Of eyes and hands: Assessing the evolutionarily stable strategy of human handedness by means of eye-tracking.

An evolutionarily stable strategy (ESS) suggests that the 1:9 ratio of left- to right-handers in humans could reflect a frequency-dependent equilibrium: the minority of left-handers and the majority of right-handers may have an advantage in competitive and cooperative circumstances, respectively. To evaluate this account, we recorded eye movements during a 'friend or foe' variation of a Go/NoGo task in 24 left- and 24 right-handed participants. Stimuli were silhouettes carrying either a food item (cooperation with a 'friend') or a weapon (competition against an 'enemy') in one hand, and participants were asked to save the friend or to hit the enemy. A better performance emerged in competitive than cooperative trials, confirming the 'weapon effect' (highest efficiency in detecting threatening stimuli), and for right- than left-handed stimuli, confirming an advantage in responding to the most frequent handedness phenotype. Left/right-handed stimuli were better categorized in the competition/cooperation trials and first saccades were faster towards left- than right-handed stimuli, particularly when these were males. We interpret this pattern as an attentional bias favouring earlier orienting to potentially more threatening targets, left-handed males. Results are discussed as partially supporting the ESS and other evolutionary biases, which might be associated with the handedness distribution of humans.