MONDAY, June 30 (HealthDay News) -- Genetics and random environmental factors each play a major role in determining both gay and heterosexual behavior, say British and Swedish researchers.

"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 -- the factors which influence sexual orientation are complex. And we are not simply talking about homosexuality here -- heterosexual behavior is also influenced by a mixture of genetic and environmental factors," study co-author Dr. Qazi Rahman, a leading scientist on human sexual orientation, said in a prepared statement.

Environmental factors are specific to an individual and may include biological processes such as different hormone exposure in the womb, the researchers noted.

The researchers studied more than 3,800 same-gender twin pairs, ages 20 to 47, in Sweden. The twins were asked about the total numbers of opposite sex and same sex partners they had ever had.

"Overall, genetics accounted for around 35 percent of the differences between men in homosexual behavior and other individual-specific environmental factors (that is, not societal attitudes, family or parenting which are shared be twins) accounted for around 64 percent. In other words, men become gay or straight because of different developmental pathways, not just one pathway," Rahman said.

Among the female twins, genetics explained about 18 percent of the variation in sexual orientation, non-shared environmental factors 64 percent, and family environment 16 percent.

It's important to note that heredity and shared environment had roughly the same influence in women, while shared environment had virtually no impact on men's sexual behavior, the researchers said.

The study, which was published in the journal Archives of Sexual Behavior, shows that while genetic factors are important, non-shared environmental factors are dominant in determining sexual orientation.

"This study is not without its limitations -- we used a behavioral measure of sexual orientation which might be okay to use for men (men's psychological orientation, sexual behavior, and sexual responses are highly related) but less so for women (who show a clearer separation between these elements of sexuality)," Rahman noted. "Despite this, our study provides the most unbiased estimates presented so far of genetic and non-genetic contributions to sexual orientation."

More information

The American Psychological Association has more about sexual orientation.