It has long been known that early language development is mildly delayed in twins. However, despite accumulated research on the cause and the mechanism of the delay, two important issues remain unknown : whether there is any gender difference in the delay and the universality of the delay across languages. Further, item analysis in twins is not also performed. To assess these issues, we compared the expressive vocabulary sizes of Japanese-speaking twins and singletons at 18 months and 24 months by collecting a large amount of vocabulary checklist data, using the MacArthur-Bates Communicative Development Inventory, and performing analyses using a structural equation model and correspondence analysis. We did not observe a twin vocabulary delay at 18 months, but one was observed between 18 and 24 months, especially twin boys. Further, characteristics of words expressed by twins differed from those by singletons. This is the first evidence of a more detailed developmental pathway in twin vocabulary delay for Japanese-speaking twins. Our results also provide evidence that the vocabulary delay is larger for male twins than female twins

特集 : 教育学
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