The high incidence of reading disability in twin boys and its implications for genetic analyses

In 1975 the Australian Council of Educational Research (ACER) conducted a nationwide survey of literacy and numeracy in 10- to 14-year olds. A total of 297 of the 12875 children involved were twins. By age 14, only 42% of the twin boys achieved adequate standards of literacy compared with 71% of single-born boys. The deficit in twin girls was much less and twins of both sexes were only moderately behind in numeracy. A survey of 9-13-year-old twin boys in the La Trobe Twin Study (LTS) produced similar results with 75% being below average in reading skills and 23% behind by 18 months or more, despite above average IQs. The ACER data are corroborated by teachers' reports obtained in the same survey, which indicate also how few of the twins with problems are receiving remediation and the high incidence of classroom problems in spelling and reading reversals. The pattern of mistakes twins make on specific items in the ACER survey can be explained as resulting either from specific cognitive deficits or from problems in concentration. The same factors influence performance on different tasks, so that literacy and numeracy are much more closely interrelated in twins than in singletons, and also correlate more with a measure of verbal intelligence. Implications for genetic analysis of scholastic achievement are examined, centering around the different factor structure of abilities in twins and common family environmental effects which are unique to twins.