Sex ratios at birth in a Micronesian Atoll population

Abstract

The recent suggestion that typically masculinized sex ratios at birth in Micronesian populations may be related to a distinct "Micronesian pattern" of life-course coital behavior is applied to data on the sex ratio of livebirths on Butaritari Atoll in Kiribati. The data show that sex ratios on Butaritari are highly masculinized and do not vary significantly with changes in maternal age. However, there is a discernible relationship between the length of closed intervals preceding male and female births. The lack of age-related change in sex ratios in the Butaritari sample is inconsistent with ethnographic data regarding levels of marital coital activity in relation to increasing age and marriage duration. The Butaritari sex ratio data is argued to support the suggestion of a "Micronesian pattern," although it is posed that further tests of this association are required.

PIP: This paper tests Underwood's proposition that there is a distinctive Micronesian pattern of continuing high levels of sexual activity regardless of age or marital duration. Data were obtained on secondary sex ratios (SSR) at birth by parental age, birth order, and birth interval from 201 Butaritari women in Kiribati in 1990; there were 631 births of known sex. The SSRs were considered reliable and in line with other SSR data in the Micronesia-Polynesia region. Reproductive histories of this population revealed a sex ratio at birth of 111.0. The SSR for women aged 30 years was 117.7; and for women aged 30 years, 107.8. Differences in SSR were not statistically significant for either paternal or maternal age differences. The SSR for fathers aged 30 years was 112.0 and for fathers aged 30 years was 104.8. The pattern of declining SSR with increased parity was not found: the highest SSRs were among middle order births. As preceding birth interval increased, there was a trend toward increasing femininity. There was a significant difference between mean birth intervals of 28.04 months and 31.48 months for male births preceding female births. Significant differences were also found between birth intervals of 2 years and 2 or more years. The evidence of masculinized sex ratios and slight increases with age led to the inference that coital activity is being maintained with advancing maternal age and with increasing marital duration. The significant difference in SSRs with increasing birth intervals and more female births supports the proposition that more frequent acts of coitus are related to earlier conceptions and shorter birth intervals, lower sex ratios are related to longer birth intervals. The frequency of coital activity with age was also evidenced in studies of the Chamorros of Guam by Underwood. There was also the suggestion that a pronounced masculine sex ratio might be the norm for Polynesian and Micronesian regions and not just atoll-based groups.