Objective studies of homosexuality among people are difficult to conduct. There are no reliable criteria for determining how much a person in a population is actually attracted to members of the same sex.
All studies are conducted on samples where the participants themselves report their sexual orientation. However, in many societies, especially conservative-religious ones, it can still be difficult and often life-threatening to acknowledge one's choice. Therefore, in the question of studying the biological characteristics of homosexuality, scientists are forced to be content with relatively small samples of representatives of several ethnic groups living in Europe, the United States and Australia. In such an environment, it is difficult to obtain reliable data.
Nevertheless, over the years of research, enough information has accumulated to admit: homosexuals are born, and this phenomenon is common not only among humans, but also among other animals.
The first attempt to estimate the number of homosexual people in the population was made by the American biologist and pioneer of sexology Alfred Kinsey. Between 1948 and 1953, Kinsey interviewed 12,000 men and 8,000 women and rated their sexual habits on a scale from zero (100% heterosexual) to six (pure homosexual). He estimates that about ten percent of the men in the population are "more or less homosexual". Later, colleagues stated that Kinsey's sample was biased and that the real percentage of homosexuals is more likely three to four for men and one or two for women.
Modern polls of residents of Western countries on average confirm these figures. In 2013–2014 in Australia, two percent of men surveyed reported being homosexual, in France - four, in Brazil - seven. Among women, these values usually turned out to be one and a half to two times lower.
Achilles tending to the wounded arrow Patroclus, painted red-figure vase, circa 500 BC.
Are there genes for homosexuality?
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Studies of families and twin couples, continuing since the mid-1980s, indicate that homosexuality has an inherited component. In one of the pioneering statistical studies on this topic, conducted by psychiatrist Richard Pillard (who is himself gay), the likelihood that a homosexual man's brother would also be homosexual was 22 percent. The brother of a heterosexual man was gay only four percent of the time. Other similar polls showed similar odds ratios. However, the presence of brothers with similar preferences does not necessarily indicate the heritability of this trait.
More reliable information is provided by studies of monozygotic (identical) twins - people with the same genes - and their comparison with dizygotic twins, as well as with other brothers and sisters, and with adopted children. If a trait has a significant genetic component, it will be more common in identical twins at the same time than in any other child.
The same Pillard conducted a study of 56 monozygotic male twins, 54 dizygotic and 57 adopted sons, from which it was concluded that the contribution of heredity to homosexuality is from 31 to 74 percent.
Later studies, including a survey covering all Swedish twins (3,826 monozygous and dizygotic pairs of twins of the same sex), refined these numbers - apparently, the contribution of genetics to the formation of sexual orientation is 30-40 percent.
As a result of interviews, Pillard and some other researchers found that the presence of other homosexual relatives among homosexuals more often corresponds to the maternal line of inheritance. From this it was concluded that the "homosexuality gene" is located on the X chromosome. The first molecular genetic experiments, by analyzing the linkage of markers on the X chromosome, indicated the Xq28 region as a possible target element. However, subsequent studies did not confirm this connection, nor did they confirm the inheritance of homosexuality through the maternal line.
Experiments with the sex chromosome were followed by genome-wide marker linkage assays, which suggested that loci on chromosome 7, 8 and 10 were linked to homosexuality.
The most ambitious such analysis was conducted relatively recently by Alan Sanders and Pillard's associate, J. Michael Bailey. As a result of the analysis, the Xq28 region again appeared on the scene, as well as the genetic locus located near the centromere of the eighth chromosome (8p12).
Sanders subsequently conducted the first genome-wide search for associations of homosexuality in males with single nucleotide polymorphisms (SNPs). Such analysis is more informative due to the fact that polymorphism can point to a specific gene, while linkage analysis points to a chromosome region, which can include hundreds of genes.
The two marker candidates from Sanders' work turned out to be unrelated to previous searches. The first of them appeared on the 13th chromosome in the non-coding region between the genes SLITRK5 and SLITRK6. Most of the genes in this group are expressed in the brain and encode proteins responsible for the growth of neurons and the formation of synapses. The second variant was found on chromosome 14 in the non-coding region of the TSHR gene of the thyroid-stimulating hormone receptor.
The contradictory data obtained in the above studies mean, probably, only that "homosexuality genes" exist, but they have not yet been reliably found.
Perhaps this feature is so multifactorial that it is encoded by many variants, the contribution of each of which is very small. However, there are other hypotheses to explain the innate attraction to people of the same sex. The main ones are the effect of sex hormones on the fetus, "little brother syndrome" and the influence of epigenetics.
Hormones and the brain
The development of the fetal brain in a "male" or "female" pattern appears to be influenced by testosterone. A large amount of this hormone during certain periods of pregnancy acts on the cells of the developing brain and determines the development of its structures. The difference in the structure of the brain (for example, the volume of certain areas) in later life determines gender differences in behavior, including sexual preferences. This is supported by cases of change in sexual orientation in people with brain tumors in the hypothalamus and prefrontal cortex.
Studies of brain structures do show a difference in the volume of the hypothalamic nuclei in heterosexual and homosexual men.
The size of the anterior hypothalamic nucleus in women is on average smaller than in men. The partial development of the brain of gay men according to the "female" type is also indicated by the comparable size of the anterior adhesion of the brain, which is larger in women and homosexual men. Nevertheless, in homosexual men, the suprachiasmatic nucleus of the hypothalamus was also enlarged, the size of which does not differ in men and women. This means that homosexuality is not explained only by the predominance of some "female" properties of the brain; the "homosexual brain" has its own unique characteristics.
