Scientists have long been puzzled by homosexuality, as it seems to be at odds with the basic human drive to reproduce.

Various theories have been offered--from the notion that homosexual men make more diligent uncles than their heterosexual counterparts (and thus are better at ensuring the survival of their relatives) to the notion that the same gene that codes for homosexuality in men makes women more fertile.

Now researchers from the University of Portsmouth in England have put forth a controversial new theory. They say homosexuality evolved in humans and other primates because it helps us form bonds with one another.

“From an evolutionary perspective, we tend to think of sexual behavior as a means to an end for reproduction," Dr. Diana Fleischman, an evolutionary psychologist at the university and one of the researchers, said in a written statement. "However, because sexual behavior is intimate and pleasurable, it is also used in many species, including non-human primates, to help form and maintain social bonds. We can all see this in romantic couples who bond by engaging in sexual behavior even when reproduction is not possible."

For the study, 92 women were asked to indicate the extent to which they agreed or disagreed with various hypothetical statements about homosexual behavior, such as: "The idea of kissing a person of the same sex is sexually arousing to me" and "If someone of the same sex made a pass at me I would be disgusted."

Then the researchers measured levels of the hormone progesterone in the women's saliva. Progesterone is linked to social bonding.

What did the researchers find? Women with high progesterone levels were more open to engaging in homosexual activity. The researchers theorize that progesterone may make people want to bond with others--and since sexual activity is one form of bonding, homosexual as well as heterosexual behavior is encouraged.

In another experiment, 59 men did word completion puzzles, filling in the blanks of words from one of the following three categories: friendship (for instance, "fr...ds" becomes "friends"), sex ("br...ts" becomes "breasts"), or neutral ("sq.ar." becomes "square").

The researchers found that the men who completed the friendship puzzles were 26 percent more likely to be open to the idea of having sex with other men compared to the men in the other two groups. In other words, when men were led to think about forming bonds with others, they were more open to homosexual as well as heterosexual behavior, Fleischman told The Huffington Post in an email.

"It’s very complex, but it’s clear there’s a continuum between affection and sexuality, and... the ability to engage sexually with those of the same sex or the opposite sex is common," Fleischman said in the statement. "In humans, much, if not most of same-sex sexual behavior occurs in those who don’t identify as homosexual.”

An intriguing theory, for sure. But not everyone is buying the new research.

“It is a plausible theory that there is a societal benefit from homosexual behavior, but the link to progesterone is probably spurious," Dr. Gerard Conway, professor of reproductive endocrinology at University College, London, who was not involved in the study, told The Telegraph. "It’s a long way from proving cause and effect.”