Q. According to Dr. Helen Fisher in her book “The First Sex” divorce patterns often run in families. She writes that there may be some inherited physiological tendency that contributes to increased susceptibility to restlessness and divorce. This could include overstimulated receptor sites and/or a reduced production of oxytocin, vasopressin / testosterone. If that’s true, couldn’t patterns of homosexuality run in families?

A. Yes, a growing body of knowledge is indicating that homosexuality can run in families, although this often has been hidden in many cases and not discussed openly.

According to authors Barbara and Allan Pease, there is a greater chance of a male being gay if he has brothers, uncles, cousins, or parents (more on the mother’s side) who are also gay. A friend of mine recently discovered—to his great surprise—that his mother was lesbian and that two maternal cousins are gay (one male and one female) along with a paternal uncle. So in his case, homosexuality shows up on both sides of his family in at least two generations.

Psychologist Anthony Bogaert of Brock University in Ontario Canada reported that the risk of being gay increases with the number of older brothers. Some mothers may develop antibodies to male fetuses and, in subsequent pregnancies, the antibodies may impact portions of the fetal brain that determine sexual orientation.

A 1995 news release from Stanford University indicated that the probability that the brother of a gay man is gay is about four times higher than normal. Similarly, the odds that the sister of a lesbian is also a lesbian is significantly higher than normal. However, male homosexuality and lesbianism tend to run in different families: sisters with gay brothers are not more likely than normal to be lesbian. A 1993 study that traced the pedigree of pairs of gay brothers found that homosexuality tends to run on the maternal side of the family tree: the brothers had a higher than average number of maternal nephews and uncles who are gay.

Scientists, led by Dean Hamer, an Aids researcher at the US National Cancer Institute, studied the family histories of 114 gay males and found that 13.5 percent of the gay men's brothers were also homosexual, compared with 2 percent in the general population. They also found that maternal uncles and maternal male cousins were more likely to be homosexual. In some of the families, gay relatives could be traced back three generations.