Focusing on testable hypotheses leaves a stack of research papers concentrating in two areas, those that try to determine definitely if genetics plays a role in sexual orientation, and those that try to find what that genetic influence might be. For the first, twins provide an important resource in the search for genetics, with many studies based on finding identical sexual orientations in identical twins with identical genomes (c.f. Whitman et al., 1993; King &;McDonald, 1992; Bailey &;Pillard, 1991). Family histories, when the sexual orientation of relatives can be definitively traced, can also shed some light on sexual inheritance by looking for a large incidence of homosexuality in some part of the family tree (c.f Bailey &;Bell, 1993; Bailey &;Benishay, 1993).
While research continues to try to determine how much, if any, of sexual orientation is genetically determined, the search for that gene is in full swing. Hamer et al. (1993) think they've found a gene locus; at the same time, Macke et al. (1993) found that the gene they'd been studying seemingly had no influence whatsoever. Structures, as the visible expression of genes, have also been studied in some detail. LeVay (1991) looked at hypothalamic differences between homosexual and heterosexual men to be followed by Allen &;Gorski (1992) who did the same with the human brain's anterior commissure. Finally, animal models, with homologous brain structures to humans that seem to have the same function, help a great deal in finding which structures to look at, as shown by the hypothalamus of gay ferrets that Cherry &;Baum (1990) have been poking at.
This long list of articles presents a large amount of data about a huge number of subjects, but the results do not always agree when they can be compared. Definite proof is usually what is wanted from any study that has to do with a politically explosive topic, something that is very hard to offer. Studies that have been replicated, in whole or in part, multiple times with large sample sizes come closer to that definite proof that activists and lawyers want, but even those leave room for doubt.
The figure of 50% pops up in many studies, but there are just as many that report a wildly different number (c.f. King &;McDonald, 1992). Without mentioning the issue of the bias of a researcher, differences in number could illustrate the difficulty of obtaining a random and unbiased sample of a homosexual population. By far the most common means of obtaining a sample for a study is through advertising in gay identified magazines and newspapers, requiring volunteers to self identify their sexual orientation and that of their siblings, information not always verified. Using these means, it would be impossible to obtain a completely random sample unless there were some way to prove that the population of self-identifying homosexuals or bisexuals who respond to ads in gay-identified newspapers and volunteer to participate in a study was no different than the population of homosexuals who may not self-identify as such and may not read gay-identified newspapers or at least respond to ads placed in one.
Bisexuality is also a confounding factor when a bisexual cannot be categorized to be at least mostly homosexual or heterosexual. Bailey &;Pillard (1991) redefined their groups to be heterosexual and non-heterosexual to account for bisexuals, and Bailey &;Bell (1993) record the Kinsey scores of each person and then divide groups by assigning certain numbers to each. Both methods, however, do not allow for bisexuality to be its own trait of sexual orientation, instead seeing it as a degree of either hetero- or homosexuality. Data is lacking either view.
Finally, bias on the part of a researcher can definitely play a role in the results of a study. The researcher controls all parts of a study, many of which can be manipulated to cause the likelyhood of a certain set of data to be higher. Manipulations of data, also, are places where it is possible to favour one interpretation over another. Whereas one researcher may be asking questions using a confidential questionnaire, another may question subjects under a glaring white light in a small cubicle, which may produce different data. Then, an answer to a question that does not exactly match one of the researcher's categories may cause the researcher to leave out that question, or may have the response put into a subjectively determined nearest match. There is a great deal of room for tweaking of data, something that may be extremely attractive to researchers with strong political views, so papers need to be looked at in great detail for any sign of bias.
Despite the confounding factors, the fact that the analyses of the data gathered in so many studies agree so closely would point to it likely being valid. Even studies that found different numbers, such as the King &;McDonald (1992) study, did find the same pattern of a higher number of monozygotic probands matching in sexual orientation than dizygotic. The fact that 50% of monozygotic twins did not match in sexual orientation would also seem to rule out environmental factors being a sole influence in determination of sexual orientation, when twins are raised by the same family in the same environment; so some part of sexual orientation would be genetic.
