The book suggests two main reasons for this.

1. The reformation increased literacy/education in the populace to a greater extent in protestant areas, because you no longer needed clergy to talk to God, or understand the bible. Protestant countries have had better education for longer and it has a compounding effect.

2. The "Marriage and Family Program" (the "MFP")... protestant areas discouraged cousin marriage and levirate marriage much earlier than catholic countries, and it is still very common in the rest of the world. Consanguineous marriage is ludicrously prevalent in the middle east, it makes most of the rest of the world more tribal and you end up with compounding genetic defects. By making cousin marriage taboo, it encouraged children to move to a different town and made people less clannish.