mother completed high school and whether she completed college or beyond but do not include family income the omitted maternal education category is less than high
school, while Panel B reports estimates from models that contain both family income and maternal education indicators. The results show that when maternal education is
entered without family income, its association with child educational success is stron- ger among children without positive MAOA status than among children with positive
MAOA status. For example, among those without positive MAOA status, having a mother who completed high school as opposed to dropping out of high school is as-
sociated with a 16.3 percentage point increases in the probability of attending college, and having a mother who completed college is associated with a 33.4 percentage point
increase in the probability of attending college. However, the analogous associations for children with positive MAOA status are considerably weaker, only 7.1 and 26.4
percentage points, respectively. When household income is added to these specifi ca- tions in Panel B of Table 6, the association between child educational outcomes and
both maternal education and household income are much stronger for children without positive MAOA status than those with positive MAOA status.
28
In short, Table 6 shows that the results from the baseline models that used house- hold income also apply to another reasonable measure of family background, maternal
education. Discerning precisely which aspects of socioeconomic background interact with MAOA status to determine educational outcomes will require additional work
using research designs that rely on well measured and plausibly exogenous varia- tion in family background measures, and unfortunately such designs are not possible
with the data used in this study. Given this, at present household income is probably best viewed as an imperfect index of general socioeconomic background, as opposed
to having a well- identifi ed interactive effect in its own right. The fact that maternal education displays interactive effects that are broadly similar to those of household
income reinforces this view.
V. Further Issues
A. Gene- Gene Interactions
A common concern in observational gene- environment interaction studies is that any documented statistical interactions between genetic and environmental factors could
actually refl ect interactions between multiple genes or so- called gene- gene interac- tions. This concern arises in the present context because educational attainment is
a complex phenotypic trait that undoubtedly involves hundreds if not thousands of genes, and it is quite likely that the effect of many of these genes depend to some
degree on the presence of others. Furthermore, it is entirely plausible that specifi c
28. A potential concern is that the models in Table 6 contain several control variables that are strongly correlated with parental education or are themselves plausible measures of socioeconomic background, in
particular school fi xed effects and racial group indicators. Table A2 of the web appendix shows results from complimentary models that contain only parent and child age indicators as controls, and the results closely
resemble those from Table 6. Also, Table A3 of the web appendix shows results from sibling fi xed- effects models similar to those in Table 5 but which use parental education indicators in place of family income, and
the results indicate that MAOA- parental education interactions are robust to the use of sibling fi xed- effects, as was the case for household income.
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
