Gender structured: features each partner playing a different gender role. This is exemplified by traditional relations between men in the Mediterranean Basin, the Middle East and Central and South Asia, as well as two-spirit or shamanic gender-changing practices seen in native societies. Albania also has a similar practice where a woman may choose to be an “Albanian Virgin" and thus given all the rights and entitlements of a man. In North America, this is best represented by the butch/femme practice.

Age structured: features two partners of different ages, usually spanning at least one generation. This is exemplified by pedastry among the Classical Greeks or those engaged in by novice Samurai, with more experienced warriors; southern Chinese boy-marriage rites; and ongoing Central Asian and Middle Eastern practices.

Incidence

Estimates of the modern prevalence of homosexuality vary considerably. They are complicated by differing or even ambiguous definitions of homosexuality, and by fluctuations over time and according to location.

Recent estimates of the number of homosexuals (not including bisexuals) in Western countries, where egalitarian relationships predominate, range from 1% to 10%, confined to a self-identified subculture. In the United States during the 2004 elections, exit polls indicated 4% of all voters self-identified as gay or lesbian. However, many who are homosexual may not be open about it in public and are therefore difficult to count.

In North America, the Middle East and Central Asia, where gender- and age-structured relationships are the rule, male homosexual practices are reported to be widespread and engaged in by many individuals who do not regard themselves as homosexual.

Biology

Prenatal hormonal theory

One recent hypothesis on the formation of sexual orientation is the prenatal hormonal theory. It holds that prenatal (pre-birth) exposure to particular levels of circulating sex hormones determines whether a fetus will acquire male or female traits, so similar exposure determines sexual orientation.

Physiological differences in homosexuals

Several recent studies, including pioneering work by Simon LeVay, demonstrate that there are notable differences between the physiology of a heterosexual male and a homosexual male. These differences are primarily noted in the brain, inner ear and olfactory sense. LeVay discovered in his double-blind experiment that approximately 10% of human male brains were physiologically different than their heterosexual counterparts. Studies in women have not produced similar findings to date.

There are some methodological problems with the studies. One of the biggest problems was differentiating brain changes that could have occurred in gay men’s usage of different parts of their brain, and potential clues to origins of sexual identity. It is a bit of a ‘chicken or the egg,’ dilemma.

Psychology

Behavioral Studies

Contemporary scientific research suggests that the majority of the human population is bisexual; adhering to a fluid sexual scale rather than a category, as Western society typically views sexual nature. Approximately four percent of adult Americans have been found to be exclusively homosexual for their entire lives, and approximately 10 percent were homosexual in their behavior for some portion of their lives. Conversely, even smaller minorities of people appear to have had equal sexual experiences with both genders indicating an attraction scale or continuum. However, social pressures influence people to adhere to categories or labels rather than behave in a manner that more closely resembles their nature as suggested by this research.

The consensus of psychologists is that sexual orientation, in most individuals, is shaped at an early age; and is not voluntarily changeable.

Nature versus nurture

Considerable debate exists over what biological and/or psychological factors produce sexual orientation in humans. Candidates include genes and the exposure of fetuses to certain hormones (or levels thereof). Freud and many others psychologists, particularly in psychoanalytic or developmental traditions, speculate that formative childhood experiences help produce sexual orientation. Other scientists and medical professionals, particularly those in biology-oriented disciplines, tend to believe that in-born factors, whether genetic or acquired in-utero, produce characteristically homosexual childhood experiences (such as atypical gender behavior experiences), or at the least significantly contribute to them.

Modern Developments

Marriage

As of 2005, four countries have enacted same-sex marriage, and other countries, including the majority of Europe, enacted civil unions. In Asia, the conflict between homoerotic tradition and a resurgent Islamic fundamentalism continues. Liaquat Ali, a 42 year old Afghan refugee, and Markeen Afridi a 16 year old Pakistani boy, reportedly fell in love and got married in a very public ceremony in October of 2005. There are efforts to refute the original reports that were authored by a reporter from the tribe where the wedding occurred.

Political developments

Publicly gay politicians have attained numerous government posts, even in countries that had sodomy laws or outright murder of gays in their recent past.

Religious developments

The overall trend of greater acceptance of gay men and women in the latter part of the 20th Century was not limited to secular institutions; it was also seen in many religious institutions. Reform Judaism, the largest branch of Judaism outside Israel had begun to facilitate religious weddings for gay adherents in their synagogues. The Anglican Communion, the world's second largest Christian Church in terms of membership, encountered discord that caused a rift between the European and North American Churches when American and Canadian churches ordained gay clergy and began blessing same-sex unions against the wishes of the Anglican archdiocese. Other Churches such as the Methodists had experienced trials of gay clergy who some claimed were a violation of religious principles resulting in mixed verdicts dependent on geography.

These developments have been accompanied by a response from certain conservative religious organizations, especially in the United States. In various instances, this movement has succeeded in overturning some of the aforementioned legislation and has had an influence on academia. In late 2005, Haworth Press withdrew from publication a volume on homosexuality in classical antiquity titled Same-Sex Desire and Love in Greco-Roman Antiquity and in the Classical Tradition of the West. This was in response to criticism from American conservative groups that objected to the discussion of positive aspects of classical pederasty, as well as to a chapter by the American academic Bruce Rind that was branded by the critics as advocating pedophilia. The publisher, in a letter to the editors, exonerated Rind from the accusation and conceded that his article was sound, but stood by its decision to withdraw it "to avoid negative press" and "economic repercussions."

Art and literature

One of the primary social manifestations of same-sex love has been through the medium of literature and art. The aspects of these expressions range across the gamut of the main artistic disciplines. Homoerotic sensibilities are at the foundation of art in the west, to the extent that those roots can be traced back to the ancient Greeks. Homer’s Illiad was regarded to contain a homosexual relationship by many of its contemporaries. Plato’s Symposium also gives readers commentary on the subject, at one point putting forth the claim that homosexual love is 'superior' to heterosexual love.

The European tradition was continued throughout the ages in the works of William Shakespeare, Michelangelo and Leonardo da Vinci. In Arab populations it was present in figures such as Abu Nuwas. The Tale of Genji, called the "world's first real novel", fostered this tradition in Asian societies.

Icons such as Madonna and Elton John have followed this tradition in modern times. Presently the Japanese anime sub-genre, yaoi, commonly features the theme. Artistic nudes have prominently displayed lesbianism. Playwrights have penned popular works with homosexual themes. These sentiments have pervaded many movies. A popular television series exploited these perceptions with Queer Eye for the Straight Guy, equating gay men with the ancient Greek Muses.