Political and social attitudes, ranging from divorce to the death penalty, were found to have a strong genetic influence in one Australian study. A Swedish study found genes significantly influenced two of the so-called “big five” personality traits “openness to experience” and “conscientiousness” while environment had little impact. In contrast, environment influenced “agreeableness” more than genes did. (The two other traits are “neuroticism” and “extroversion.”) Another study, at the University of Texas at Austin, found that personality in identicals correlated 50 percent, in fraternals about 25 percent.
Body fat is under genetic influence. Identical twins reared together will have the same amount of body fat 75 percent of the time; for those reared apart it’s 61 percent, showing a heavy genetic and mild environmental influence, according to a 1991 study.
Both optimism and pessimism are heavily influenced by genes, but shared environment influences only optimism, not pessimism, according to a study of 522 pairs of middle aged identical and fraternal twins. Thus family life and genes can be equal contributors to an optimistic outlook, which influences both mental and physical health. But pessimism seems largely controlled by genes.
Religiosity is influenced by genes. Identical and fraternal twins, raised together and apart, demonstrate that 50 percent of religiosity (demonstrated by religious conviction and church attendance) can be attributed to genes.
Sexual orientation is under genetic influence, though not solely, according to studies by Michael Bailey, Ph.D., associate professor of psychology at Northwestern University. In one study he found that if one identical twin is gay, the other is also gay 50 percent of the time. However, when Bailey analyzed a sample of 5,000 twins from the Australian twin registry, the genetic impact was less. In identical male twins, if one was gay the likelihood of his twin being gay was 20 percent; in fraternal twins the likelihood was almost zero. In women, there was little evidence of heritability for homosexuality.
When substance abuse was studied in 295 identical and fraternal twin pairs, year of birth was the most powerful predictor of drug use. Younger twins were most likely to have abused drugs, reflecting widespread drug use in the culture at large. Alcoholism, however, has a significant genetic component, according to Andrew Heath, Ph.D., at the Virginia Institute for Psychiatric and Behavioral Genetics at Virginia Commonwealth University School of Medicine.
Attention deficit disorder may be influenced by genes 70 percent of the time, according to Lindon Eaves, M.D., director of the Virginia Institute for Psychiatric and Behavioral Genetics. Eaves and colleagues studied 1,400 families of twins and found genetic influence on “all the juvenile behavior disorders,” usually in the range of 30 to 50 percent.
Twins tend to start dating, to marry, and to start having children at about the same time. David Lykken, Ph.D., and Matthew McGue, Ph.D., at the University of Minnesota, found that if an identical twin had divorced, there was a 45 percent chance the other had also. For fraternals, the chance was 30 percent. The researchers think this is due to inherited personality traits.
Schizophrenia occurs more often in identical twins, and if one twin suffers from the disorder, the children of the healthy identical sibling are also at greater risk, according to psychiatrist Irving Gottesman, M.D., of the University of Virginia. The risk is about twice as high for the children of a twin whose identical counterpart is ill, as it is for the children of a twin whose fraternal counterpart is ill.
