Nature and Nurture

By Judy Woodward

In our Department of Psychology, Minnesota Twins refers not to baseball, but to revolutionary studies of behavioral genetics that have changed what we know about the heritability of physical, mental, and psychological traits. The pioneers of these fields continue to be honored for their work: Professor Irv Gottesman was named one of the Top 30 Most Influential Psychologists in 2013, as well as being named an Honorary Fellow of King's College in London. On August 8, Professor Thomas Bouchard received the American Psychological Foundations Gold Medal Award for Life Achievement in the Science of Psychology.

Learn more about the pathbreaking findings of Professors Bouchard and Gottesman through the Minnesota Twins studies.

There was a time when few scientists believed that genetics influenced human personality traits. Some of those who did -- Britain's Francis Galton, for example -- sullied their first-rate science with their enthusiasm for the social engineering and eugenics movements of the late 19th century. In the post-World War II era, the association with Nazi experiments almost completely stymied legitimate research in the field.

Almost -- but not quite. At the University of Minnesota, researchers not only have been studying behavioral genetics since the 1940s, but also have helped to revolutionize the field.

The pioneer: Irving Gottesman

In his last year at the Illinois Institute of Technology in the early 1950s, Irving Gottesman decided that "physics has no future."

His predictive powers may have been flawed, but Gottesman's decision to give up on physics was probably one of the best things that ever happened to the field of behavioral genetics.

As an undergraduate -- in spite of his physics major -- Gottesman had been fascinated with abnormal psychology and the works of Freud. He applied to the graduate program in psychology at the University of Minnesota and began one of the most fruitful careers in the history of modern psychology.

At the time, the University was virtually alone in its support for genetic and biological research in psychology. When Gottesman did his first study with twins, "it was a classic way to determine if genetics had any influence on human traits," he says. "I was fully prepared to find that there was no influence."

Instead he found that certain personality traits in adolescents were, in fact, under strong genetic control. But even after Gottesman received his Ph.D. and moved on to begin his groundbreaking studies on the heritability of schizophrenia among twins, the medical and mental health establishment remained skeptical. "The vast majority of American psychiatrists at the time believed that the nature versus nurture battle had been won -- by nurture," he says.

That attitude began to change after Gottesman was invited back to the University in 1966 to launch the Program in Behavioral Genetics. Over the years, researchers in the program began to offer incontrovertible evidence for the first law of behavioral genetics: every trait is heritable.

In retirement, after teaching at the University of Virginia, Gottesman came back to Minnesota, where he now holds a dual appointment in the psychology and psychiatry departments. He has little use for the whole notion of a battle between nature and nurture. "At Minnesota," he says, "the attitude has been that it should never have been a battle ... but a wide-open search for the causes for human behavior."

Learning from twins: Thomas Bouchard

If Gottesman laid the groundwork for studying twins to identify the underpinnings of traits, Thomas Bouchard pushed it to new frontiers. Indeed, when the history of behavioral genetics in the late 20th century is written, no name will loom more prominently than Bouchard's.

Now semiretired from his post as head of the Minnesota Center for Twin and Adoption Research, Bouchard was trained as a social psychologist and had limited exposure to behavior genetics when he came to the University. Here he met the pioneering Gottesman, who became his mentor.

Studying twins is "the most simple and direct way to answer the nature versus nurture question," says Bouchard. "It's straightforward and easily understandable. With monozygotic twins raised apart, Mother Nature has given us a powerful experiment in society. It's not a pure experiment, but it's a reasonable approximation."

The actual research began with the "Jim twins." Bouchard had read a newspaper story about a pair of adult identical twins who displayed uncannily similar behavior patterns and lifestyle choices despite their having been raised apart. He called the reporter to find out more.

"More publicity followed," Bouchard says. "Other twins wrote to us. Twins have an intuitive understanding of the project. I was also lucky because the working environment at the University of Minnesota was so great."

Starting in 1979, Bouchard located 139 pairs of twins who had been raised separately. Because identical twins share all their genetic material, Bouchard says, it is the perfect population in which to study the heritability of physical, mental, and psychological traits.

By carefully measuring

correlations between the twins, he could determine precisely the varying roles played by heredity and environment. Bouchard later refined his results by studying a group of fraternal twins raised separately. The findings produced by the Minnesota Study of Twins Reared Apart have been influential in establishing the genetic basis for human behavior. The data gathered in the study, which lasted until 2000, continue to be a rich resource for researchers in the field.

In the beginning, says Bouchard, he believed that "social attitudes were all caused by environment. It never dawned on me that we would discover that 50 percent of their causes were genetic."

His research has shown that the heritability of many traits in identical twins is substantial, but not total, and that twins raised together were no more and no less like each other than twins raised apart. So where do the differences in identical twins come from?

"Why aren't the twins who grew up together more like each other than the twins who were raised apart? That was so surprising. It's a real puzzle ... but it's an environmental question that will have to be answered by the students to come. That's what science does, after all," he says.

Twins and beyond

Bouchard_0194.jpgWe now know that the influence of genetics on human personality and behavior is "an empirical reality," says Regents Professor of Psychology Matt McGue. "We've convinced the field of the importance of genetics. Now we want to show how genetics and environment work together."

McGue cites research on adolescents to illustrate the connections between the two. He has found that kids who engage in risky behaviors before the age of 15 "are at the highest risk for mental health problems in later life," he says. "Why does this correlation exist? Risky genotypes. They've inherited genes that make it more difficult to inhibit their behavior."

But the young risk takers are also living in a specific environment.

"If kids are engaging in these behaviors," McGue continues, "that affects their experiences in adolescence. They become less connected to home and school, and more affiliated with deviant peers."
With his colleagues William Iacono and the late David Lykken, McGue established the Minnesota Twin Family Study in 1988 to explore the complex interplay of genetics and environment. Where Thomas Bouchard had studied twins raised apart, McGue and his colleagues focused on twins raised in the same environment. Over the years they've put together a rich database of the physical and psychological characteristics of as many as 90 percent of the twins born in Minnesota.

McGue_8238_v2.jpgTwins are at the heart of McGue's work, but they are not his only interest. In collaboration with Danish colleagues, he has drawn on Danish national birth records for an intriguing study of nonagenarians. While most study projects focus on youth, the subjects in the Danish 1905 Study -- so-called because of the birth year of the participants -- were 92 or 93 when research began in 1998. McGue hopes that this work will highlight factors "that contribute to successful aging ... and increase public awareness of successful aging strategies."

Indeed, his findings underscore the intertwined influences of genetics and environment. "The older you get," says McGue, "the less important genetics seem to be. Roughly 25 percent of how long someone lives is associated with genetic factors. The rest is how you live."

Iacono and McGue anticipate their field's next frontier: the unraveling of the specific DNA codes that control genetic predispositions to human characteristics. Maybe as soon as the next decade, says Iacono, "we will be able to access hundreds of relevant genes and determine their effects on human behavior ... . The biggest surprise to me is the incredibly rapid progress made with the human genome. It will revolutionize the field."



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