studies show genetic factors are 25 to 50 percent of the cause for many psychiatric disorders, and about 60 percent of the cause for drug abuse and dependence, he says. Environmental factors are the remaining risk factors.