The relationship between testosterone and aggression: a meta-analysis
Section snippets
Rationale for the relationship
Testosterone is an androgen that has been implicated in the development and maintenance of masculine characteristics in a variety of species (Mazur & Booth, 1998). It has been documented that the females of most species are less aggressive and have far lower testosterone levels than do males; this is taken as evidence of a link between testosterone and aggression (Archer, 1991). How exactly testosterone affects aggression and dominance behavior is unknown (Mazur, 1983), but multiple pathways
Evidence for the relationship in non-human animals
In non-human animals, the relationship between testosterone and aggression (where aggression is operationalized through observable aggressive behavior) has been demonstrated through correlational and experimental studies, involving manipulation of testosterone levels through castration and injection of testosterone. Results overwhelmingly indicate that testosterone and aggression are related (Turner, 1994).
Individual correlational studies demonstrate this relationship in a wide range of birds
Evidence for the relationship in humans
As mentioned previously, results from human studies are widely discrepant, although the relationship between testosterone and aggression in humans has been investigated a great deal (Archer, 1991). Some studies find a strong positive relationship, others a negative one, and still others find no effect whatsoever (Archer, 1991). This lack of consensus is sometimes evident even within studies. A good example is Turner (1994), where testosterone was positively correlated with norm-violating
Possible moderating variables
The research literature suggests many possible moderators of the relationship between testosterone and aggression, including metabolism, age, sex, circadian rhythm, stress, past experience, and social rank (Rubinow & Schmidt, 1996). Hormonal moderators, such as cortisol, have also been proposed Brain, 1983, Leshner, 1983. Although it would be useful to investigate the effects of these moderators on the relationship between testosterone and aggression, typically, insufficient data are available
Adaptationist reasoning behind the proposed relationship
It has been suggested that testosterone levels interact with environmental influences to produce behavior (Vom Saal, 1983) and that aggression is primarily an adaptive behavior in response to stimuli that could be construed as threatening to one's fitness, such as territory violations, and/or survival (Daly & Wilson, 1994). In support of these ideas, for example, the aggressive behavior of birds increases in frequency when resources are scarce (Harding, 1983). In the context of this research,
Predictions
First, it is predicted that the relationship between testosterone and aggression would be weak and positive based on the results from past meta-analyses and the expectation that the relationship is moderated by other variables (Archer, 1991). The rest of the predictions relate to the effects of those moderator variables. For example, it is predicted that the relationship between testosterone and aggression should decrease with increasing age. This is expected because of the range restriction in
Studies
One hundred and six articles were examined to determine the nature and strength of the relationship between testosterone and aggression. Articles were located utilizing a number of databases (PsycInfo, Biological Abstracts, General Science Index, and Current Contents), as well as through referrals from colleagues. Reference sections were also perused for additional studies to include. Aggression was broadly defined, involving competition, self-report measures of state and trait aggression
Effect sizes
The aggregate unweighted mean correlation across all the included studies was 0.230. When effect size was weighted by number of participants, the mean correlation was significant (Zc=13.730, p<0.01), but of smaller magnitude (r(53)=0.140). Table 3 gives additional descriptive information for the weighted and unweighted solutions. Out of 54 effect sizes, 45 were in a positive direction (Fig. 1). In other words, a positive relationship was observed in 83% of the cases.
Effects of moderator variables
Before an analysis of
Overall relationship
The finding of a weak, positive correlation between testosterone and aggression is consistent with past meta-analyses (Archer, 1991). However, it should be noted that only studies that reported a p-value or effect size were included in the analyses. A possible consequence is that this sample underestimates the proportion of non-significant findings in the population.
Moderator variables
Two variables were found to influence the size of the relationship between testosterone and aggression: (1) age of participant and
Acknowledgements
We wish to thank Grant Harris, Martin Lalumière, and Marnie Rice for their comments on an earlier version of this paper, Martin Daly, Lee Fabrigar, Kevin Parker, and John R. Reddon for their valuable advice, and Kusum Sharma for assisting with collection of the data. This research was supported by a contract from the Kingston Psychiatric Hospital and a Senior Research Fellowship from the Ontario Mental Health Foundation to the third author.
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