Girl or Boy: Interbirth Intervals and Offspring Sex among the Wichí
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Abstract
From an evolutionary perspective, the interbirth interval is a measure of parental investment in a child. Parents actively make decisions regarding the length of intervals between births, even in societies without access to contraception. This parental investment may vary according to the sex of the child. Sex preferences vary across societies according to the ecological constraints they face. Traditional subsistence-based societies exhibit fertility close to natural levels and have limited access to modern health care.
Birth intervals among Wichí mothers are investigated in order to estimate the time required by maternal effort for each offspring and to determine whether there are sex differences in the temporal regulation of births. Mothers spaced their births at mean intervals of 38.05 months. In 60.8% of cases, intervals were shorter than 36 months. The secondary sex ratio was biased toward females. However, intervals following the birth of a male child were significantly longer than those following the birth of a female child. No sex effect was detected for interval closure.
The Wichí reproductive strategy is directed toward ensuring greater maternal investment in male children, either because mothers consciously seek the survival of the scarcer sex, which precisely for that reason increases its reproductive value, or because the birth interval is biologically adjusted and mothers take longer to recover some level of physiological condition necessary for a subsequent pregnancy. These mechanisms are not mutually exclusive, and both may be operating at the same time.
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