able to disentangle genetic and environmental effects. But if the placement of the adoptive child is not random, then the break is not as clean and estimates will be biased. I believe, however, that using the data from the EICV that include information both about the type of relationship between the child and his new family and about the education of the biological parents can control for non-random placement and remove the bias associated with it.

The adoption strategy also, implicitly, assumes that adoption occurs just after birth, so that all the environmental effects can be attributed to the adoptive parents. In the current study, there is no way to know when the “adoption” occurred, but it can be assumed that in most cases adoption did not occur immediately after birth. Indeed, as suggested by the higher orphanhood rates for children born before 1994 in figure 1, many of the children might have lost their biological parents during the Genocide in 1994. HIV/AIDS orphans are also not very likely to have lost their parents immediately after birth. Although the duration is difficult to estimate for each child, it is likely that many children lived for some years with their biological parents before being placed in a new household. In that case, the environmental effect should be assigned both to the biological parents, for the first pre-adoption years, and to the “adoptive” parents for the subsequent years. In that situation, in a regression including both the schooling levels of biological and “adoptive” parents, the nature component, if represented by the coefficient for the biological parents, would be upwardly biased, while the nurture component, if represented by the coefficient for “adoptive” parents would be downwardly biased. Since most of the coefficients on the education of “adoptive” parents are nevertheless positive and significant, despite this potential downward bias, it actually reinforces the conclusion from this study that there is a nurture component in the intergenerational transmission of human capital.

The third objection to the internal validity of estimates derived from the adoption strategy is self-selection. If the same individuals who have a taste (or a discount factor) such that they like education for themselves are also the ones who are the most likely to provide a good learning environment for their children, then parental education is not causal in improving their children’s schooling outcomes. The adoption strategy does not remove this potential source of bias.

The first objection to the external validity of the results of this study is clear: the type of household structure, the average schooling levels of parents and children are very different in the African context than in developed countries where the adoption strategy has usually been implemented. One hypothesis that would reconcile the results of this study with the conclusions of Behrman and Rosenzweig (2002) and Plug (2004) that there is no environmental effect of mothers’ education would be that the mother’s education might matter during the first years at school, which are still the relevant margin for most children in Africa, while it might be less crucial for high school or college graduation, which are the margins of interest in developed countries.<sup>12</sup> Moreover, even if estimates from the adoption strategy are unbiased, they might be peculiar to the relationship between adoptive parents and

> 12 However, it is generally recognized that the first school years have a large impact on subsequent academic achievement.

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