How much do the decisions of parents matter? Most parents believe that even the most mundane acts of parenting—from their choice of day care to their policy on videogames—can profoundly influence the success of their children. Kids are like wet clay, in this view, and we are the sculptors.

Yet in tests measuring many traits, from intelligence to self-control, the power of the home environment pales in comparison to the power of genes and peer groups. We may think we're sculptors,...