What study or studies have been done demonstrating a causal link between their family structure and success?

What is their average household income?

What countries in Africa do most of them come from?

The grandparent post literally linked to a write up about a Harvard study showing that a two parent family is the biggest predictor for economic mobility. Other studies have shown that time spent studying (not race, not household wealth) is the biggest predictor of SAT scores and thereby university admission.