MAOA Induced Differences in Environmental Conditions

genes or sets of genes could directly impact both the earnings of parents and the edu- cational attainment of their children. Taken together, these factors raise the possibility that what I have interpreted as an interaction between MAOA status and childhood income actually may be an interaction between MAOA status and other genes. Put differently, childhood income may be standing in for unobserved genetic traits, leading to spuriously nonzero interaction terms. While there is some evidence that the effects of different gene loci on complex phenotypes are largely additive in nature Hill et al. 2005, gene- gene interactions remain an important concern. However, the sibling fi xed- effects approach reduces the likelihood of major bias re- sulting from gene- gene interactions. This is due to another basic principle of genetics, known as the principal of independent assortment, which holds that the combining of parental alleles at any given gene locus is independent of reproductive cell formation at all other gene loci. If genes are inherited independently of one another, then because full biological siblings share the same parental gene profi les, if a given male sibling inherits positive MAOA status while their brother does not, this difference is orthogo- nal to any other genetic differences that may exist between the two. As a result, it is unlikely that the observed interaction between childhood income and MAOA status is merely an artifact of MAOA interacting with other genetic traits. It should be noted that while the principle of independent assortment is fundamen- tal to Mendelian genetics, there may be important exceptions, and this remains an active area of genetic research. In particular, it may be the case that genes located close together on the same chromosome are sometimes inherited as a group. Using the same data as this study, Fletcher and Lehrer 2009 conduct formal tests for link- ages between all of the available genetic markers and fi nd no evidence that the law of independent assortment is violated in this case. However, the data contains only a small number of genes and this evidence cannot be considered conclusive. If MAOA is indeed inherited with genes that jointly impact educational outcomes, then this paper’s core fi ndings could be partially attributable to gene- gene interactions. While the sibling fi xed- effects approach reduces the chances that the basic results are due to gene- gene interactions, this is not to say that such interactions do not ex- ist or that they do not affect educational attainment. Gene- gene interactions almost certainly do exist to some degree, and the single gene models presented here may mask substantial heterogeneity in the income- education association across children who have the same MAOA status but who differ with respect to other genetic charac- teristics. Investigating such interactions will require larger genetic samples than are currently available in the Add Health data, and are an important direction for future research. This potential heterogeneity notwithstanding, the robustness of my results to sibling fi xed- effects specifi cations reduces the possibility that the interaction studied here is driven by the omission of other genetic traits.

B. MAOA Induced Differences in Environmental Conditions

As noted, initial MAOA differences between full siblings are as good as randomly as- signed, and therefore cannot be the result of environmental circumstances. However, this does not preclude the possibility that MAOA status is correlated with the envi- ronmental conditions children experience after conception. In particular, it is possible that children’s MAOA status could induce differential treatment from their parents, teachers, or peers, giving rise to an association between MAOA status and environ- ment even within sibling pairs. 29 There is a substantial body of economic research on the issue of how parent’s investments in their children’s human capital might respond to child endowments, especially across siblings Behrman, Pollack, and Taubman 1982; McGarry 1999; Er- misch and Francesconi 2000. From a theoretical standpoint, it is equally plausible that parents make “compensating” investments in which more resources are allocated to the less able sibling to promote equality, and that they make “reinforcing” investments in which more resources are allocated to the more able sibling to maximize marginal returns. Empirically, the majority of evidence from the United States has found that parental investments are indeed associated with child endowments, and that compen- sating investments predominate. There is less existing work on how child endowments may infl uence treatment within the formal educational system or by their peers, but at least some degree of differential treatment is highly plausible. With respect to the current study, it is important to note that if children with posi- tive MAOA status are treated differently than those without positive MAOA status, then the effects of this differential treatment will be absorbed by the MAOA terms in the models estimated above, including the sibling fi xed- effects models. In other words, the estimated effect of MAOA status in these models includes both the effect of MAOA status itself, and the effect of any environmental conditions that were ex- perienced as a result of having MAOA status. This is not necessarily problematic, as treatment experienced as a direct result of MAOA status can reasonably be considered an effect of MAOA status. Still, it is important to note that the estimates above incor- porate both the direct and induced effects of MAOA status, and that there is no readily available method of disentangling the two.

C. MAOA Inhibitors