The science of sexuality

Yet, what determines a person's sexual orientation is largely a mystery.

For years scientists have searched for the answer, exploring the influence of siblings and specific genes, and a new study points out their quest has been clouded by how sexuality is measured, as well as political agendas. 

Over the past 50 years, political rights for lesbian, gay, and bisexual (LGB) individuals have significantly broadened in some countries, while they have narrowed in others. 

In many places, political and popular support for LGB rights hinges on the causes and consequences of homosexuality, making research into why it occurs important.

Researchers wrote in the study, published in the journal Psychological Science in the Public Interest:' Individuals' political attitudes about sexual orientation tend to correlate with their views of the causes of sexual orientation.

'Those who hold positive attitudes (i.e., that there is nothing inherently wrong with non-heterosexuality or its open expression) have tended to believe that sexual orientation is due to non-social causes such as genetics.

'Those who hold negative attitudes (i.e., that non-heterosexuality is undesirable or immoral and that society should restrict its free expression) have tended to believe that homosexuality has social causes, such as early sexual experiences and cultural acceptance of non-heterosexuality.'

The experts, from seven universities continued: 'The question of whether or not people choose their sexual orientations has clouded rather than clarified thinking.'

They believe various biological factors, including prenatal hormones and specific genetic profiles are likely to contribute to sexual orientation, but are probably not the sole cause.

Scientific evidence suggests biological and non-social environmental factors jointly influence sexual orientation, in other words, a combination of nature and nurture.

One interesting idea is that homosexuality may be due to an immune attack on an unborn son - an idea first proposed by Ray Blanchard, a professor of psychiatry at the University of Toronto in 2001.

He claimed the 'fraternal birth order effect' means the more older brothers a man has, the more likely he is to be gay.

However, the effect doesn't hold 'true' for women or younger brothers.

Male foetuses carry male-specific proteins on their Y chromosome, called H-Y antigens and he theorised that some of the antigens may influence homosexual orientation in men.

As H-Y antigens are not present in a mother's body, they may trigger the production of antibodies that bind to the H-Y antigens to stop them from working properly, perhaps not 'switching on' the 'straight' signal in the foetus' brain, The Atlantic explained.

He proposed the effect grows stronger with every son a woman bears, so that an only child has a 2 per cent chance of being gay, but the odds rise to six per cent for a man with four older brothers.

Last week, a study claimed that around half of straight men and women potentially carry so-called homosexuality genes that are passed on from one generation to the next.

This has helped homosexuality exist among humans throughout history, even though gay men tend to have fewer descendants who directly inherit their genes.

Researchers from Ilia State University in Georgia came to this conclusion using a computational model that, among others, included aspects of heredity and the tendency of homosexual men to come from larger families.

The study, published in Springer's journal Archives of Sexual Behaviour, did not, however, identify any specific genes.

The review paper says the number of people who say they are gay could also be growing because of more tolerance in some societies.

'Homosexual orientation does not increase in frequency with social tolerance, although its expression (in behaviour and in open identification) may do so,' they write.

While this reasoning has been negatively used by countries such as Russia to support anti-gay laws, the review says nothing can change the fact homosexuality is natural and ubiquitous whether it's caused by antibodies, genes or any other biological mechanisms.

Lead author, J Michael Bailey of Northwestern University said: 'Sexual orientation is an important human trait, and we should study it without fear, and without political constraint.'

Various biological factors - including prenatal hormones and specific genetic profiles - are likely to contribute to sexual orientation, though they are not the sole cause. Scientific evidence suggests that biological and non-social environmental factors jointly influence sexual orientation.

(www.dailymail.co.uk/sciencetech/article-3563192/The-science-sexuality-having-older-brothers-certain-genes-experts-study-really-influences-sexual-orientation.html#ixzz47CGaaaOz)