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Understanding Human Variation in Initiation of Breastfeeding: A Mixture Model Analysis

Establishment of lactation has important biological consequences for the health and development of the newborn. Substantial variation within cultures and among different cultures is seen in the onset of breastfeeding. We used a series of parametric mixture models to explore this variation and to uncover general human patterns for the initiation of breastfeeding. The model components reflect two hypothesized patterns of behavior. The first is a "natural" pattern of breastfeeding that has a rapid onset, and reflects, to some extent, a general mammalian behavior. The second behavior arises through culturally mediated delays in the initiation of breastfeeding. Candidate models were fit to observation from 25 previously published studies of breastfeeding behavior, resulting in interval- or right-censored observations of early breastfeeding behavior on 26,220 mother-infant pairs. Maximum likelihood estimation revealed that two-component models are clearly identified, and cultural and geographic covariates were found to have significant effects in the model, including the first component. Although there is clear evidence for two distinct behaviors associated with the initiation of breastfeeding, learned behaviors play an important role in mediating even the "early onset" behavior. We conclude that the increased reliance on learned rather than preprogrammed behaviors characterize the evolution of early maternal behaviors that must have accompanied the re-emergence of atricial infants over the course of human evolution.