Genetic link to sex crimes found

Fathers or brothers of men found guilty of committing a sexual offence are four to five times more likely than average to commit a similar offence themselves, a new study claims.

The study, carried out by scientists from the University of Oxford in the UK and the Karolinska Institute in Sweden, suggested that genetic factors, rather than environmental influences, were responsible for this effect.

Researchers analysed data from around 22,000 male sex offenders convicted in Sweden between 1973 and 2009. They found that 2-2.5 percent of brothers or fathers of convicted sexual offenders had themselves committed a sexual offence. This is approximately five times higher than the offending rate in the general population of around 0.5 percent.

Professor Niklas Långström from the Karolinska Institute and lead author of the study, said: 'Although sex crime convictions are relatively few overall, our study shows that the family risk increase is substantial.'

In order to separate genetic factors from environmental influence, the authors compared families from similar backgrounds, looking at fathers with sons, full siblings as well as differences between paternal and maternal half-brothers - likely reared in separate environments but with the same number of shared genes.

Based on this, the authors determined that just two percent of the risk of committing a sexual offence is accounted for by shared environmental influences, whilst 40 percent is accounted for by genetics. The remaining risk is likely due to 'non-shared' environmental factors, which could include things such as head injuries, peer group and social influences.

However, Emily Underwood notes in Science that most previous studies have found that experiencing abuse early in life is the most important risk factor for later committing abuse crimes. The offenders' previous record of exposure to abuse was not taken into account here, a limitation in attempting to quantify the role of environmental influences. Further, relying on conviction records could lead to a bias in the data, as many more sexual crimes are committed than reported.

Dr Fred Berlin, a psychiatrist at Johns Hopkins University in Baltimore, USA, who was not involved in the study, told Science that the group 'may be overestimating the role of genes' because their assumptions were incorrect.

The authors, however, are 'definitely not saying that we have "found a gene for sexual offending" or anything of the kind', said Professor Seena Fazel from the University of Oxford. Instead, they suggest a combination of genes relating to impulse control, sex drive and intelligence are likely to be involved. They are cautious about this research leading to the ostracising of offenders' families, said Professor Långström, who made clear that 'this does not imply that sons or brothers of sex offenders inevitably become offenders too.'

The authors argue that this new information could help better predict those at high risk of offending. It could then be possible for social services to intervene and 'target education and preventative therapies where they could do the most good', said Professor Fazel.