male homosexuality Concordance rates MZ twins 57% DZ twins Non -twin brothers 13% In early 1990S, Dean Hamer and colleagues at NIH embarked on genetic studies of male homosexuality. They phenotyped the individuals by asking them to answer a number of questions about their sexuality: self-identification, attraction, fantasy, and behavior > bimodal distribution of scoresmale homosexuality Non-twin brothers DZ twins MZ twins Concordance Rates In early 1990's, Dean Hamer and colleagues at NIH embarked on genetic studies of male homosexuality. They phenotyped the individuals by asking them to answer a number of questions about their sexuality: self-identification, attraction, fantasy, and behavior  bimodal distribution of scores. 57% 24% 13%