Scientists suggest homosexuality is genetic, decode Darwinian paradox

New Delhi: Will homosexuality eventually be extinct? 

With new evidence suggesting that homosexual behaviour is governed primarily by genetic influences and gay couples reproducing significantly less than heterosexuals, that's a question scientists across the world are pondering. 

While environmental factors play a part in the expression of a homosexual phenotype, or physical trait, scientists suggest their influence is not powerful enough to cause an otherwise heterosexual organism to become homosexual. 

Scientists are also debating how such behaviour fits with British naturalist Charles Darwin's theory of evolution. 

"The Darwinian paradox suggested that it is impossible to maintain genes which do not promote reproduction, as in the case of homosexuality. Since homosexuals reproduce significantly less than heterosexuals, the genes promoting this trait should rapidly go extinct,? Andrea Camperio Ciani, professor of evolutionary psychology in Italy's University of Padova, told PTI.

Ciani, who has pioneered efforts to solve why homosexuals have not become extinct from human population, notes that this paradox for a long time excluded the genetic hypothesis and suggested that homosexuals behave so due to a sin and a misbehaviour that can be eradicated through therapy?

Over the past few decades, evidence has begun to accumulate that homosexuality is a behaviour that appears primarily due to biological or genetic influences, scientists said.

"The predisposition to mate with the opposite sex is aimed at reproduction and passing genes to the subsequent generations. However, homosexual behaviour is quite widespread and not a new phenomenon in the animal kingdom," said Manaswini Sarangi, a PhD research scholar in Evolutionary Biology at Bangalore's Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR).

"Through several studies across a variety of species, displaying homosexual behaviour has been shown to give an evolutionary advantage to the organisms," she told PTI.

According to T N C Vidya, associate professor, Evolutionary and Organismal Biology Unit at JNCASR, some gene variants are more common amongst gay men.

"As far as I know, it is not clear to what extent homosexuality is heritable," she said.

"Whether gene variants that influence homosexuality propagate over generations will depend on whether homosexuals get to reproduce. Even if they themselves do not reproduce, such variants may be transmitted if their relatives who carry copies of the genes but do not express them (are not homosexuals themselves) reproduce," Vidya said.

If homosexuality is indeed heritable, she said, it does not necessarily contradict the theory of natural selection.

Ciani noted that genes promote fecundity, or ability to produce offspring, in some individuals but promote homosexuality in others. In this way, they are preserved in the human gene pool.

By studying the pedigree of hundreds of homosexuals and their families, he said he has resolved the paradox and discovered that the genes influencing homosexuality indeed exist.

Scientists also say that homosexual behaviour is not specific to humans and has been documented in over 500 non-human species, including bonobos, penguins, and even grizzly bears. 

According to Sarangi, it is quite widespread in the animal kingdom, ranging from insects to birds to primates.

"Therefore, saying that homosexuality is unnatural is factually incorrect. It is another behavioural variant like in many other traits," she said.

The Indian Supreme Court's recent verdict decriminalising adult, consensual homosexual acts has been widely welcomed.

"I am pleased to learn that the Supreme Court of India has de-penalised homosexuality since it is a great acquisition in civil rights. Most homosexuals are born with this orientation, under the influence of genes that belong to the normal human gene-pool," Ciani said.

"Decriminalising homosexuals was long overdue. It will probably help to some extent against harassment of homosexuals, but society itself also has to change so that people are more welcoming of those who are not like them in some way or the other. I hope this judgment will lead the way to more changes, such as allowing gay/lesbian marriages," said Vidya.

Sarangi added that the historic judgement decriminalising homosexual behaviour among consenting adults is a progressive step for the country.
Ciani said his findings are an outcome of a series of different research studies conducted in the past 20 years, starting with the first publication in 2004 and a series still ongoing with statisticians, mathematicians and psychologists and students in laboratories across the world.

"Our last research, just published last year in Human Nature on a very large number of lesbian, bisexual and heterosexual females and their families, pointed to the same mechanism as we discovered in gays," Ciani said.

An important precursor of Ciani's studies is Dean Hamer from the US National Institute of Health who first suggested genetic inheritance in the 1990s. But most colleagues disproved his pioneering researches at the time.

Ciani hopes that his studies contribute to increased tolerance for diversity. 

"I am happy that my studies and their scientific results contributed to this historical event in your large and great county, which I am particularly fond of since I started my scientific career doing field research in India studying the social behaviour of monkeys in Shimla, Himachal Pradesh," he said.