Behavioral Genetics: Methods and Madness

The research discussed so far searches for genes implicated in specific problems. But research relating behavior and genetics rarely involves actual examination of the genome. Instead, psychologists, psychiatrists, and other nongeneticists calculate a heritability statistic by comparing the similarity in behaviors among different sets of relatives. This statistic expresses the old nature–nurture division by representing the percentage of a behavior due to genetic inheritance versus the percentage due to environmental causes.

Such research purports to show a substantial genetic component to alcoholism. For example, some studies have compared the incidence of alcoholism in adopted children with that of their adoptive parents and with their natural parents. When the similarities are greater between the offspring and absent biologic parents, the trait is thought to be highly heritable.

But children are often adopted by relatives or people from the same social background as the parents. The very social factors related to placement of a child — particularly ethnicity and social class — are also related to drinking problems, for example, thus confusing efforts to separate nature and nurture. A team led by University of California sociologist Kaye Fillmore, Ph.D., incorporated social data on adoptive families in the reanalysis of two studies claiming a large genetic inheritance for alcoholism. Fillmore found that the educational and economic level of the receiving families had the greater influence, statistically erasing the genetic contribution from the biological parents.

Another behavioral genetics methodology compares the prevalence of a trait in monozygotic (identical) twins and dizygotic (fraternal) twins. On average, fraternal twins have only half their genes in common. If the identical twins are more alike, it is believed that genetic inheritance is more important, because the two types of twins are supposedly brought up in identical environments. (To eliminate the confounding influence of gender differences, only same-sex fraternal twins are compared.)

But if people treat identical twins more similarly than they treat fraternal twins, the assumptions of the heritability index dissolve. Much research shows that physical appearance affects how parents, peers, and others react to a child. Thus, identical twins — who more closely resemble one another will experience a more similar environment than fraternal twins. University of Virginia psychologist Sandra Scarr, Ph.D., has shown that fraternal twins who resemble one another enough to be mistaken for identical twins have more similar personalities than other such twins.

Heritability figures depend upon a number of factors, such as the specific population being studied and where. For example, there will be less variation in weight in a food-deprived environment. Studying the inheritance of weight in deprived settings rather than an abundant food environment can greatly influence the heritability calculation.

Heritability figures in fact vary widely from study to study. Matthew McGue, Ph.D., and his colleagues at the University of Minnesota calculated a zero heritability of alcoholism in women, while at the same time a team led by Kenneth Kendler, M.D., at Virginia Medical College calculated a 60 percent heritability with a different group of female twins! One problem is that the number of female alcoholic twins is small, which is true of most abnormal conditions we study. As a result, the high heritability figure Kendler’s team found would be reduced to nothing with a shift in the diagnoses of as few as four twins.

Shifting definitions also contribute to variations in the heritability measured for alcoholism. Alcoholism may be defined as any drinking problems, or only physiological problems such as DTs, or various combinations of criteria. These variations in methodology explain why heritability figures for alcoholism in different studies vary from zero to almost 100 percent!

The Inheritance of Homosexuality

In the debate over homosexuality, the data supporting a genetic basis are similarly weak. One study by Michael Bailey, Ph.D., a Northwestern University psychologist, and Richard Pillard, M.D., a psychiatrist at Boston University, found that about half the identical twins (52 percent) of homosexual brothers were homosexual themselves, compared with about a quarter (22 percent) of fraternal twins of homosexuals. But this study recruited subjects through ads in gay publications. This introduces a bias toward the selection of overtly gay respondents, a minority of all homosexuals.

Moreover, other results of their study do not support a genetic basis for homosexuality. Adopted brothers (11 percent) had as high a “concordance rate” for homosexuality as ordinary brothers (9 percent). The data also showed that fraternal twins were more than twice as likely as ordinary brothers to share homosexuality, although both sets of siblings have the same genetic relationship. These results suggest the critical role of environmental factors.

One study that focused on a supposed homosexual gene was conducted by Dean Hamer, Ph.D., a molecular biologist at the National Cancer Institute. Hamer found a possible genetic marker on the X chromosome in 33 of 40 brothers who were both gay (the number expected by chance was 20). Earlier Simon LeVay, M.D., a neurologist at the Salk Institute, noted an area of the brain’s hypothalamus that was smaller among gay than heterosexual men.

Although both these findings were front-page stories, they provide quite a slender basis for the genetics of homosexuality. Hamer did not check for the frequency of the supposed marker in heterosexual brothers, where it could conceivably be as prevalent as in gay siblings. Hamer has noted that he doesn’t know how the marker he found could cause homosexuality, and LeVay likewise concedes he hasn’t found a brain center for homosexuality.

But for many, the politics of a homosexual gene outweigh the science. A genetic explanation for homosexuality answers bigots who claim homosexuality is a choice that should be rejected. But to accept that non-genetic factors contribute to homosexuality does not indicate prejudice against gays. David Barr, of the Gay Men’s Health Crisis, puts the issue this way: “It doesn’t really matter why people are gay… What’s really important is how they’re treated.”

    Last reviewed: By John M. Grohol, Psy.D. on 30 Jan 2013
    Published on PsychCentral.com. All rights reserved.