Researchers at Queen Mary's School of Biological and Chemical Sciences and Karolinska Institutet in Stockholm, Sweden, say that overall, genetics accounted for around 35 percent of the differences between men in homosexual behavior and other individual-specific environmental factors accounted for around 64 percent.

Individual-specific environmental factors may include biological processes such as different hormone exposure in the womb -- not societal attitudes, family or parenting which are shared by twins, the researchers say.

"This study puts cold water on any concerns that we are looking for a single 'gay gene' or a single environmental variable which could be used to 'select out' homosexuality," study co-author Dr. Qazi Rahman of Queen Mary's School of Biological and Chemical Sciences said in a statement

The researchers tracked all adult twins ages 20 to 47 in Sweden -- 3,826 same-gender twin pairs, who were asked about opposite sex and same sex partners.

For women, genetics explained roughly 18 percent of the variation in same-sex behavior, non-shared environment roughly 64 percent and shared factors, such as family environment, explained 16 percent, the researchers say.

The study, published in Archives of Sexual Behavior, shows that genetic influences are important but modest, and that non-shared environmental factors, such as different hormone exposure in the womb, dominate.