Brothers of men convicted of sexual offences are five times more likely than average to commit the same types of crimes, scientists have found.
The study suggests that genetic factors are largely responsible for the effect and that environmental factors, such as sons learning from fathers, have only a minor influence. The authors urged authorities to consider interventions for the male relatives of sexual offenders, including counselling on appropriate sexual behaviour or even offering medications designed to lower sex drive.
Niklas Långström, professor of psychiatric epidemiology at the Karolinska Institute in Stockholm and lead author, said: “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.”
There is no evidence for a “sex offending gene”, he added. Instead, a constellation of genes linked to factors such as impulse control, intelligence and sexual appetite are likely to influence the risk of a person committing an offence.
The study is the first major investigation of the genetic basis of sexual crime – a subject that had proved too challenging for previous survey-based research. “It’s pretty sensitive to ask about these things, so we tried to use officially available data” said Prof Långström.
The scientists used the records of 21,566 men convicted of sexual offences in Sweden between 1973 and 2009. Around 2.5% of brothers or fathers of convicted sex offenders were themselves convicted of sexual offences, compared to an offending rate of about 0.5% of men in the general population. The authors looked at both rape and child sexual offences and found similar patterns for both.
About 40% of sexual offending risk is explained by genetic factors, a statistical analysis found, and about 2% of the risk was attributed to environmental factors shared between siblings, such as parental attitudes, neighbourhood and education. Unique environmental factors, such as head injuries, peer group influence and social experiences, are likely to account for the remaining differences in risk between people.
The authors make the case for targeting interventions at the male relatives of sex offenders. “We’re not saying you should lock up the brothers,” said Seena Fazel, professor of forensic psychiatry at the University of Oxford and co-author. “It may well be that something extra can be offered to these high risk families in terms of teaching about boundary setting, relationship skills, conflict management.”
However, they acknowledge that the issue would have to be handled with extreme sensitivity and said it was too early to say whether pharmacological interventions would ever be helpful.
“Obviously one has to be careful about not ostracising people,” said Långström. “It’s important that it does not become a public thing, where not only has your father committed a sexual offence, but you’re forced to attend mandatory courses.”
The idea that sexual offending tends to be clustered in families is not new, but typically has been put down to a “cycle of abuse” – an idea that has probably been given too much weight, the authors argue. Instead, they say, the effect is mostly explained by genetics.
To demonstrate this, they considered the case of maternal half-brothers (typically brought up in the same family home) and paternal half-brothers (who normally live with different mothers). If environmental factors were largely responsible, a much higher risk would be expected in maternal half-brothers whose siblings had committed crimes. But both groups were roughly twice as likely to offend if their half-sibling had previously committed a sexual offence. The link was also about half as strong as for full siblings, again supporting the idea that genetic factors were more powerful.
Dr Rajan Darjee, a consultant forensic psychiatrist in Edinburgh, who was not involved in the work, said: “Genes influence brain development, and brain functioning underpins psychological functioning, so it should not be surprising to find that genetic factors play a role in sexual offending.”
That genes play a role does not mean that a person is “less responsible” or that crimes are inevitable, he added. “It just emphasises that genes are an important part of a complicated jigsaw.”
A spokesman for the National Organisation for the Treatment of Abusers, said: “The possibility of carefully targeted interventions and support for families in which the relative risks are higher is an exciting prospect for a committed professional community.”
The risk was found to be slightly higher for brothers (fives times the average) compared to fathers (four times), which could simply reflect brothers being closer in age and therefore more likely to both offend during the study period.
“People don’t commit sex offences from zero until they die. You have windows of risk,” said Långström.