ASU Research E-Magazine: Supporting the Biological Link

Frederick Whitam thinks that there may be a biological basis for sexual orientation. Results of an 11-year study conducted by the ASU professor of sociology, combined with similar findings in work conducted by other researchers, continue to provide support for the idea.

Whitam’s research focused on pairs of twins in which at least one twin was homosexual. He says that the study’s findings are welcomed by many gay and lesbian people.

“Homosexual people believe the biological position supports their claims to civil rights protections,” Whitam says. “Also, it impacts the on-going debate concerning gays in the military.”

The questions are complicated and the stakes are high. As a result, the debate often gets heated. Whitam says, “One key to the inclusion of groups as ‘protected classes’ in civil rights legislation and rulings is the principle of immutability.” This is the notion that certain groups, such as women and blacks, merit protection under the law because they were born that way.

“I studied pairs of twins in which at least one twin was homosexual,” Whitam explains. “I sought participants through announcements in the gay press and through personal referrals from 1980 to 1991. One or both twins filled out an 18-page questionnaire that focused on obtaining information about the sexuality of twins.”

When studying twins, scientists try to describe similarities. They use the term “concordance” to describe the level of similarity that exists for different characteristics. For example, body build, eye color, hair texture, hair color, and other physical traits tend to be 100 percent concordant in identical twins.

“But 100 percent concordance for more complex behavioral traits is rare, even in identical twins,” Whitam says. Most researchers would say that a concordance rate of 60 percent is high enough to suggest some sort of biological basis for the behavior.”

Whitam and his colleagues studied 38 pairs of identical twins (34 male pairs and four female pairs). Their results showed a concordance rate of 66 percent for homosexual orientation, which suggests that if one twin was homosexual, there was a 66 percent chance the twin brother or sister also would be homosexual.

The researchers also studied 23 pairs of fraternal twins. Those results showed a concordance rate of 30 percent for homosexual orientation for that group.

Is sexual orientation biologically determined, socially learned, or the result of some type of interaction? Whitam says the biological question has been debated by sex researchers for more than 150 years.

In 1952, F. J. Kallman’s research with twins jarred the widely held notion that homosexual orientation was socially determined. In 1962, research by German scientist W.W. Schlegel supported Kallman’s finding that homosexual orientation in identical twins had a 100 percent concordance rate.

More recently, in 1991, Whitam says that J. M. Bailey and R. C. Pillard’s large study of male twins caused a stir among many social scientists.They found a concordance rate of 52 percent for homosexual orientation in 56 pairs of identical twins and 22 percent concordance for 54 pairs of fraternal twins.

The findings of our study are more consistent with Bailey and Pillard’s than with the early work of Kallman and others,” Whitam says. “In both recent studies, the rates of concordance, while not 100 percent, are still sufficiently high to suggest a strong biological basis for sexual orientation.”—John Matthews