Sexual Preference: Nature or Nurture?


Staff member
Jun 3, 2003
Sexual Preference: Nature or Nurture?
Ultimately, is sexual preference largely determined by genetics, or does the environment (i.e. family upbringing) matter? That is an interesting and important question. Twenty years ago, the vast majority of psychologists believed that the environment under which a child grew up played a major role in determining sexual preference. Distant or absent fathers along with overbearing mothers were often blamed for same-sex preferences of, particularly, male children.

These theories of an environmental (family) basis for sexual preference have been increasingly discounted by researchers, as more credence has been placed on genetics. But even if genetics plays a major role in determining ultimate sexual preference, the rules under which the genetics laws must work are not simple.

Not too many years ago, the laws governing genetic inheritance were viewed by scientists as very simple, largely following rigid rules laid out by Gregor Mendel, a monk who studied genetics by observing characteristics of successive generations of peas, and other plants. Mendel basically concluded that there were dominant and recessive genes, with dominant genes ruling unless the individual inherited recessive genes from both parents. Individuals with but one dominant gene could still transmit recessive genes to their progeny. The inheritance of eye color is an example, with brown eye color being the dominant trait and blue eyes being recessive. If the individual inherited one gene for brown eyes from either parent, the eye color would be brown. Only if both parents provided the blue, recessive gene, then eye color would be blue.

These simple rules apply in most instances. But they did not explain the occasional green-eyed person, or the even rarer exception of people with one brown eye and one blue eye. These exceptions to the rigid laws--situations that didn't quite conform to expectations--were often discounted by scientists as being unimportant. But in recent year, these "sloppy" exceptions to the rules have assumed increasingly importance, as more frequently, exceptions to the established laws were discovered. In many instances, for example, the effect of a recessive gene might not be completely masked by the dominant gene. Or, certain combinations of genes may have impacts on the person intermediate between their individual impacts. Eyes are not only brown or blue--they may be slightly brownish or slightly bluish. Or complicated interactions among several different genes may be involved.

Another controversial illustration is the comparative role of cigarette smoking (an environmental factor) and genetics in the development of lung cancer. Many people who smoke heavily all their lives never develop lung cancer. However, many other smokers die of lung cancer, some at a relatively young age. Those who do not develop lung cancer likely have a genetic factor that protects them, or, alternately, those who develop lung cancer have a genetic factor that interacts with the environmental influence (smoking) to result in lung cancer. No one knows for sure, but clearly, the incidence of lung cancer is likely tied to both genetic and environmental factors.

There is increasing evidence pointing to a genetic basis for sexual preference, but just as in the cancer-smoking connection, that doesn't mean that the role of environmental influences can be completely ruled out. The evidence supporting the genetic basis includes (1) work by researchers pointing to a specific gene identified as present in many, if not most, gay men; (2) studies of identical twins which indicate that if one twin is gay the likelihood of the second twin being gay is increased; and (3) studies suggesting physical differences in brain structure between gays and straights. Interestingly, in the case of the identical twin studies, the probability that the second twin is gay if the first twin is gay is not 1.0, despite the fact that both twins inherited the identical genes from their parents.

A simple model would suggest that being straight is the dominant trait (as is having brown eyes) whereas being gay is a recessive trait, from which recessive genes must come from both parents. But that model is far too simple. Only a model that incorporates an incomplete manifestation of genes or complex interaction among genes, would be consistent with all the variations seen in human sexual preference. Further, while there may be a sexual preference programmed into our genetic makeup, the environment could still play a role, just as it likely does in the cigarette smoking-lung cancer connection.

A simple Mendel-like model of the inheritance of sexual preference would suggest that nearly everyone would be either straight or gay. In this Mendelian world, virtually all straights would be absolutely disinterested in same-sex activities, and nearly all gays would be totally disinterested in different-sex activities. There would be few, if any, bisexuals. But this does not conform to the real world, and reality instead suggests that there are a lot of people--both men and women, where sexual preference is not clear-cut.

In addition, this opens up the possibility that while genetics may be a factor in determining sexual preference, other environmental factors may still play a role, at least for a portion of the population. It is widely believed and probably well documented that men in prisons, students in single-gender schools etc. tend to engage in more same-sex activities than occurs within the population at large. Interest in same-sex versus different-sex activities is likely, in part, determined by the available options, and part by genetic programming. Further, for some individuals, sexual preference is not static but dynamic, with comparative preferences for same-sex versus different-sex activities changing over time. I am aware not only of married men who are seeking to move into a same-sex relationship instead, but also of men with gay partners who are seeking to dissolve the gay relationship and marry a female lover. So making the assumption that for everyone, sexual preference if rigidly preprogrammed and static oversimplifies the case as well. Psychologists have long known that it is possible many individuals who can be aroused by either same-sex or by different-sex relationships to "develop" either the straight or the gay sides of their personalities and have a degree of control over sexual preference. Thus, the basis (cause) for human sexual preference remains marvelously complex and interesting.