Dr. Trayce Hansen, licensed clinical and forensic psychologist, compiled an overview of various comprehensive research studies performed regarding the genetic v. environmental debate surrounding homosexuality.
A closer look at the research
Twin study investigations of homosexuality were recently conducted in both Sweden and Finland. Such twin studies compare rates of homosexual behavior between different sibling groups who share varying degrees of genetic similarity (ie, identical twins versus non-identical twins). By comparing such rates, twin studies help sort out the extent to which homosexual behavior is genetic and/or environmental. For instance, if homosexuality is genetic, then in cases where one identical twin is homosexual the co-twin should be homosexual nearly 100 percent of the time because identical twins share 100 percent of their genes.
But that is not what these two large-scale Scandinavian studies found. Both studies revealed that when one identical twin was homosexual the other twin was homosexual only 10 percent or 11 percent of the time. Such findings indicate that homosexuality is not genetically determined.
A Danish research investigation studied two million adults living in Denmark, a country where same-sex marriage has been legal since 1989. This study uncovered a number of specific environmental factors that increase the probability an individual will seek a same-sex rather than an opposite-sex partner for marriage.
For Danish men, the environmental factors associated with higher rates of homosexual marriage include an urban birthplace and an absent or unknown father. Significantly, there was a linear relationship between degree of urbanization of birthplace and whether a man chose homosexual or heterosexual marriage as an adult. In other words, the more urban a man's birthplace, the more likely he was to marry a man, while the more rural a man's birthplace, the more likely he was to marry a woman.
For Danish women, the environmental factors related to increased likelihood of homosexual marriage include an urban birthplace, maternal death during adolescence, and mother-absence.
Finally, an American research study—the most comprehensive and representative survey of sexual behavior in America—reported its findings concerning homosexuality. The results of this study also support an environmental theory of homosexuality, not a genetic one. In particular, this survey identified specific types of environments that increase the likelihood of homosexual behavior. The authors describe these environments as "congenial" to the development of homosexuality.
For American men, the environmental factor most related to homosexual behavior was the degree of urbanization during the teenage years. Specifically, boys who lived in large urban centers between the ages of 14 and 16 were three to six times more likely to engage in homosexual behavior than were boys who lived in rural communities during those same ages. The authors offer the following possibility: "an environment that provides increased opportunities for and fewer negative sanctions against same-gender sexuality may both allow and even elicit expression of same-gender interest and sexual behavior." Note the word "elicit." These researchers believe that growing up in a more pro-homosexual region may evoke or draw out homosexual behavior in young men. The implication is that some homosexual men who were reared in urban centers would not have become homosexual if reared in non-urban centers. The authors explain, "the environment in which people grow up affects their sexuality in very basic ways."
This map of the CA Prop 8 results would seem to support the "urban influence" findings.
For American women, the environmental factor most associated with a homosexual or bisexual identity was a higher level of education. And though that was also true for men, the pattern for women was more dramatic. For instance, a woman with a college degree was nine times more likely to identify herself as non-heterosexual than a woman with only a high school diploma.
For more information about the college education influence on homosexual determination, click here.
To read the entire Trayce Hansen research compilation with reference and source citation, click here.