Shauna Miller goes through the archives:
The DoD has funded studies on the impact of gay servicemembers as far back as 1957, when the Navy's Crittenden Report found "no factual data" to support the idea that they posed a greater security risk than heterosexual personnel ... In 1988, the Defense Personnel Security Research Center -- a DoD agency -- conducted its own study on gay soldiers to determine whether their service under current policies created security risks, for instance in terms of blackmail. It also discussed, based on the military and wider social data available, whether the military's policies were sustainable. The study returned again and again to the facts of conduct: "Studies of homosexual veterans make clear that having a same gender or an opposite-gender orientation is unrelated to job performance in the same way as is being left or right-handed."