I wish researchers would also look at the effect the availability of a good preschool has on the life of low-income parents.
The parent dropping off or picking up the child now sees a classroom full of books and toys. Usually a parent can also have a few words with the teacher during drop-off and pick-up. The parents are seeing what an enriched environment for children looks like. They are hearing how professionals deal with common childhood problems.
Parents are also now able to get, and hold, a stable job. They don't have to worry about their children being propped in front of a television all day with a caretaker who may be unreliable. Perhaps the parents can use the opportunity to go back to school. Perhaps seeing and talking with the preschool teachers helps parents see they can improve their own lives.
I understand that the preschool's main job is to help the children gain the strengths and abilities to live productive lives. But I bet the results spread out beyond just the individual children.