Sex ratios at birth in Micronesia: reply to James.

In response to comments of William H. James on the authors' articles, it is stated that Micronesian populations are now and have been in the past characteristically highly masculinized at birth (i.e., secondary sex ratios [SSR] of over 108-110). It is fascinating to examine how this finding may reflect microevolutionary change in this island setting. Aspects of reproductive behavior in Micronesia were examined for clues to proximate/remote mechanisms that may promote this pattern of male versus female births. One of the most singular defining features of the reproductive pattern of these groups is high coital rates, maintained against age and marriage duration. It is argued that SSR patterns in Micronesia are a corollary of the unique biobehavioral reproductive features of these island groups, which includes high coital rates. Professor James' concerns relate to 2 considerations: 1) the statistical aspect of results from small scale populations, and 2) the veracity and reliability of results from ethnographic research. Methodologies employed by modern ethnographers entail extensive and intensive cross-checking, multiple verification, and repeated re-evaluation of information collected from a large number of independent sources. In the Butaritari case, conclusions are drawn from several very different but consistent lines of evidence that show coital rates are maintained with age and marriage duration in this group: ethnographic data showing the marriage relationship maintains a highly sexualized environment, demographic data showing no effective reduction in fertility through the first 2 decades of marriage. In the case of the sex ratios, almost all births in these populations over the last century have been covered, and sample sizes in the thousands are available. These are acceptable for use of chi-square and sufficient to make inferences about variations in sex ratios by age group as they relate to coital activity.