Sex offending is written in the genes, an unprecedented study has shown, with the brothers of men convicted of sex crimes five times more likely to commit a rape or assault than the general public.

The controversial finding suggests that some men are born with an increased risk of molesting children or carrying out a violent sexual attack.

Researchers at Oxford University and the Karolinska Institute in Sweden say the fathers and brothers of sex offenders could be offered psychotherapy to teach them relationship skills such as respecting boundaries and curbing aggression as a way of protecting the public.

Around 40 per cent of the risk of committing a sex crime is genetic, the research found, with the remaining 60 per cent down to personal and environmental factors, such as being abused as a child, upbringing, wealth and education.

“We are definitely not saying that we have found a gene for sexual offending’ or anything of the kind,” said Professor Seena Fazel, of Oxford University’s Department of Psychiatry.

“What we have found is high quality evidence from a large population study that genetic factors have a substantial influence on an increased risk of being convicted of sexual offences.

“At the moment genetic factors are typically ignored when it comes to making risk assessments of those at high risk of sexual offending.