Boys who are left handed and have lots of brothers more likely to be gay, research finds 

Scientists claims to have uncovered the three major factors that make a boy more likely to be gay.

The researchers suggest being left-handed, having several older brothers and having other gay people in the family are all significant markers.

Gay men are around 34 per cent more likely to be left-handed than heterosexual men, the team said.

Having one or more older male brothers, known as the ‘fraternal birth order effect’ increases the likelihood of being gay by around 14.8 per cent.

Having gay members in the family – on either the mother’s or father’s side – increased the likelihood of being gay, but was largely separate to the other factors, the researchers said.

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Scientists claims to have uncovered the three major factors that make a boy more likely to be gay. The researchers suggest being left-handed, having several older brothers and having other gay people in the family are all significant markers

But the factors do not add together, the authors said.

In other words, being left-handed and having several older brothers did not add together to make it more likely that someone is gay.

As evidence of this, gay men who had several older brothers were ‘primarily’ right-handed, the authors from the University of Toronto write.

The research was based on a study of more than 800 gay men.

The largest, ‘sub group’ of men in the survey, comprising 63 per cent, did not have any of the three ‘markers’ – and were very similar in their profiles to straight men.

The different sub-groups of gay men also had different psychological profiles.

For example, the researchers found that the gay men with none of the three ‘markers’ were the most likely to be gender-conforming.

This means acting in a way typical of a man, rather than acting like a typical woman, for example working in more typically ‘male’ jobs such as a garage mechanic or firefighter.

However, gay men with many older brothers were more likely to have jobs most typically carried out by women.

The researchers suggest being left-handed, having several older brothers and having other gay people in the family are all significant markers. Gay men are around 34 per cent more likely to be left-handed than heterosexual men, the team said

Writing in the journal PNAS, the authors write ‘We present evidence that non-heterosexual men can be categorized into at least four subgroups’.

Explaining why males become gay because they have several older brothers, the authors say that it is thought that every time a woman gives birth to a male boy, it triggers an immune response in the mother’s body.

This immune response increases with each boy born, and this affects brain development in subsequent boys, including that of the boy’s future sexual preference.

Left-handedness is a trait more common in men than women: men are 20 per cent more likely to be left handed than women. 

The causes of handedness are not yet known, but it is thought to be linked to genetic, immunological, and hormonal factors which affect the developing brain, the researchers asid.

Being gay is known to be likely to be inherited – with genetics considered to account for around 32 per cent of whether someone is gay or not.