What explains the gender gap in ideology, i.e. the observation that women tend to be more leftist than men? We provide new evidence showing that personality traits play a key role. Using a novel high-quality data set, we show that the mediating (i.e. indirect) effects of gender operating through personality traits by far dominate the direct effects of gender. They also dominate other potential differences between the sexes like income or education as explanatory factors. Our findings suggest that women tend to be more leftist than men mainly because they have different personalities, which, in turn, shape their expressed ideology. Taking such mediating effects of personality traits into account explains over three quarters of the observed gender gap in general ideological preferences.