Sex ratio is the number of men per 100 reproductive-age women within a specified mating pool. We generated and tested two hypotheses about the cross-cultural relationships between sex ratio and mate preferences using preference ratings of 18 characteristics provided by 9809 participants and corresponding sex ratio data secured from an international organization. The Classical Sex Ratio Mate Preference Shifts Hypothesis predicts that in imbalanced sex ratio societies, the more numerous sex will lower their standards, to facilitate acquisition of a partner of the less numerous sex. The Alternative Sex Ratio Mate Preference Shifts Hypothesis predicts that in lower sex ratio societies, men will lower their standards to secure more short-term matings, whereas women will raise their standards to avoid deception by men seeking short-term relationships. Results supported the Classical Sex Ratio Mate Preference Shifts Hypothesis for men, and the Alternative Sex Ratio Mate Preference Shifts Hypothesis for women. Discussion addresses limitations of the current research and highlights future directions for research on the relationships between sex ratio and mating psychology and behavior. Copyright © 2006 John Wiley & Sons, Ltd.