All good points; which is why I was careful to call it correlation instead of causation. For example, distance from the Canadian border is also well-known to co

All good points; which is why I was careful to call it correlation instead of causation. For example, distance from the Canadian border is also well-known to correlate with education outcomes (with larger distances correlating with worse outcomes). That said, by far the strongest correlation is with dual parents. I don't know if anyone has studied the details you ask for, but it does stand to reason that a single parent -- of either sex and regardless of cause -- has a much tougher life and therefore less time to spend one on one with their child.