The full set of regressors is presented in appendix table A2. Lagged cognition is a significant predictor of math across the different specifications (i), (ii), and (iii). In addition, all four lagged readiness outcomes are significantly associated with literacy at school age in specification (i).

# IV. CONCLUSIONS

There are wealth gradients in child development that start early, change over time, and accumulate with age. Data from a longitudinal cohort of children living in Madagascar are used to document that, even in the context of a very low income country where more than 2/3 of the population lives below the poverty line, wealth gradients are substantial. The gradients are robust to the inclusion of a large set of explanatory variables, from lagged outcomes and early conditions to maternal education and endowments to the inclusion of parenting inputs.

Our four main findings are as follows. First, the SES gradients observed at preschool age are shown to persist into school age across all developmental domains tested. Substantial gradients in school readiness outcomes by household wealth are evident when the children were 3–4 years old; these gradients widen with increasing child age and flatten out by ages 9–10 for all domains. Second, the magnitude and evolution of the wealth gradient are found to vary by domain, widening earlier for receptive vocabulary and sustained attention (between ages three and six) and later for the cognitive composite and memory of phrases (between ages 7–8). As reported previously, when the children were 3–6 years, vocabulary and sustained attention demonstrate the largest gradients when these same children are 7–10 years after controlling for lagged outcomes, maternal endowments, and home environment. Third, an estimated 12–18% of the predicted wealth gap in school readiness at ages 7–10 can be accounted for by differences in home stimulation at the same age, even when controlling for early skills and initial endowments. Finally, there are also significant SES gaps in learning outcomes (math and early literacy) when the children are 7–10 years old, mapping closely onto the gaps for our school readiness outcomes. As with the readiness measures, the significance of the wealth gradients in learning outcomes is robust to the inclusion of lagged outcomes and initial endowments.

Our results align with previous evidence that measures of executive function and language would be potentially more sensitive to the effects of household wealth or maternal education. The differential widening gradient may be due, in part, to the differential timing of development of various regions of the brain (Lenroot and Giedd 2006), although we do not have the data to understand these mechanisms.

The timing and widening of the SES gradients confirms the importance of investing during the early years, because of the brain’s greater plasticity (i.e., capacity to benefit from environmental interventions) and the timing of physiologic development (Shonkoff 2011). These developmental benefits of early intervention are supported by a strong economic argument for intervening as early as possible (Doyle et al. 2009). During the first five years of life, children’s language, early understanding of mathematics and reading, and self-control emerge. The extent to which children master these skills during this sensitive period has implications for success in school (Lerner 1998). There is also evidence that performance in early measures of executive function is associated with performance in later measures of academic skill, such as early math and reading ability (Blair and Razza 2007).

In addition to highlighting the importance of addressing poverty in early childhood, we find that SES continues to play a critical role into middle childhood. Our estimates from Madagascar 17