A study conducted by the University of Portsmouth believes it has found the evolutionary reason for same-sex behavior.

In evolutionary terms, homosexuality should have died out generations ago as, according to Darwin, any trait that makes an animal less likely to reproduce will die out.

The study is based around the hormone ‘Progesterone’; heterosexual men and women with a higher level of this hormone are more likely to have homoerotic thoughts. This hormonal bond seems to help human beings bond with others of the same sex.

The study wanted to review why there is a presence of homosexuality if it doesn’t help carry on a race from an evolutionary perspective.

Dr Diana Fleischman, the report’s author, told The Telegraph: “From an evolutionary perspective we tend to think of sexual behaviour as a means to an end for reproduction.”

She goes on to explain just why, then, homosexual behavior is recorded in human and non-human primates.

“However, because sexual behaviour 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 behaviour even when reproduction is not possible.”

Due to the scale of sexuality, then, it is possible for there to be extremes of bonding behaviour.

“Having some degree of attraction to the opposite sex is a type of adaptive behaviour, and in any adaptive behaviour you will see extremes of the spectrum, hence some people will only be attracted to members of the same sex.

“But the research suggests that having exclusively heterosexual thoughts is a disadvantage – it’s better to be a little bit attracted to the opposite sex.”

Dr Fleischman said her research did not point to any correlation between environmental levels of synthetic progesterone – as is in the contraceptive pill – and sexual orientation.She added that while environmental levels of progesterone have increased, there is no evidence that the percentage of people who are gay or lesbian has gone up.

The researchers concluded that homosexual thoughts can go hand in hand with the need to forge same-sex alliances, which can be traced back to the teamwork of the earliest hunter-gatherers.