Study of Twins Emphasizes Importance of Heredity : Science: Genes are called far more influential in shaping behavior than family environment.
A landmark study on identical twins raised apart offers new reassurance to parents who worry about the adequacy of their child-rearing efforts and rekindles the age-old debate of nature vs. nurture.
The long-awaited study, the most definitive in a long series trying to separate the effects of genetics and environment in a child’s development, comes down heavily on the side of genetics. It indicates that the broad outlines of personality and behavior are put in place in the brief instant when the mother’s and father’s genes mix during conception, establishing the basic route that the child will take during the rest of its life.
Nurture--the family environment--plays a much smaller role, according to the study, published today in the journal Science.
“For most every behavioral trait so far investigated, from reaction time to religiosity, an important fraction of the variation among people turns out to be associated with genetic variation,” wrote the University of Minnesota researchers, led by psychologist Thomas J. Bouchard Jr. This work “does not show that parents cannot influence those traits, but simply that this does not tend to happen in most families.”
Psychologist Robert Plomin of Pennsylvania State University called the Minnesota twin study “the single most important finding in behavioral genetics in the last decade.” The results in general agree with many previous studies of the role of genetics in producing behavior.
But the new research finds a stronger connection between genetics and behavior than the previous studies, said Norman Krasnegor of the National Institute of Child Health and Human Development. “It’s a powerful statement and one which people will debate for a long time,” he said. “It will cause some good controversy and make people work to come up with new data or alternate explanations.”
Psychologist David Rowe of the University of Arizona agreed heartily with the new study. “Parents probably deserve less credit for when things go well, and much less blame for problems,” he said.
But the study “does not imply that parenting is without lasting effects,” the group wrote. “Parents can produce . . . effects if they grossly deprive or mistreat all their children. It seems reasonable that charismatic, dedicated parents determined to make all their children share certain personal qualities, interests, or values, may sometimes succeed.”
The researchers found that 70% of the intelligence quotient--IQ--is accounted for by genes, the strongest correlation found for any characteristic. Genetics also accounts for about 50% of personality differences, including traits such as extroversion; 50% of religiosity, including how often someone attends religious events, and about 40% for job interest variations.
Environment accounts for the rest of the differences, but the researchers note that even two children in the same family may not share the same environment. “The environment molds your personality, but your genes determine what kind of environment you . . . seek,” said Minnesota psychologist David T. Lykken, one of the study’s authors.
Studies of genetically identical twins--which occur about once in every 240 births when a fertilized egg splits into two embryos during development--are especially valuable to researchers. If the twins are adopted into different families at birth and raised separately, then the study provides a unique tool to separate the closely entwined effects of nature and nurture.
Three previous studies of such twins have been conducted, but the Minnesota study, begun in 1979, is by far the largest, now enrolling more than 100 sets from around the world. The study has been widely profiled in the news media because of the eerie similarities that have been observed in reunited twins.
Jerry Levey and Mark Newman, for example, were twins who did not meet until the age of 30. When they were reunited, both had similar mustaches and hairstyles, aviator glasses, big belt buckles and big key rings. Each was a volunteer firefighter and made his living installing safety equipment. Each drank Budweiser and crushed the cans when he finished.
Levey and Newman are the rule, not the exception, Bouchard has found.
“We think of each pair of identical twins as one piece of music played by two different musicians,” Bouchard said. “The music can be played fantastically, or it may not run right. But you’ll always be able to recognize the piece. That’s because nature writes the score. Environment is responsible for the playing technique.”
Now, the Minnesota researchers report in today’s Science paper, they have been able to quantify the contribution of genetics.
But Bouchard also cautions: “Just because something’s genetically influenced doesn’t mean it’s chiseled in stone. . . . If I see a child who has problems with aggression, I don’t curse genetics. I recommend a therapist.”
