Identifying the mechanisms that compel prosocial and antisocial behaviors is essential when seeking to develop effective social policies. While selfishness is ever present, in many situations cooperation is the norm. Testosterone increases a variety of antisocial behaviors but can also induce cooperation and honesty. We tested whether testosterone influences taking money from others, a measure of proactive aggression. We administered synthetic testosterone to participants (N = 163) in a double-blind placebo-controlled study before they made decisions in the power-to-take (PTT) game in which participants can take resources controlled by another while the other person can destroy resources so they are not taken. Three measures of testosterone were obtained from blood samples before and after drug administration to assess parametric effects. The data showed that an average 62% increase in total testosterone had no effect on average taking or destroying decisions in the PTT game. But taking money from others increased linearly in participants with supraphysiologic levels of testosterone. At the same time, testosterone improved participants’ moods and those with positive affect destroyed fewer resources. Our results show that testosterone should be viewed as a nuanced modulator of social interactions rather than the cause of antisocial behaviors.