Brothers of men convicted of sexual offences are five times more likely to commit similar crimes
Brothers of men convicted of sexual offences are five times more likely than average to commit similar crimes, research has shown.
Sons of fathers with a criminal record for sexual offending had a nearly four-fold increased risk of following in their parent's footsteps, scientists found.
Genes play a leading role in the link, according to the study of 21,566 men - suggesting that certain individuals are born with a propensity to commit sexual offences.
Statistical analysis indicated that 40% to 50% of the differences in risk seen between close relatives of offenders and men from the general population were genetically driven.
But the researchers stressed this in no way meant that someone with a brother or father convicted of rape was fated to become a sex offender
Lead scientist Professor Niklas Langstrom, from the Karolinska Institute in Sweden, said: "It's important to remember that it's nothing mystic. People get worried about the fact that there's a strong genetic component in problematic human behaviour.
"Of course, you don't inherit in some kind of automatised robotic way so that you will grow up to be a sexual offender."
Yet the results showed that genetic factors had a "substantial" influence on the risk of being convicted of a sexual offence, said the researchers.
The scientists looked at data on all men convicted of sexual offences in Sweden between 1973 and 2009.
For the purposes of the study, sexual crime was defined as a conviction for any sexual offence described by the Swedish penal code.
Of the 21,566 offenders, nearly half had convictions for adult rape or child molesting. Other crimes included possession of child pornography, indecent exposure, and sexual harassment.
In terms of absolute risk, being closely related to an offender only led to a small increase in the chances of committing a sexual crime.
Just 2.5% of brothers of sex offenders were themselves convicted of similar crimes, but this compared with a mere 0.5% of men in the general population.
Evidence from half-brothers sharing either a mother or father and being raised in different family environments supported the idea that genetics played an important role in sexual offending.
Co-author Professor Seena Fazel, from Oxford University, said: "We are definitely not saying that we have found a gene for sexual offending or anything of that kind. 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.
"It tells us something about why if we take two sets of brothers, whose backgrounds might look identical, one set has a higher risk of sexual offending than the other - a large proportion of this difference is likely to be due to genetic factors."
Writing in the International Journal of Epidemiology, the researchers concluded: "We found substantial evidence of moderate to strong excess familial risk for sexual offending among men.
"Having a father or brother convicted of a sexual offence increased the risk of being convicted oneself four to five times compared with age-matched control men without a sexually aggressive father or brother.
"These familial aggregation effects are comparatively large in relation to familial risks for other studied behaviours."
Children of male violent offenders were about 3.5 times more likely than average to commit violent crimes themselves, for instance.
The findings may point the way towards providing better social help to at-risk families, many of which were already known to social services, said the authors.
"At the moment genetic factors are typically ignored when it comes to making risk assessments of those at high risk of sexual offending," said Prof Fazel.
"Many of the families we are talking about may already be known to social services for other reasons, and if we can predict those at high risk of offending with greater accuracy then it may be possible to shape these interventions and target education and preventative therapies where they could do the most good."
Interventions might include help with setting boundaries, and improving conflict resolving and social skills, he said.
A logical next step would be a large-scale study that took DNA samples and looked for genetic patterns that may contribute to problem sexual behaviours, Prof Fazel added.
But pinning down the precise genetic variants involved in sexual offending would be next to impossible, requiring samples from many thousands of individuals.