How much of this gap is explained by the fact that low-income earners have to work harder and thus have both less time to interact with their children, and less money to hire other humans to interact with them?

Consider a wealthy "Leave it to Beaver" one-income family: dad works, mom stays at home and drags the tots to an endless series of carefully selected enrichment activites, maybe drops them off at a Montessori day care a few days a week.

While at the other end of the social scale, single mom works two jobs to make ends meet and is barely ever home, so kids are left with a relative who's taking care of a whole brood of children, or plunked down in front of the TV and told to keep quiet.