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.