Inequality of Opportunity in Japan: A behavioral genetic approach

This study examined the extent to which the inequality of opportunity exists in Japan, using a classical twin design. Participants were 1,006 working, male twins (738 identical and 268 fraternal twins; age 20–60) recruited via a web-based survey. Participants responded to the questions of academic achievement at the ninth grade, years of education, and annual income. Univariate genetic analyses revealed that individual differences in each of the three variables are substantially influenced by the following factors: genetic (27%–35%), family environment (34%–47%), and individual-specific environment (26%–30%). Gene-environment interaction analyses revealed that genetic and environmental etiologies of education and income differ across age/cohort; for education, genetic influences are weaker and family environmental influences are stronger in the older age/cohort, whereas for income, the former is stronger and the latter is weaker for the older age/cohort. Finally, multivariate genetic analyses revealed that family environmental factors unrelated with academic achievement exert strong influences on income, in part, via education. These results suggest that inequality of opportunity exists among Japanese males even when genetic influences are controlled for. This paper discusses the need for future research on the inequality of opportunity, using a genetically informative design.