Antibodies and the brain
In 1996, psychologists Ray Blanchard and Anthony Bogaert found that gay men often have more older brothers than heterosexual men. This phenomenon has received the Sexual orientation, fraternal birth order, and the maternal immune hypothesis: A review the name fraternal birth order effect, which can be loosely translated as “younger brother syndrome”.
Over the years, statistics have been repeatedly confirmed, including on populations of non-Western origin, which made its authors submit a hypothesis as the main one explaining the phenomenon of homosexuality. Nevertheless, critics of the hypothesis point out that in reality it explains only one or two cases of homosexuality out of seven.
It is assumed that the basis of the "little brother syndrome" is the immune response of the mother against proteins associated with the Y-chromosome. Probably, these are proteins that are synthesized in the brain precisely in the departments associated with the formation of sexual orientation and listed above. With each subsequent pregnancy, the amount of antibodies against these proteins increases in the mother's body. The effect of antibodies on the brain leads to a change in the corresponding structures.
Scientists analyzed genes on the Y chromosome and identified four main candidates responsible for immunizing mother against fetus - genes SMCY, PCDH11Y, NLGN4Y and TBL1Y. Most recently, Bogart and colleagues tested two of them experimentally (protocadherin PCDH11Y and neuroligin NLGN4Y). Mothers whose youngest son has a homosexual orientation actually showed a higher concentration of antibodies to neuroligin 4 in the blood. This protein is localized in the postsynaptic membrane at the sites of interneuronal contacts and is probably involved in their formation.
"The Kiss", painting of a red-figure vessel, master of Briseis, circa 480 BC.
Hormones and epigenetics
Epigenetic labels - chemical modification of DNA or proteins associated with it - form the gene expression profile and thus create a kind of "second layer" of hereditary information. These modifications can appear as a result of environmental influences and even be transmitted to offspring within one or two generations.
The idea that epigenetics plays a significant role in the formation of homosexual behavior was prompted by the fact that even in monozygotic twins, the highest level of concordance (the same manifestation of a trait) was only 52 percent. At the same time, in numerous studies, the influence of environmental conditions after birth - upbringing and other things - on the formation of homosexuality was not registered. This means that the formation of certain types of behavior is rather influenced by the conditions of intrauterine development. We have already mentioned two of these factors - testosterone and maternal antibodies.
The epigenetic theory suggests that the influence of certain factors, in particular hormones, leads to a change in the gene expression profile in the brain due to changes in DNA modifications. Despite the fact that twins inside the womb should be equally exposed to signals from outside, in reality this is not the case. For example, twins have different DNA methylation profiles at birth.
One of the confirmations of the epigenetic theory, albeit indirectly, was the data on the selective inactivation of the X chromosome in mothers of homosexual sons. Women have two X chromosomes in their cells, but one of them is randomly inactivated precisely due to epigenetic modifications. It turned out that in some cases this happens in a directional way: the same chromosome is always inactivated, and only the genetic variants presented on it are expressed.
The hypothesis of William Rice and colleagues suggests that epigenetic markers that cause homosexuality are transmitted along with the germ cells of the father or mother. For example, some DNA modifications that exist in the egg and determine the development of the "female" model of behavior, for some reason, are not erased during fertilization and are transmitted to the male zygote. This hypothesis has not yet been experimentally confirmed, nevertheless, the authors are going to test it on stem cells.
Homosexuality and evolution
As we can see from the statistics given at the beginning of the article, a certain percentage of homosexual people are consistently present in different populations. Moreover, homosexual behavior has been recorded for one and a half thousand species of animals. In fact, true homosexuality, that is, the tendency to form stable same-sex couples, is observed in a much smaller number of animals. Sheep is a well-studied mammalian model. Approximately eight percent of the males in sheep are involved in homosexual relationships and show no interest in females.
In many species, same-sex sex performs certain social functions, for example, it serves to assert dominance (however, in people in certain groups, it serves the same purposes). Similarly, in human societies, episodes of sexual relations with members of the same gender are not necessarily indicative of homosexuality. Polls show that many people who have had similar episodes in their lives consider themselves heterosexual and are not included in the statistics.
Why did this type of behavior persist in the process of evolution?
Since homosexuality has a genetic basis, some genetic variants continue to be passed on from generation to generation, without being rejected by natural selection.
Thanks to this, the phenomenon of homosexuality was called the "Darwinian paradox". To explain this phenomenon, researchers are inclined to think that such a phenotype is a consequence of sexual antagonism, in other words, the “war of the sexes”.
"War of the sexes" implies that within the same species, representatives of different sexes use opposite strategies aimed at increasing reproductive success. For example, it is often more profitable for males to mate with females as much as possible, while for females it is too costly and even dangerous strategy. Therefore, evolution can choose those genetic variants that provide some kind of compromise between the two strategies.
The theory of antagonistic selection develops the hypothesis of sexual antagonism. It implies that options that are disadvantageous for one sex may be so beneficial for another that they still persist in the population.
For example, an increase in the percentage of homosexual individuals among males is accompanied by an increase in females' fertility. Such data were obtained for many species (for example, N + 1 talked about experiments on beetles). The theory is also applicable to humans - Italian scientists have calculated that all the available data on compensation for male homosexuality of some members of the genus by increased female fertility would be explained by the inheritance of only two genetic loci, one of which must be located on the X chromosome.