Men are up to five times more likely to be convicted of sexual offences if they have a brother or father who is a convicted sex offender, according to a new study which suggests that genetic factors may be responsible.
Researchers from Oxford University (UK) and the Karolinska Institute (Sweden) used statistical methods to analyse data on all 21,566 men convicted of sexual offences in Sweden between 1973 and 2009.
The researchers looked at the share of sexual offences perpetrated by fathers and brothers of convicted male sex offenders and compared this to the proportion among Swedish men from the general population with similar age and family profiles.
They found that around 2.5 per cent of brothers or fathers of convicted sex offenders are themselves convicted of sexual offences. This compares to convicted sex offenders making up about 0.5 per cent of men in the general population.
Having a brother convicted of a sexual offence led to five times the risk of a man being convicted of a sexual offence compared to a man whose brother had not been convicted of this type of crime.
Meanwhile, being a father of a man convicted of a sexual offence led to almost four times the risk of being convicted of a sexual offence.
Statistical modelling suggested that around 40 per cent of the difference in risk seen between brothers and fathers of convicted sexual offenders and brothers and fathers of those without such a conviction is due to genetic factors.
Around 58 per cent of the difference in risk appears linked to 'non-shared' environmental factors (such as perinatal complications, head injuries, childhood sexual victimisation, and peer group attitudes) that affect an individual but are not shared with other family members.
Evidence on the difference in risk between maternal and paternal half-brothers (presumably reared in separate family environments) supports the idea that genetics makes a substantial contribution to increased risk.
"Importantly, this does not imply that sons or brothers of sex offenders inevitably become offenders too. But although sex crime convictions are relatively few overall, our study shows that the family risk increase is substantial," said Niklas Langstrom, Professor of Psychiatric Epidemiology at Karolinska Institute and the study's lead author.
"Preventive treatment for families at risk could possibly reduce the number of future victims," Langstrom said.
"We are definitely not saying that we have 'found a gene for sexual offending' or anything of the 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," added Professor Seena Fazel of Oxford University's Department of Psychiatry, an author of the paper.
The research is published in the International Journal of Epidemiology.
