Among the identical twins, who share the same genetic makeup, 48 percent of the gay or bisexual subjects had a gay twin. Among the fraternal twins, who share no more genetic inheritance than ordinary sisters, the rate was 16 percent; and among the genetically unrelated adoptive sisters it was 6 percent.

His data confirm earlier studies indicating that homosexuality appears to run in families. [Dean Hamer] said the gay men in his study were much more likely than average to have gay brothers. In addition, he found that gay men were more likely to have gay uncles and cousins, but predominantly on the maternal side. "There does appear to be, in at least a fraction of the population, a transmissible, presumably genetic, factor," Hamer said.

At the Harvard meeting, [Richard Pillard] reviewed the landmark study he and [J. Michael Bailey] did among gay male twins. As in the lesbian study, the researchers advertised in the gay media for gay male subjects - a method that critics charge, and the authors agree, makes a truly random sample impossible. They produced a pool of 167 gay men who had identical or fraternal twins or adoptive brothers.