Men are up to five times more likely to commit a sex crime than the average male if they have a brother or father who has also been convicted of a serious sexual offence, the largest study of its kind has shown.

A survey of 21,566 men convicted of rape and other sexual crimes in Sweden between 1973 and 2009 has revealed a strong genetic component to sex offending which appears to play a significant influence on the risk of close male relatives going on to commit a sex crime, researchers said.

The study found that about 2.5 per cent of the brothers or fathers of criminals convicted of sexual crimes are themselves convicted of similar crimes – compared to 0.5 per cent of the general male population who become convicted sex offenders.

Scientists said the findings should not be used to excuse sex offending, to restrict the freedom of the male relatives of sex offenders, or to suggest that there are genes for rape or paedophilia. However, they believe the results could lead to better prevention strategies for the sons or brothers of known sex offenders.

Future studies may discover the genes that influence the risk of sexual offending, and if they do it is likely that these genes are “mediators” of aberrant behaviour rather than contributing directly to the cause of offending, Professor Fazel explained. (Getty Images)

“If interventions can be provided that are not harmful, this is an opportunity….We’re not saying you should lock up the brothers of sex offenders, because there is a very small proportion of brothers who go on to offend,” said Seena Fazel, professor of forensic psychiatry at Oxford University.

“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,” Professor Fazel said.

“It tells us something about why if we take two sets of brothers whose backgrounds look identical [and] 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,” he said.

Future studies may discover the genes that influence the risk of sexual offending, and if they do it is likely that these genes are “mediators” of aberrant behaviour rather than contributing directly to the cause of offending, Professor Fazel explained.

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. (Getty Images)

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.

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. (alamy) (Alamy)

“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 of 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.

Violent crime in the genes?

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.