Research focusing on twins is categorized into research "about" twins and research "by" twins. This paper includes both types of research using the same data set. In study 1, I demonstrated that the development of vocabulary size was about 1 3 months delayed in twins, as compared to that in singletons at 18 and 24 months of age. On the other hand, the pattern of vocabulary development in twins, examined in terms of content, was similar to that in singletons. In study 2, I showed that the individual differences in vocabulary size at 18 months of age were explained primarily by a shared environmental factor, whereas those at 24 months of age were explained by both a genetic and a shared environmental factor. In addition, the longitudinal analysis showed that a new genetic factor and a new shared environmental factor appeared to affect individual differences in vocabulary size during 18 and 24 months of age, whereas the other shared environmental factor seemed to exert a continuous influence during both periods. The integration of findings from research "about" and "by" twins was also discussed.

特集 : 教育学の射程
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