Sex differences in the relationship between aggressiveness and the strength of handedness in humans

Abstract

Cerebral lateralisation, the partitioning of cognitive functioning into one hemisphere of the brain, was once considered unique to humans; however, recent research in a variety of taxa suggests that lateralisation is an evolutionarily ancient adaptation. Handedness is the most obvious manifestation of cerebral lateralisation in humans. Much of the literature on handedness has focused on the direction, rather than the strength, of this lateralisation. From both genetic and evolutionary perspectives it may be more informative to study degrees of cerebral lateralisation rather than direction. Strong evidence suggests that the strength may be more closely associated with individual differences in behaviour in humans than the direction, and individual variation in the degree of lateralisation has been found to correlate with personality-like characteristics such as aggressiveness in fish. The association between different patterns of lateralisation and personality characteristics may help explain how variation in the strength of lateralisation is evolutionarily stable in natural populations. The present study investigated the relationship between aggression and strength of handedness in humans. We found a significant interaction between sex and lateralisation with respect to aggression. In males, trait aggression was significantly higher in strong-handers than in mixed-handers, while no difference was seen in females. This finding highlights the importance of considering sex as a factor when investigating relationships between cerebral lateralisation and personality characteristics. Potential causes and consequences of the sex interaction as well as future directions for research are discussed.

Keywords:

• Cerebral lateralisation
• Handedness
• Aggression
• Sex differences
• Humans

Acknowledgements

Adam Reddon is now at McMaster University, Hamilton, ON, Canada. Thank you to Kathryn Vaillancourt and Aaron Wu for their assistance with data collection and entry. This research was supported by an NSERC (Canada) Discovery Grant to PLH.