[notahacker]

Yeah. Certainly if the US regular pre-school system looks anything like the UK one, the difference between non-Montessori and Montessori pre-schools in the sorts of play actually encouraged probably isn't that big. The authors attempt to control for notable differences in the demographics of the treatment group, but they're there (and in this case, the higher incomes of the parents in the treatment group probably not just a wealth effect, but a correlate of other systematic differences with the parents who didn't...)