text: For two years, these men studied the number of occurrences of homosexuality in both monozygotic, or identical twins (a set of twins coming from the same fertilized egg and thus having identical DNA), and dizygotic, or fraternal twins (a set of twins from too separate zygotes, causing them to have similar DNA, but not any more so than that of two ordinary siblings), in addition to adoptive brothers of gay males (meaning that all of these males would have been raised in the same environment).