Analyses of Sex Ratios among Residents of the Khumbu of Nepal Support the Trivers-Willard Hypothesis

Abstract

The Trivers-Willard hypothesis predicts a sex ratio bias contingent on maternal condition in species characterized by variation in male reproductive success. A male-biased sex ratio among mothers in good condition, and a female-biased sex ratio among mothers in poor condition is expected. Studies in humans have thus far provided mixed answers to the question of whether or not sex ratio is affected by maternal condition. The present study assessed whether or not the introduction of a western cash economy influenced the observed secondary sex ratio in Nepal's Khumbu region. Because acculturated villages provided better access to the cash economy and to health facilities, residence in an acculturated village was used as a proxy for "good" maternal condition. I analyzed demographic data gathered by survey in 1971 and 1982. The sample included 734 children from the 1971 survey and 1598 children from the 1982 survey. Using Poisson regression I analyzed the extent to which the sex ratios in age-stratified groups differed between the acculturated and unacculturated villages. In the 1971 dataset, the younger women in the acculturated villages displayed a significantly higher (p=.014) proportion of male offspring. It is likely that older women were subjected to minimal acculturation effects during their child-bearing years and among these data there was a lack of significant deviation between acculturated and unacculturated post-menopausal women. The rapid overall increase in acculturation between 1971 and 1982 likely made conditions in the two sets of villages much more similar by 1982. The results of this study underscore the impact that the transition to a market economy had on women in Nepal's Khumbu region.