Within the fields of social stratification, the sociology of education, child development, and economics, there is clear evidence that poverty is negatively correlated with cognitive skills, test scores, and other measures of intellectual ability and academic achievement. However, researchers have not unpacked the full explanation for this (Guo and Harris 2000). Some argue that income alone provides the financial capital necessary for children to meet their growing needs (Smith et al. 1997). Others argue that higher incomes allow parents to provide a greater investment in children's cognitive growth, by doing things such as reading to their children and involving them in educationally enriched activities (Guo and Harris 2000; Mayer 1997).
One explanation for the class based gap in child cognitive skills that has not been fully explored is breastfeeding. Breastfeeding has the potential to explain at least part of these gaps considering the large body of literature claiming that breastfed babies score higher on IQ tests (Anderson et al. 1999; Kramer et al. 2008; Oddy et al. 2003), and that upper and middle class mothers are more likely to breastfeed (Dubois and Girard 2003). Additionally, public health officials have been putting increasing emphasis on rising rates of breastfeeding, particularly because of its purported ability to reduce health inequalities (Rippeyoung 2009; Wolf 2010).
To address this gap in the literature, I examined 3,521 Canadian children using the National Longitudinal Survey of Children and Youth (Cycles 6-8) to assess the relative mediating impacts of breastfeeding and home environment on poverty gaps in child IQ, as measured by the standardized scores of the Peabody Picture Vocabulary Test (PPVT) when they were four to five years old. Understanding the causes of these test score gaps is important because cognitive skills are related to a number of other measures of well-being. According to Haskins and Rouse (2005:2), "children who score poorly on tests of intellectual skills during the preschool years do less well in elementary and high school and are more likely to become teen parents, engage in criminal activities, suffer from unemployment, and become clinically depressed as adults." Thus, by uncovering ways in which these early disparities in academic achievement can be reduced, we can better address a number of interrelated social problems throughout the life course.
Review of the Literature
Income and Cognitive Skills Gaps
Income has been clearly shown to have a positive impact on test scores, both when measured in terms of poverty (e.g., poor or not poor) or in terms of income (e.g., on a scale) (Guo and Harris 2000; Janus and Duku 2007; Phipps and Curtis 2000). For instance, Phipps and Curtis (2000) found that depending on how poverty is measured, poor children score between 3.154-4.586 points lower than children who are not poor on the PPVT using the first cycle of the NLSCY.
Smith et al. (1997) examined the role of income and poverty on a number of achievement and intelligence tests in the United States. Using both the Infant Health and Development Program (IHDP) data on low birth weight 5 year olds and the Children of the National Longitudinal Survey of Youth (C-NLSY) in the United States, they found that when looking at mean differences on a variety of intelligence, achievement, and verbal ability tests (the Bayley Scales, the Stanford-Binet IQ test, the Peabody Picture Vocabulary Test-Revised [used in this analysis], Peabody Individual Achievement Test, and the Wechsler Preschool and Primary Scale of Intelligence), poverty (measured by income to needs and the duration of poverty) has a consistently negative effect on children's cognitive achievement. They also found that the 2-7 year olds who were "very poor" scored 7-12 points lower than "near-poor" children. They interpret this to mean that increasing incomes to near-poor levels among the very poor could have a dramatic impact on children's cognitive scores. …