From my article on Steroid.com on "roid rage"
Increased aggressiveness is often claimed to occur with anabolic steroid use. Although it´s highly rare (less than 5%), significant psychiatric symptoms have been found in some steroid users, including aggression and increased violence, mania, and even psychosis. However, it must be noted that in the studies done without a control group, it can safely be assumed that naturally aggressive people simply just be more inclined to use steroids (type-A personalities, if you will). This would probably have an effect on possibly skewing the results. Certainly, if someone takes the risk to use steroids to improve their performance in a sport or their physique, they have certain aggressive traits. Can steroids enhance them? Possibly. Can steroids be to blame for anti-social, psychotic, "roid-rage" type behavior? Probably not. The evidence just isn´t there to support that.
In fact, a landmark study was performed which examined different doses of testosterone administration on men aged 20-50, who had a variety of experience with steroids from having used them previously to not at all prior to the study. A variety of psychological tests were performed at the outset of the study as well as at the end. What was found was that no participant in the study had become violent as a result of the testosterone injections they had been receiving, although some said they felt more aggressive. This clearly indicates that there is a high level of control over possible violent or aggressive behavior that can result from steroid use. The researchers also noted that in terms of the psychological tests performed, some subjects showed little or no response to testosterone, with regards to psychological measures, while others experienced significant changes. Thus, general temperment clearly plays a large role in how one responds psychologically to steroid administration. In addition, when this study was compared with others, similar results were found:
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Out of 109 cases studied, only 5 people exhibited Psychological (Manic or Hypomanic) effects. (*Archives of General Psychiatry, Volume 57, February 2000.)