The size of the INAH 3 nucleus would seem to be a product of the expression of the genes that may also cause the expression of a homosexual orientation. The fact that they are smaller in gay men than heterosexual would point to some kind of linkage, at least, with the cause for a sexual orientation. Either that, or any one of the many factors that might confound this experiment caused a false result.
The most obvious problem with LeVay's study is that all the homosexual men died after a long period of being infected with HIV, which could have affected the size of the INAH 3 nucleus, addressed by showing that heterosexual men who died from AIDS did not show the smaller nucleus that gay men did. Statistical comparisons of data also showed significant differences even with the extremely small sample sizes that LeVay worked with. Defining sexuality was difficult, as the heterosexual brains could only be presumed so, and one brain known to be from a bisexual was simply added to the sample of homosexual. If there were a large problem with that classification, however, an ANOVA would likely have shown no difference in the two samples.
Whether or not the results from LeVay's study could be considered valid, Allen &;Gorski's 1992 study of the anterior commissure (AC) found similar results, in this case gay men and women (all presumed heterosexual) having a larger AC than heterosexual men. Their study did not have the same problem with sample size, although homosexual women were not included because only for the gay men was there a way to determine sexual orientation before death, it being recorded in their charts. They also this time had homosexual subjects who did not die from AIDS related causes. The fact that this study was able to address much better many of the possible confounding factors LeVay faced would point to there being some kind of correlation between structure of the AC and sexual orientation, whether or not the size of the INAH 3 nucleus of the hypothalamus correlates as well.
Finding the gene, at the genome level, seems to most often involve simply poking around until an interesting bit of genetic code appears, attempting to figure out what it codes for, and then testing men and women of varying sexual orientations for differences in sequencing. One such failed experiment, done by Macke et al. (1993), demonstrates this. They hypothesized that the gene coding for androgen receptors could cause some variation in sexual orientation. When samples of about 200 gay and presumed heterosexual subjects were tested for sequence variation of the gene, no significant variation was found. Although the gene that Hamer et al. (1993) were testing did show difference in homosexual and heterosexual men (only men were tested), both show the same approach, in which a gene may simply be found by chance.
Studies searching for a certain gene that would cause the expression of a certain sexual orientation, although attractive, may not be as productive as those searching for definite biological structures that are somehow linked to that expression. Once there are found all the structural analogues to sexual orientation, then going back and finding genes that affect those structures would likely be much easier and more focused than the point and shoot approach that Macke et al. seemed to have practiced. Other linked behaviors, such as possible differences in a number of cognitive abilities that Tuttle &;Pillard (1991) tested, could lead to a very long list of behaviors and structures that seem to differ with sexual orientation.
Animal models can aid in finding structures, being easier to manipulate than humans with long life spans and laws. Poking at a ferret's brain is legal, whereas lesioning a human's hypothalamus usually isn't, meaning that the effects of artificial changes in brain structure can only really be researched in animals. Cherry &;Baum (1990), for example, showed that by lesioning a sexually dimorphic nucleus of a ferret's hypothalamus, homosexual orientation could be artificially produced.
The search for genetics in structure and in the genome itself is less conclusive, but still often convincing. Harder to carry out, the studies that have been done really require replication more than any modification of their original design to be any more conclusive. The problems with these studies are acknowledged and attempts are made to control for them, with varying degrees of success, but with little chance of circumvention. The conditions under which the experiments were conducted were not changeable without resources not available at the time; but now that the confounding factors have been identified, future researchers carrying out replications can better be aware of them.
Somewhat conclusive is not enough for a lawyer when these studies are presented, even though that is a major purpose of many. The issue of gay rights is brought into play here, with the possible proof of the genetic determination of sexual orientation a major foundation of argument, even if one used by both sides. But more than that, this is an issue that can cause bias in the people conducting a study as well as those reading it. Science becomes hard when emotions are involved, and every study that comes out has been examined for political motives, before being used for them. Proof of a definite genetic component of sexual orientation may change the opinions of some of those who do not believe it, but it is impossible to tell what would happen until, or if, that proof appears. The way it stands now, for every study there is an equal and opposite study, and Science has neither advanced nor hindered the cause of gay rights.
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