In the preceding chapters we discussed the process of learning to read. As we pointed out there, despite some variation in teaching methods and cultural approaches to teaching reading, most children do learn to read quite well. However, this process is a struggle for many children, and a number of these do not learn to read very well and may even reach adulthood without being able to read profi ciently. Part of the problem for some children with reading diffi culties may be tied to general cognitive aptitude (or IQ). Low verbal IQ does tend to be associated with defi cits in comprehension and vocabulary. But the biggest hurdle that most children with reading diffi culties must overcome involves word decoding. Most of the children who have diffi culty learning to decode words experience problems with reading speed and/or comprehension, despite scoring in the average range or better on intelligence tests. To complicate matters further, the correlation between IQ and reading diffi culties tends to appear later in elementary school, which suggests that years of poor reading can suppress performance on aptitude tests (Hoskyn & Swanson, 2000; Siegel, 1992; Stanovich, 1986; but see B.A. Shaywitz et al., 1995). Thus the causal relationship between IQ scores and reading diffi culties may be bidirectional, since reading becomes a main avenue for acquiring new vocabulary and general knowledge around third grade (Chall, 1967, 1983).