No personality differences between oral contraceptive users and naturally cycling women: Implications for research on sex hormones.

Oral contraceptives (OCs) are the most widely used contraception method, and there is increasing interest in their effects on cognition, affect, and brain structure and function. Since women are not randomly assigned to OC use or non-use, it is unclear if previously-reported differences between OC users and naturally cycling (NC) women are due to sex hormones or to personal characteristics associated with pill use. The aim of this study was to fill this knowledge gap by comparing OC users and NC women on the Big Five personality factors. Participants came from two independent, large samples. The first contained 148 OC users and 93 NC women, and the second contained 247 OC users and 148 NC women. Consistent across both samples, multivariate analyses of covariance (with the five personality factors as dependent variables and age as a covariate) revealed no significant differences between OC users and NC women in personality, with absolute values of univariate effect sizes averaging d = .09. The pattern of results did not change when reason for OC use was considered, and users of different types of OCs did not differ from each other in personality. This well-powered study did not find personality differences between OC users and NC women in two independent samples, suggesting that previously-reported differences in cognition, affect, and the brain may be linked to neuroendocrinology.