Genes play a large part in determining the sexual orientation of men, scientists have shown.

Genetic factors account for between 30% and 40% of what decides whether a man is gay or straight, according to the largest investigation conducted into the subject.

A group of US researchers stressed environmental forces, such as hormones in the womb, played a more important role.

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But this did not imply upbringing or other social factors, or individual choice, had a bearing on sexual orientation.

"Sexual orientation has nothing to do with choice," said Dr Michael Bailey, from Northwestern University in Chicago, who co-led the study.

"Our findings suggest there may be genes at play - we found evidence for two sets that affect whether a man is gay or straight.

"But it is not completely determinative; there are certainly other environmental factors involved."

The research involved testing the DNA in blood samples taken from more than 409 gay brothers and their heterosexual relatives.