Their sample is relatively narrow: 455 kindergartners and first-graders, all twins, who live in Ohio and western Pennsylvania, nearly all of them white and middle-class. The researchers divided the kids in two groups: those with mothers who have above-average reading skills and those whose mothers are average readers. For both groups, they controlled for socioeconomic status, meaning that their results can't be explained away by class differences among the kids. (Fathers are absent from this study, like many of its kind. The research was done only with mothers, because double interviews cost more and also, Martin says, because the mother is "usually the best recorder" of family events.)

Both groups of mothers were asked about how often their children are read to—and also how often they amuse themselves with books. Then the mothers were asked a separate set of questions about order at home, designed to get at what researchers call "executive function." A few sample responses: "It's a real zoo in our home," "The children have a regular bedtime routine," and "We are usually able to stay on top of things." A shout-out to all my endearingly, creatively messy friends (but not to my husband, who still shouldn't leave his shoes in the middle of the front hall): It's clear that by an "ordered home," Johnson and Martin do not mean a spotlessly neat and clean one.

Surprisingly, the amount of shared parent-child reading time did not matter, on average, for the reading skills of either group of kids. What mattered instead, for the kids of average-reader mothers, was how often a child amuses herself with books. What mattered for the kids of the high-reading moms was how orderly the family's home was.