The study was possible because of the detailed registers kept in Sweden on convicted sex offenders and their relatives. The scientists looked at offending in the sons, brothers, half-brothers and cousins of men who had been convicted of any sexual crime over the 37-year period, including adult rape and child molestation.
By comparing fathers with sons, and full brothers with paternal half-brothers, who are usually reared in separate families by two different mothers, and maternal half-brothers, who are normally brought up together by the same mother, the scientists were able to tease out the relative influence of shared environment and shared genes on the risk of sexual offending.
This suggested that about 40 per cent of the difference in risk seen between brothers and fathers of convicted sexual offenders and brothers and fathers of those without a similar conviction is due to genetic factors. In other words the heritability is 40 per cent while the rest of the risk is due to shared environmental factors, which can include anything from medical complications during birth and child sexual abuse, to upbringing and family background.
“Importantly, this does not imply that sons or brothers of sex offenders inevitably become offenders too…Just because something is genetically determined, it does not mean that we cannot use the environment to change it,” said Niklas Langstrom, professor of psychiatric epidemiology at the Karolinska Institute in Stockholm and lead author of the study published in the International Journal of Epidemiology.
“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,” Professor Langstrom said.
Rajan Darjee, a consultant forensic pathologist in Edinburgh, who was not involved in the study, said that it is interesting but not surprising to find a strong genetic component to sexual offending.
“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,” Mr Darjee said.
“The fact that genes play such a role does not mean that a person is less responsible for their offending or that offending is inevitable in someone at higher genetic risk, it just emphasises that genes are an important part of a complicated jigsaw,” he said.
“We should not interpret the findings of this study to indicate that any male relative of a sexual offender is going to commit a sexual offence so they should be put under special restrictions. We cannot know who will or will not commit sexual offences in this way,” Mr Darjee added.
The researchers accept that a major limitation of their study is that it only focused on men who were convicted of sexual crimes and ignored the estimated 80 per cent of sexually abusive acts that are never reported to the police, or the offences that did not result in successful prosecutions.
However, one of the strengths of the study is that it represented the more severe end of the sexual offending spectrum and did not have to rely on the inherent bias of self-reported cases of sexual abuse, the scientists said.
The suggestion that sexual offending has a strong genetic component is broadly in line with other research indicating a similar inherited link with violent crime. Although researchers do not suggest there are “genes for rape”, there is some evidence that certain, defined genetic traits may be linked to extreme violent behaviour.
Last year, researchers in Finland suggested that between 5 and 10 per cent of severe violent crime in that country could be attributable to two genes, each of which can modify the activity of the brain in different ways.
However, the important point was that many people carry these gene variants and are not violent. You may inherit a genetic predisposition, but it is possible to control its influence.
