The results, published online this month in the Archives of Sexual Behavior, confirm earlier findings that identical twins are more concordant for same-sex behaviors than fraternal twins are but only modestly so: In men, genetic effects appeared to explain 34% to 39% of the differences between the two twin groups, whereas in women, genetics accounted for only about 18% to 19% of the difference--a finding consistent with other research showing that sexual orientation in women is not as rigidly determined as it is in men.