That genes strongly influence how we act is beyond question. Several decades of twin, family, and adoption studies have demonstrated that roughly half of the variation in most behavioral traits can be chalked up to genetics. But identifying the causal chain in single-gene disorders such as Huntington’s disease is child’s play compared with the challenges of tracking genes contributing to, say, verbal fluency, outgoingness, or spiritual leanings … All we really know so far is that behavioral genes are not solo players; it takes many to orchestrate each trait. Complicating matters further, any single gene may play a role in several seemingly disparate functions. For example, the same gene may influence propensities toward depression, overeating, and impulsive behavior, making it difficult to tease out underlying mechanisms.
Each gene comes in a variety of flavors, or alleles, with varying degrees of sequence variation. One allele might contribute to a winning personality whereas another may raise the risk of mental illness. Environment plays a strong hand, bringing out, neutralizing, or even negating a gene’s influence. And genes interact with one another in unpredictable ways. …
… A team led by Hasse Walum of the Karolinska Institute in Stockholm looked at the DNA preceding the AVPR1a gene in about 500 pairs of adult same-sex Swedish twins, all of them married or cohabiting for at least 5 years, and their partners. One short variant of a stretch of DNA in this region–there are several variants–was associated with less stable relationships. Answers to questions such as “How often do you kiss your mate?” and “How often are you and your partner involved in common interests outside the family?” reflected slightly lower feelings of attachment on the part of men with this variant, researchers reported in the 16 September issue of the Proceedings of the National Academy of Sciences. These men were less likely to be married and, among those in relationships, more likely to have experienced recent marital strife. … Another paper published last spring showed a different link between AVPR1a and how people treat others. Richard Ebstein and colleagues at Hebrew University in Jerusalem reported that the length of the variant predicted how human subjects would respond in the “dictator game,” a way to assess altruism. The researchers divided 200 volunteers into groups “A” and “B.” The “A’s” received $14 each and were told to share as much as they wished with a “B” whom they had never met. About 18% kept all the money, and 6% gave it all away, with the rest somewhere in the middle. The people who behaved more selfishly–or, as the headlines proclaimed, more ruthlessly–had the same variant as the people with the less stable relationships in the study mentioned earlier. Ebstein speculates that in these people, vasopressin receptors were distributed in such a way that they provided less of a sense of reward from the act of giving (or loving)…
… resilience may lie in a gene for a protein that regulates serotonin, a brain messenger that has been associated with emotional ups and downs. The gene is called SERT for serotonin transporter.
In a classic paper published in Science in 1996, Klaus-Peter Lesch of the University of Würzburg, Germany, and colleagues at the U.S. National Institutes of Health reported that the length of the regulatory DNA at the beginning of SERT affected human behavior. Lesch’s team found that among 505 adults, those scoring high on various tests measuring “neuroticism”–depression and anxiety–tended to have one or two copies of a short variant whereas those who were more laid back had only the long form. The short version translates into more serotonin in the synapse, and too much serotonin leads to anxiety, in both animals and humans…
… the effect of the gene depends on life experiences. In Dunedin, New Zealand, researchers led by Avshalom Caspi of the Institute of Psychiatry in London tracked 847 people over more than 20 years from the age of 3. The researchers counted stressful life events occurring between the ages of 21 and 26 and asked subjects if they had been depressed in the past year.
Among people who had not reported any major life stresses, the probability of depression was low regardless of their SERT alleles. But among people who had been through four or more stressful experiences, 43% of those with two short alleles reported a major depressive episode–more than double the proportion of subjects with two long alleles. The study also showed that almost two-thirds of people with a history of abuse as children experienced major depression as adults if they had two short alleles. But child abuse didn’t raise the risk of adult depression in people with two long alleles…
… they’ve been trying to nail down the role of dopamine receptors, in particular one called the D2 dopamine receptor, in addictions to alcohol, drugs, smoking, or gambling, as well as eating disorders and obesity. … in a group of 195 student subjects at Binghamton University in New York state, those with A1 alleles were more likely to engage in early sexual activity but were less inclined to develop steady relationships… people with two A2 alleles (and no A1) were 8% more likely to form political attachments. Fowler called it “the first gene ever associated with partisan attachment.”… boys with one A1 allele tended to have higher delinquency rates than those with two copies of the A2 allele. But the rates were also higher than in those boys with two A1 copies, suggesting that there is not a simple relationship between the amount of dopamine and behavior…
… the gene for catechol O-methyltransferase (COMT), an enzyme that breaks down dopamine in the prefrontal cortex, the seat of higher cognitive functions such as planning and reasoning. … 497 undergraduates personality tests and found that those with low-activity COMT genes were more neurotic and less extraverted. … a difference in a simple test that has come to be recognized as a reliable indicator for anxiety: the startle reflex, as manifested in involuntary eye blinking in response to a sudden noise or unpleasant pictures. Among 96 female psychology students, individuals with two copies of the low-activity COMT had the most exaggerated startle responses … Yet other work evaluating how well individuals organize their thoughts found low-activity COMT to be an asset. Psychiatrist Daniel Weinberger and colleagues at NIMH think they know why. Brain-imaging studies of 100 normal adults found that those with the low-activity COMT have denser nerve connections. Weinberger and others speculate that the elevated dopamine in the prefrontal cortex may bolster temporary connections, leading to better concentration but reduced ability to shift focus and more behavioral rigidity. As a result, a person may dwell excessively on stressful thoughts. So the gene seems to come with a tradeoff–better cognitive function but more anxiety–the scientists conclude. …
