Birth interval is a major determinant of rates of fertility, and is also a measure of parental investment in a child. In this paper the length of the birth interval in a traditional African population is analysed by sex of children. Birth intervals after the birth of a boy were significantly longer than after the birth of a girl, indicating higher parental investment in boys. However, in women of high parity, this differential disappeared. Birth intervals for women with no son were shorter than for those with at least one son. All these results are compatible with an evolutionary analysis of reproductive decision-making. First born sons have particularly high reproductive success, daughters have average reproductive success and late born sons have low reproductive success. The birth interval follows a similar trend, suggesting that longer birth intervals represent higher maternal investment in children of high reproductive potential.
PIP: This study examines variation in birth intervals among a nomadic population from northern Kenya. Data are obtained from a survey conducted in 1993 of about 850 Gabbra pastoralist families. The total fertility rate was 5.63 children per woman in this largely non-polygynous society. The mean length of birth interval of ever married Gabbra women of all ages ranged from about 2.9 years to 3.2 years. The analysis excluded all intervals over 5 years and included all women who had at least 2 births. The mean length of the birth interval of 3170 birth intervals was shorter for female followed by male births and two female births. The statistically significant difference was about 2 months. The birth intervals after boys were longer, but the sex of the child closing the birth interval had no significant effect. Hazard model analysis of only married women under 49 years old indicates that a male child was followed by a longer birth interval for all of the first five birth intervals, but it was significant only for the second birth interval. A Kaplan-Meier plot of all birth intervals after the first one for women with children of both sexes, daughters only, and sons only reveals that women with children of both sexes had the longest birth intervals. Women with only daughters had the shortest intervals. Women with only sons had shorter birth intervals than women with at least 1 son and 1 daughter. All differences were significant. A hazards analysis that included age of the mother finds that the total number of sons, but not parity, had a highly significant effect on the birth interval length. Findings suggest that the determination of birth interval is not simply the effect of maternal age; that parents make an active decision about the length of the birth interval; and that maternal investment in male infants, except at higher parities, was greater than in female infants as an investment strategy favoring children with the highest reproductive potential.