New research from Sweden's Karolinska Institutet in collaboration with Oxford University, UK, shows that close relatives of men convicted of sexual offenses commit similar offenses themselves more frequently than comparison subjects. This is due to genetic factors rather than shared family environment. The study includes all men convicted of sex crime in Sweden during 37 years.

"Importantly, this does not imply that sons or brothers of sex offenders inevitably become offenders too," says Niklas Langstrom, Professor of Psychiatric Epidemiology at Karolinska Institutet and the study's lead author. "But although sex crime convictions are relatively few overall, our study shows that the family risk increase is substantial. Preventive treatment for families at risk could possibly reduce the number of future victims."

The report is published in the International Journal of Epidemiology and based on anonymzsed data from the nationwide Swedish crime and multigeneration registers.The research included all 21,566 men convicted for sex offenses in Sweden between 1973 and 2009, for example rape of an adult (6,131 offenders) and child molestation (4,465 offenders). The researchers looked at the share of sex crimes perpetrated by fathers and brothers of convicted male sex offenders and compared this to the proportion among comparison men from the general population with similar age and family relationships.

The results suggested familial clustering of sex offenders, about 2.5 percent of brothers or sons of convicted sex crime offenders are themselves convicted for sex crimes. The equivalent figure for men in the general population is about 0.5 percent. Using a well-established statistical calculation model, the researchers also analyzed the importance of genetic and environmental factors for the risk of being convicted of sexual abuse.

"We found that sex crimes mainly depended on genetic factors and environmental factors that family members do not share with one another, corresponding to about 40 percent and 58 percent, respectively," says Niklas Langstrom. "Such factors could include emotional lability and aggression, pro-criminal thinking, deviant sexual preferences and preoccupation with sex."

Self-reported sexual victimization rates in Sweden are largely similar to those in other Western and central European nations, Canada and the USA. Other cross-national comparisons of police-reported offenses should be done cautiously because of differences in legal definitions, methods of offense counting and recording, and low and varying reporting rates of sexual violence to the police.