Learning to read seems to enable the mind to work better at a categorical level

Timothy Bates comments on a Bryan Caplan post, The Hours and Academic Achievement.
Caplan notes:

Adults love controlling the way kids spend the hours of the day. What's the payoff for all their meddling? Hofferth and Sandberg's "How American Children Spend Their Time" (Journal of Marriage and the Family) provides some fascinating answers for kids ages 0-12.

After compiling the basic facts about kids' time use from the 1997 Child Development Supplement to the PSID, H&S regress measures of academic achievement on time use, controlling for child's age, gender, race, ethnicity, head of household's education and age, plus family structure, family employment, family income, and family size. All test scores have a mean of 100 and a standard deviation of 15, and all time use is expressed in hours per week.

[snip]

The big result is the lack of results. Controlling for family and child background, time in school and studying barely help - and television viewing barely hurts. Contrary to wishful assertions that exercising the body improves the mind, sports don't matter either. Out of nineteen activities, only two predict greater academic success across the board: reading and visiting.

The estimated effect of visiting is modest. Reading, however, is a huge deal. Ceteris paribus, 10 extra hours of reading per week raise letter-word comprehension by .5 SDs, and passage comprehension, applied problems, and calculations scores by .4 SDs. Despite obvious worries about reverse causation - smart kids enjoy books more - much of this is plausibly causal. After all, many smart kids don't read much, and H&S include a lot of solid control variables. And you really can learn a lot from books.

Timothy Bates responds in the comments.

You might be interested in two papers where we hypothesised that aquiring the ability to read (as opposed to reading books) would raise IQ.

In a large cohort, controlling childhood SES, reading (and math) were associated with big gains in adult social status, through pretty complex links, including later IQ.

Collaborating with the TEDS cohort of several thousand identical twins studied in multiple waves from age 3 to age 16, we tested if one of the twins got better on a test of reading, would they score higher on IQ in later years? The Design includes the initial (moderately strong) association of these two traits, and nests all analyses within families to avoid those confounds).

The answer was yes: If one of two genetically identical twins acquires better reading skills, these are realised not only as enduring gains in reading, but in changed (increased) IQ in later waves.

These were not explained by reading of books: Rather learning to read seems to enable the mind to work better at a categorical level.

So, people differ greatly in IQ for genetic reasons, and literacy makes people smarter than they would have been.

Links to the papers are here:

All this goes to my interest in whether recreational reading (as opposed to purposeful reading) has any correlation with life outcomes. We commonly associate enthusiastic reading with good life outcomes but fail to control for other confounding variables such as IQ, SES, parental education attainment, etc. In other words, recreational reading may simply be the product of underlying variables rather than an independent variable itself.

Bates' work doesn't directly address that question but obliquely seems to indicate that recreational reading volumes are not an independent variable and that it is IQ and purposefulness that are the drivers of life outcomes.