The study is the largest in the world so far, and was performed in collaboration with the Queen Mary University of London. More than 7,600 Swedish twins (men and women) aged 20-47 years responded to a 2005 – 2006 survey of health, behaviour, and sexuality. Seven percent of the twins had ever had a same-sex sexual partner.

“The results show, that familial and public attitudes might be less important for our sexual behaviour than previously suggested”, says Associate Professor Niklas Långström, one of the involved researchers. “Instead, genetic factors and the individual’s unique biological and social environments play the biggest role. Studies like this are needed to improve our basic understanding of sexuality and to inform the public debate.”

The conclusions apply equally well to why people only have sex with persons of the opposite sex as to why we have sex with same-sex partners. However, the conclusions are more difficult to transfer to countries where non-heterosexual behaviour remains prohibited.

Overall, the environment shared by twins (including familial and societal attitudes) explained 0-17% of the choice of sexual partner, genetic factors 18-39% and the unique environment 61-66%. The individual’s unique environment includes, for example, circumstances during pregnancy and childbirth, physical and psychological trauma (e.g., accidents, violence, and disease), peer groups, and sexual experiences.

Publication: ‘Genetic and Environmental Effects on Same-sex Sexual Behaviour: A Population Study of Twins in Sweden’, Niklas Långström, Qazi Rahman, Eva Carlström, Paul Lichtenstein, Archives of Sexual Behaviour, 7 June 2008, doi 10.1007/s10508-008-9386-1.

For further information, please contact:

Associate Professor Niklas Långström
Centre for Violence Prevention, Karolinska Institutet
Tel: +46(0)8-517 70560 or +46(0)70-425 4133
E-mail: Niklas.Langstrom@ki.se
Web: www.cvp.se

Press Officer Katarina Sternudd
Tel: +46 (0)8-524 838 95 or +46 (0)70-2243895
E-mail: Katarina.sternudd@ki.se

Karolinska Institutet is one of the leading medical universities in Europe. Through research, education and information, Karolinska Institutet contributes to improving human health. Each year, the Nobel Assembly at Karolinska Institutet awards the Nobel Prize in Physiology or Medicine. For more information, visit ki.se