Introduction Several theories, including psychodynamic theories, sexual imprinting and early conditioning have been formulated to explain sexual development. Empirical data, however, remain insufficient for a thorough evaluation of these theories. Aim In this study, we test the hypothesis that a critical period exists for the acquisition of sexual preferences, as suggested by empirical findings in birds and mammals (sexual imprinting). Methods An Internet questionnaire was used. Main Outcome Measures We gather data from individuals with a sexual preference for pregnant and/or lactating women, under the hypothesis that pregnancy or lactation may become sexually attractive in adulthood following an exposure to pregnant or lactating women in infancy. Results We find that these preferences are more common in older siblings, i.e., in individuals who have been exposed to more maternal pregnancy and lactation. This result is independent of respondent and sibling sex. In addition, only maternal pregnancies and lactations experienced between 1.5 and 5 years of age are associated with the preferences. Conclusions We discuss our findings in relation to theories of sexual development and to earlier reports of birth order effects on sexual behavior. We suggest that this age range may constitute a sensitive period for the acquisition of sexual preferences.

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