Call it the reading income gap: Children from low-income households average just 25 hours of shared reading time with their parents before starting school, compared with 1,000 to 1,700 hours for their counterparts from middle-income homes.

These oft-repeated numbers originate in a 1990 book by Marilyn Jager Adams titled, "Beginning to Read: Thinking And Learning About Print." Ms. Adams got the 25-hours estimate from a study of 24 children in 22 low-income families. For the middle-income figures, she extrapolated from the experience...