Sex ratios at birth in a Micronesian Atoll population
Brewis, A.A.
Social Biology 40(3-4): 207-214
1993
ISSN/ISBN: 0037-766X PMID: 8178189 DOI: 10.1080/19485565.1993.9988847Document Number: 334779
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.