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Biological basis of sexual orientation

STANFORD -- Research into the biological basis of sexual orientation "presents a clear double message. Yes, genetics plays a part. No, it is not all genetics," Dora B. Goldstein, professor emeritus of molecular pharmacology, told the audience that attended the first in a series of public lectures sponsored by the Medical Center's Lesbian-Gay-Bisexual Community on March 9.

"This shouldn't be too surprising because that is what all kinds of behavioral studies indicate. Genes determine everything. The environment affects everything. Then there is this big area where the two interact," she added.

In her noon presentation, Goldstein reviewed relevant research in a number of different areas: population studies, family studies, twin studies, genetic research, childhood behavior, and brain differences.

  • The often used statistic that about 10 percent of men are homosexual comes from a study performed by Kinsey. But the subjects were selected in a non-representative fashion, Goldstein said. More recent studies have found that the proportion is between 3 and 5 percent. There is less data on lesbians, but they appear to make up about 1 to 2 percent of the female population. "Of course these are lower bounds, because many people will not admit that they are gay," she pointed out.
  • Several studies also have shown that homosexuality tends to run in families. 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.
  • Identical twin studies shed additional light on the genetic underpinnings of sexual preference. If there are differences in preference between identical twins, who share the same genes, then that difference cannot be genetic. Here, the research indicates that in cases where one identical male twin is gay, about half the time the other twin is gay as well. "This is way above 4 percent, so it's got to be genetic, but it is nowhere near 100 percent," Goldstein said. The percentages for lesbian twins are slightly lower, but generally comparable.
  • DNA studies have identified the general location of at least one "gay gene." The maternal heritability of male homosexuality narrowed the region where such a gene must reside to the X chromosome, because sons get this chromosome from their mother. Analysis of DNA markers on the X chromosomes of sibling pairs has further narrowed the search to a region called XQ28. It consists of hundreds of genes and is located near the tip of the X chromosome. However, there is some indication that genes located elsewhere may have a similar effect on sexual orientation, Goldstein said.
  • Fetal development studies suggest how such a gene might influence such a complex behavior. The development of a fetus into a male is accomplished by the development of the testes, which produce testosterone, which has a wide range of physiological effects. During the perinatal period, a week before and after birth, testosterone has an irreversible organizing effect on the body and brain of males. If the hormone is absent during this period, the individual's anatomy and behavior never can become wholly male. A testosterone surge during puberty activates male sexual development and behavior.
  • Differences between gay and straight sexual orientation appear at a very early age. In a study, a group of openly homosexual men were asked when they first became aware of their attraction to men and boys, when they realized that they were gay and when they "came out" to others. The group reported becoming aware of their attraction at a very young age, between 5 years old and puberty. Self-awareness of their sexual orientation took place around puberty, and coming out of the closet took place much later.

The results of this survey are supported by studies of "gender non-conforming children." In little girls, this behavior, acting as tomboys, bears no social stigma. In little boys, cross-dressing, playing with dolls and behaving like girls is socially damaging. A larger than average number of such "sissy boys" become gay adults, she said.

  • Finally, there is some evidence that the brains of homosexuals may be different from those of heterosexual men and women. The differences have been found in the hypothalamus, which controls eating, drinking, temperature regulation and sexual behavior. Studies done in the Netherlands and in Southern California have found such differences in several areas within the hypothalamus. One region, the midsagittal area of the anterior commissure, is larger in females than in males, but also appears to be larger in homosexual males. Another area, the suprachiasmatic nucleus, which controls circadian rhythm, is larger in heterosexual males and females than it is in homosexuals.



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