ARE LOW FEMALE WAIST-TO-HIP RATIOs A COSTLY SIGNAL? EXPLICATING THE COMMUNICATIVE FUNCTION OF FEMALE WAIST-TO-HIP RATIOs

Abstract

The male preference for low female waist-to-hip ratios (WHRs) has been documented across cultures and time. According to costly signaling theory, this male preference could not have evolved unless low female WHRs reliably communicate information about females that increase men’s reproductive success. Previous research suggests that, relative to high female WHRs, low female WHRs signal the availability of valuable reproductive fat and nubility (i.e., a woman is ready to reproduce but has not been pregnant yet). This line of work begs an important question: what has maintained the reliability of low female WHRs as a biological signal? As the first attempt to address this question, I hypothesize in this thesis that growing and maintaining low female WHRs suppress native immunity (indicated by, e.g., lower white blood cell count, or WBCC) such that women with lower WHRs will tend to be slower in responding to infections. To test this hypothesis, I analyzed two reasonably large, existing datasets respectively from China (CHNS) and the U.S. (NHANES III) and observed three major findings after controlling for fat deposition in other body parts (e.g., upper-arm) and the body-mass index (BMI). First, lower female WHRs significantly correlated with lower WBCC not in CHNS but in NHANES III. Second, smaller waists significantly correlated with lower WBCC in both CHNS and NHANES III. Third, and most notably, thicker thighs (but not hips) significantly correlated with lower WBCC in NHANES III (CHNS did not provide thigh measures). These findings largely support my immunity-suppression hypothesis and help launch a new phase of research on the communicative function of female WHRs.