Decoded: Homosexuality is genetic, homosexual couples reproduce less

New Delhi : India's Supreme Court historic judgement on September 6, 2018, making gay sex legal in the country has become the talk of the town. Besides common people, scientists are also striving hard to study the behaviour of homosexuality and its root cause. A study on homosexuality reveals that homosexual couples reproduce much less than heterosexual couples and is a genetic cause.

Besides the gene factor, it is the environment which plays a role in how an individual expresses the homosexual phenotype or physical trait. However, the findings need powerful evidences, meaning these two factors is not enough to make an otherwise heterosexual organism to become homosexual, scientists say.

Scientists are studying and questioning themselves how homosexual behaviour fits with 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," Andrea Camperio Ciani, professor of evolutionary psychology in Italy's University of Padova, told PTI.

"Since homosexuals reproduce significantly less than heterosexuals, the genes promoting this trait should rapidly go extinct," he added. 

Ciani observes that it was a common belief for a long time that homosexuals behave so because of sin and some misbehaviour that can be eradicated through therapy. Thus, for too long, the Darwinian paradox had not taken into account the genetic factor of homosexuality.

Over the past few years it has been seen that homosexuality is a behaviour appearing primarily due to biological or genetic influences. Experts say that the inclination of animals to mate with the opposite sex 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).

In order to study the homosexual behaviour, scientists have experimented in over 500 non-human species, including bonobos, penguins, and even grizzly bears. They found that the homosexual behaviour is not specific to humans.

According to Sarangi from JNCASR, homosexuality 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.

According to TNC Vidya, an associate professor of 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.

Gene variants that influence homosexuality can propagate over generations depending on whether homosexuals get to reproduce.

Vidya explained that even if the homosexual individuals do not reproduce themselves, their relatives who carry copies of the genes but do not express them (are not homosexuals themselves) can transmit them when they reproduce.

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

"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.