by David Sirkin
15 June 2009; last modified 18 June 2009
In order for sexual reproduction to work in perpetuating a species, several things have to happen. Some of them are related to sexual attraction. First and foremost, the members of the species have to be attracted to each other, and not to the members of other species. Then, second, they should preferentially be attracted to the opposite sex. There are other things too. For example, males should be less attracted to females who are pregnant than to those who are ready to conceive.
Given that sexual behavior between members of the same sex will not result in offspring, why is homosexuality, both in animals and humans, so common? The short answer is that the outward appearances of males and females of the same species are more similar than different, and that, similarly, the brains of males and females, wherein the programs for sexual attraction and behavior are located, are also more similar than different. Consider humans and our closest relatives, chimpanzees and gorillas. A female human looks a lot more like a male human than she does like a female chimp. Similarly, the structure and organization of a male human brain are a lot closer to those of a female human brain than to those of a male chimp brain.
The brain starts out with a basic potential for attraction to other members of the species, and during development, probably primarily in childhood, hormonal and possibly other biological influences typically shape the brain towards preferential attraction to the opposite sex. The brain’s system for attraction will then become sensitive to the fairly subtle differences between the sexes in appearance, manner of movement, scent, and tone of voice.
All physical and mental attributes of an individual are subject to influence by factors that can be categorized as either “nature” or “nurture.” Sexual preference is influenced by biological factors, which might include not only genetics, but the womb environment, and possibly other factors such as diet (some of these might be classified as nurture), and to some extent by the social environment. The effect of one or more departures from the most typical circumstances may result in sexual preference shifting towards the same sex instead of the opposite sex.
Sexual preference usually means just that: a preference for one sex over the other. The shaping of the brain towards attraction to one sex, be it the same sex or the opposite sex, probably never absolutely excludes attraction to the other sex. This explains bisexuality, which could be described as an intermediate preference, or equal preference, and it also explains the frequent choosing of the less preferred sex when the preferred one is not available, as happens, for example, when persons with heterosexual preferences choose same-sex partners in prisons, on ships, in single-sex schools, and in the military. Another example is some women in middle age who find themselves mainly in the company of other women, and not receiving overtures from men (or being fed up with men), turning to women for mates. Conversely, persons with homosexual preferences may be coerced by social norms to choose opposite-sex partners.
Does learning play a significant role?
Can a person’s sexual preference be determined by early sexual experience? If a person has the experience of getting sexual pleasure from the less preferred sex, is the person’s preference likely to change? There probably is an effect of experience and also of learning from example, observation, and teaching. But these effects are limited in strength. People have strong sexual attractions before ever having sexual experience. Also, people who have taken same-sex partners because of lack of availability of opposite-sex partners generally retain their preference for the opposite sex and choose opposite sex partners again when they have the chance.
Does the persistence of homosexuality represent an evolutionary imperfection?
If evolution is such a powerful process, and has resulted in the perfection of nature, why have not humans and animals evolved to the point that only attraction to the opposite sex exists? First of all, evolution does not result in perfection. We only have to be good enough to compete successfully against rivals. Secondly, there may be a trade-off. In other words, having a significant number of individuals with sexual preference for the same sex has some disadvantage, in that those individuals will be less likely to reproduce, but reducing their numbers to a very small percentage of the population might come at a cost, resulting in a perhaps greater disadvantage of some kind.
The cost, if there is one, of a more universal preference for the opposite sex could be for the individuals, in other words, making the individuals weaker or more vulnerable in some way, or it could be for the group. In social animals, such as humans, we have to consider evolutionary pressures on groups that compete with each other, each group having its own gene pool.
One possible explanation is that some degree of attraction for the same sex is helpful for social cohesion of a group. A low level of such same-sex attraction, that is not recognized as being sexual in nature to most individuals, may be at work in human societies. In bonobo chimps, sexual behavior between each individual and numerous other individuals of both sexes in the group seems to serve a variety of social functions. Another possibility, that has been noted by others, is that it is advantageous for a group to have some members who are not involved in reproduction so that they can be more free to perform other jobs.