Too many people, women, men? The psychological effects of population density and sex ratio.

Has your environment become more crowded over time? Do you find yourself surrounded by mostly men or mostly women? Here, we review recent work on the psychological effects of two key ecological dimensions: population density-the number of people in a given space-and sex ratio-the relative proportion of men to women in a group. Higher population densities are associated with a future-oriented psychology, increased educational investment, and a focus on 'quality over quantity' in family size and relationship preferences. Unequal sex ratios are associated with increased competition and risky behaviors amongst individuals of the more prevalent sex, and a general shift toward the relationship preferences of the scarcer sex.