Each of Archer's meta-analyses included only five to six studies. The current analyses were based on 45 independent studies (n=9,760) with 54 independent effect sizes. Only studies that reported a p-value or effect size were included in the analyses; the sample may have underestimated the proportion of non-significant findings in the population. Correlations ranged from -0.28 to 0.71. The mean weighted correlation (r=0.14) corroborated Archer's finding of a weak positive relationship between testosterone and aggression. Two variables were found to influence the size of the relationship between testosterone and aggression: age of participant and time of day that the testosterone was measured. The effect size was largest in the 13- to 20-year-old males, declining with age. The time of day that the testosterone was measured was found to be a significant moderator in the relationship between testosterone and aggression. This is not surprising, given the diurnal variation in the reliability of testosterone levels. What is surprising is that so many researchers have measured testosterone in the morning, when the testosterone levels of individuals are most variable. On any given trial, then, the level of testosterone that is observed could change markedly. This would tend to attenuate the magnitude of the observed relationship. For males, the observed relationship between testosterone and aggression is highest in the afternoon, even though the mean level of testosterone is lower. This could be due to the increased reliability of the testosterone measurement. Contrary to the prediction, the nature of the measure of aggression (behavioral or self-report) did not have any effect on the relationship between testosterone and aggression. Future research might address the evolutionary hypotheses concerning inter-male competition by concentrating on inter-male aggression in competitive situations, especially in the presence of females. 4 tables, 3 figures, and 83 references