The future of genes

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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.

"We can't manipulate the genes or the environment in humans so what we have to look for is a natural experiment," says Mr. Eaves, who holds a doctorate in genetics. "They come in two kinds, identical twins and nonidentical twins."

Most researchers agree that genetics influence many behaviors to some degree. Twins studies, which have been going on for decades, continue to help define the specific ways genes can affect behavior.

Generally, researchers contrast fraternal or nonidentical twins, who develop from two eggs fertilized by different sperm, with identical twins, who develop from one fertilized egg that divides into two babies.

"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.

"We have a very strong sense that you can't ignore the genes in complex behavior, but we're a long way away from saying, 'These are the particular genes that influence particular things,'" Mr. Eaves says. "If you can really pin down one or two genes that have a clear-cut influence on the outcome, you could begin to look at how the genes affect the outcome and give better drug treatment."

Investigating adult samples of twins is equally as important as studying adolescents, says Dr. Kenneth Kendler, professor of psychiatry and human genetics at Virginia Commonwealth University. He has focused his work on psychiatric and drug abuse disorders in adults. Since the 1980s, he has conducted a set of related studies with 6,000 twins.

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.