Educational attainment is an enduring predictor of adult health. Higher educational attainment is associated with decreased mortality 
, decreased disability 
, and decreased cardiovascular disease 
. Link and Phelan have posited 
the ability of those with higher education to take advantage of resources, attain fulfilling jobs, respond to new health information, and gain access to health care. However, the connection between higher levels of education and increased cognitive functioning is another possible pathway by which increased education can lead to better health 
Educational attainment was historically severely limited among African-Americans, particularly in the South. By 1831, it was illegal for slaves to be taught to read or write in all of the Southern states 
. Restrictions on education gradually lessened but remained during the Jim Crow era. During that period, schools were racially segregated, and the quality of black schools was markedly inferior to that of white schools 
Despite such legalized barriers to education, educational possibilities for African-Americans in the South increased markedly over the course of the 20th
century. This time-period effect engendered dissimilar educational opportunities, on average, for different age cohorts now alive 
. For example, African-Americans who are currently in their 80s experienced a different educational climate when they were school-aged than did those who are now in their 40s.
Because education is an important determinant of an adult's access to resources and health, it is hard to overstate the importance of this transformation in opportunities. Social policies such as desegregation and civil rights expansion are likely responsible for some of the increases in 20th century African Americans' educational attainment, but their influence has not been examined in detail.
Twin pairs are a potentially informative population in which to examine changes in educational attainment because each pair has the same parents and childhood socioeconomic status, two known determinants of educational attainment. Because genetic variance is likely constant over time, then an increase in phenotypic variance is likely attributable to increases in environmental variance. We therefore hypothesized that twins born later in the 20th century would be less similar to each other in educational attainment than those born earlier in the 20th century and that heritability estimates would therefore decrease over time. We hypothesized that the educational attainment of older twin pairs would be more highly correlated than that of younger twin pairs because of these changes in educational access in the South over time, potentially reflecting a “ceiling effect” associated with the Jim Crow laws and discrimination. The aim of this study was to test that hypothesis.
Twin studies have long been used to compare the intra-twin similarity of identical (monozygotic) twins to that of fraternal (dizygotic) twins, as a means of estimating genetic and environmental effects. While our focus here was not exclusively on estimating genetic effects, our study design allowed us to identify the relative levels to which genetic and environmental factors contributed to educational attainment in African-Americans across age cohorts. The extent to which genetic factors influenced educational attainment provided insight into genes that affect characteristics such as personality, rather than the actual ability to attain an education.