Effects of extraversion on the McCollough Effect: a gender difference

We have previously reported reliable extraversion–introversion differences in the McCollough Effect (ME), with male extraverts reporting stronger effects than introverts and have demonstrated that ME strength within observers is affected by cholinergic agents in men and by menstrual cycle phase in women. We speculated that the ME is a sensitive indicator of central neurotransmitter activity and of more generalised changes in arousal. In the first of two experiments with extraverted and introverted women we supported the prediction that a specific effect, post-adaptation exercise, which raises cholinergic levels, significantly reduced ME strengths. However, we did not find the predicted extraversion effect. A second experiment tested the possibility that extraversion had been confounded by hormonal fluctuations by comparing ME strengths in groups of extraverted ambiverted and introverted women across five phases of the menstrual cycle and in similar groups of men. We found the predicted extraversion differences in men, but not in women, who showed a significant phase effect and concluded that the effect of extraversion on the ME may not be generalised from men to women.