A 2008 study on a nationally representative sample of young adult males in the United States found an association between lifetime and past-year self-reported anabolic-androgenic steroid use and involvement in violent acts. Compared with individuals who did not use steroids, young adult males who used anabolic-androgenic steroids reported greater involvement in violent behaviors even after controlling for the effects of key demographic variables, previous violent behavior, and polydrug use.
A 1996 review examining the blind studies available at that time also found that these had demonstrated a link between aggression and steroid use, but pointed out that with estimates of over one million past or current steroid users in the United States at that time, an extremely small percentage of those using steroids appear to have experienced mental disturbance severe enough to result in clinical treatments or medical case reports.
A trial conducted in 2000 using testosterone cypionate at 600 mg/week found that treatment significantly increased manic scores on the YMRS, and aggressive responses on several scales. The drug response was highly variable, however: 84% of subjects exhibited minimal psychiatric effects, 12% became mildly hypomanic, and 4% (2 subjects) became markedly hypomanic. The mechanism of these variable reactions could not be explained by demographic, psychological, laboratory, or physiological measures.
A 2006 study of two pairs of identical twins, in which one twin used anabolic steroids and the other did not, found that in both cases the steroid-using twin exhibited high levels of aggressiveness, hostility, anxiety and paranoid ideation not found in the "control" twin. A small scale study of 10 AAS users found that cluster B personality disorders were confounding factors for aggression.