A recent study has found that younger siblings are more likely to be gay.

Researchers at the University of Melbourne have provided new insights into the relationship between sibling order and sexual orientation, the Daily Mail reported.

Previous research had already established a connection between having older brothers and an increased chance of male homosexuality, The Post reported in April, but this study is the first to identify a similar link for those with older sisters.

The findings showed that men with older siblings are more likely to identify as gay, with the probability increasing with the number of older brothers they have.

For example, having at least one older brother raises the chances of being gay by approximately 10%, while having older sisters is cor with a 5% increase.

However, men with three older brothers exhibit a shocking 41% higher likelihood of being gay, and those with five older brothers face an 8% chance — four times that of the average man.

The researchers also found that this correlation does not extend to lesbians and older siblings or bisexuality.

However, the study did reveal a connection between sibling count and asexuality: individuals with no siblings were significantly more likely to report experiencing no sexual attraction.

The data for this research was pulled from the UK Biobank, a comprehensive study monitoring the health and lifestyles of a large British population over time.

The findings align with a previous investigation conducted in the Netherlands, which also emphasized the relationship between the number of older brothers and the probability of being gay.

The reason for this connection is still unconfirmed, but one leading theory is the “maternal immune hypothesis.”

That theory claims that after a mother gives birth to a boy, proteins enter her bloodstream and trigger the production of antibodies that may affect the sexual development of subsequent male children.

As a mother has more sons, these antibodies accumulate and may influence personality traits, potentially leading to differences in sexual orientation.

A 2017 study supports this hypothesis by showing that mothers of gay sons possess more antibodies targeting male-specific proteins compared to mothers of straight sons. However, that explanation does not account for the newfound association with older sisters.

Despite these findings, researchers stress the need for more research into why sibling order impacts sexuality.