Twins tend to lag behind singletons in their language development, though the causes of this are as yet unknown. Language development of twins is typically evaluated using the stan dards for the singletons in Japan, and the results often suggest that twins suffer from poor de velopment. The present study was performed to resolve this situation. The subjects were 937 (maternal associations group) and 1092 (school applicants group) twin mothers and their twin children of known zygosity. Relative to general population norms, twins in the maternal associations group were about one to two months delayed in terms of age at first spoken word. Nevertheless, about 95% of twins have started speaking at least one word at 18 to 19 months, as sin gletons do. According to maternal reports from the school applicants group, monozygotic males, compared with singletons, showed the largest delay throughout childhood; this delay was partly attributed to the effect of 'twin language'. Language development was more similar to t at of monozygotic pairs than that of dizygotic pairs. In conclusion, the results of present study suggested that language development of twins partly differ from that of singletons and that estimation of language development specifically for twins are needed.
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