Men are more likely to commit a sex crime if their father has been convicted for a similar offence, says a new research that places the onus on genetic factors more than family environment.

The risk increases substantially with a brother who has been convicted, says the study of over 21,000 sexual offenders in Sweden during 37 years.

Genetic factors account for 40-50% of the risk, with the remainder coming from the environment, says the team from Sweden's Karolinska Institutet and Oxford University.

The overall risk is small and only 2.5% of brothers of sex offenders are likely to commit similar crimes themselves, the authors assure, while saying that the work does not imply any genetic inevitability that relatives of sex offenders will commit crimes themselves.

"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," said Niklas Langstrom, Professor of Psychiatric Epidemiology at Karolinska Institutet and the study's lead author.

The study has not isolated a gene for sexual offence but found evidence for genetic factors having a substantial influence.

The report is published in the International Journal of Epidemiology.

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 men from the general population with similar age and family relationships.

The results suggested a familial clustering of sex offenders, with about 2.5% of brothers or sons of convicted sex crime offenders themselves convicted for sex crimes.

Genetic and environmental factors

"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% and 58%, respectively," says Langstrom.

"Such factors could include emotional instability and aggression, pro-criminal thinking, deviant sexual preferences and preoccupation with sex."

Karolinska Institutet had published a work last year linking two genes to the history of violent behaviour and increased relative risk of committing crimes.

These were the "warrior gene", the monoamine oxidase A (MAOA) gene and a variant of the cadherin 13 (CDH13) gene.

But the study had also said that people with these genes do not necessarily commit crimes but merely had a higher relative risk of doing the same.

Genetics is in a nascent state today with much still not known about the role of genes. It is well known that a gene could be responsible for more than one trait or function.