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One direct test of the hypothesis that parents have proclivities to invest in children according to their reproductive value is offered by a study of twins, of whom one in each pair was healthier. Evolutionary psychologist Janet Mann conducted a study of 14 infants: seven twin pairs, all of whom were born prematurely. When the infants were 4 months old, Mann made detailed behavioral observations of the interactions between the mothers and their infants (Mann, 1992). The interactions were observed when the fathers were not present and when both twins were awake. Among the behavioral recordings were assessments of positive maternal behavior, which included kissing, holding, soothing, talking to, playing with, and gazing at the infant. Independently, the health status of each infant was assessed at birth, at discharge from the hospital, at 4 months of age, and at 8 months of age. The health status examinations included medical, neurological, physical, cognitive, and developmental assessments. Mann then tested the healthy baby hypothesis: that the health status of the child would affect the degree of positive maternal behavior. When the infants were 4 months old, roughly half the mothers directed more positive maternal behavior toward the healthier infants; the other half showed no preference. By the time the infants were 8 months old, however, every single one of the mothers directed more positive maternal behavior toward the healthier infant, with no reversals. In sum, the results of this twin study support the healthy baby hypothesis. Another study found that the level of investment mothers devote based on the health status of the child depends on her own level of resources (Beaulieu & Bugental, 2008). Mothers lacking resources followed the predictable pattern—they invested less in high-risk (prematurely born) infants and invested more in low-risk (not prematurely born) infants. In contrast, mothers who have a lot of resources actually invest more in high-risk than in low-risk infants. The authors propose that if parents have abundant resources, then they can afford to give abundant resources to the needier child while still having enough resources in reserve to provide for their other children.
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David M. Buss (Evolutionary Psychology: The New Science of the Mind)