"Academic Achievement of Twins and Singletons in Early Adulthood: Evidence from the Analysis of a 1980s Taiwanese Cohort", British Medical Journal (BMI), 2008, (with Meng-Ting Tsou, Meng-Wen Tsou and Ming-Ping Wu), (SCI)

While it is well established that young twins exhibit inferior cognitive performance when compared with their singleton peers, a recent Danish study suggests that by adolescence twins and singletons show similar academic performance. In this study we test this hypothesis by using Taiwanese data to compare the long-term academic achievement of twins and singletons. We find that for the cohort born in 1983-1985, twins performed significantly worse than singletons on the Taiwanese college entrance examination. After controlling for a number of possible confounding factors, we find that twins were 2.2% less likely to score highly enough to attend college. Furthermore, we find that twins are particularly susceptible to the persistent negative effects of low birth-weight. While low birth-weight singletons were 3.2% less likely to attend college than other singletons, low birth-weight twins were 8.5% less likely to attend college than other twins. By separating same-sex twins and different-sex twins, we created a crude proxy for identical and fraternal twins respectively. Analyzing these groups, we find some evidence that heredity may partially cause the positive relationship between low birth-weight and low academic achievement. Our study suggests that governments should discourage practices which lead to an increased probability of conceiving twins. For instance, governments might adopt policies which lessen the use of hormone therapy and in-vitro fertilization, both of which are known to lead to higher probabilities of twin conception. Women might be encouraged to have children earlier to mitigate the need for such procedures, and couples trying to conceive might be persuaded to wait longer before resorting to them.