Married people vs divorced probably have the same distribution of mental illness. I think it has to due more with two incomes, two parents attention/time/insights/wisdom to benefit from. Maybe mom is good at math and dad is good at writing or history. It seems like more is better when it comes to raising a kid--hence the phrase 'it takes a village'. I bet if you did a study that also counted how many other extended family members are involved in the kid's life that would also show the same correlation.