Caused and effect has been replicated in a longitudinal study. 

The study variables the were controled doesnt have to be exactly the same: two black parents and one at home, and from the same neighborhood and age, poverty, similar results with perhaps different metrics of success. 

For example the original study measured recent monthly income instead of annual income. Since many of these families would have difficulty with annual income records, this was the most accurate method. Sampling more than once a year yielded a statistically significant result since the differences between the groups were so large.

Another thing is unforeseen events which affected many in both groups iirc like a plant closure.