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  • Title: Sex ratio at birth deviations in modern Venezuela: the Trivers-Willard effect.
    Author: Chacon-Puignau GC, Jaffe K.
    Journal: Soc Biol; 1996; 43(3-4):257-70. PubMed ID: 9204700.
    Abstract:
    This study evaluates the impact of the Trivers-Willard (T-W) effect on human populations, using demographic data collected from vital registration data in Venezuela. The evaluation of the sex ratio at birth (SRB) and of fetal and infant deaths supports the existence of T-W effect in the Venezuelan population in extreme conditions. This T-W effect was observable in the SRB but not at later ages and is related to the marital status of the mother. The results indicate that the investment in females associated with environmental adversity is greater than the investment in males associated with good environmental conditions. This study relies on stepwise discriminant analysis to evaluate the influence of socioeconomic status (SES) factors according to marital status on the probability of the child being born a male or a female. Data were obtained from the 1988 and 1990 Venezuelan death and birth registers in the Central Office of Statistics and Information. Fetal and infant deaths numbered 87,229. Births numbered 577,976 and were reported for a variety of demographic/socioeconomic variables. The study evaluates the Trivers and Willard (1973) hypothesis that the sex ratio at birth is correlated with socioeconomic status. The focus is on the deviations in the sex ratio at birth (SRB) in Venezuela. The authors refer to studies confirming sex biases in mortality and sex biases in allocating resources and care and refuting the Trivers-Willard (T-W) effect. Findings indicate that the SRB was 0.5124 and confirms other estimates. The SRB for married and cohabiting couples was 0.512 and 0.514, respectively. The SRB was lower for single women (0.508). Differences were all statistically significant. Findings suggest that the T-W effect may be stronger in women who do not live with a male partner. Sex ratio deviations varied by SES. Higher educational status was associated with a higher SRB. Extreme poverty was associated with lower SRB and had a stronger impact on SRB than high SES. The T-W effect appeared stronger prior to conception. The T-W effect varied by maternal marital status. Females were more advantaged when mothers were unmarried. The sex ratio of neonatal deaths was 0.562; that for infant deaths was 0.574. The sex ratios for mortality did not differ for any of the SES indicators. There were differences by type of births and gestation time. Single births and early gestational times had higher male mortality. Infant deaths among mothers aged 30-34 years showed a higher sex ratio.
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