Is it that marriage itself is beneficial or that people capable of maintaining a relationship make better parents? Divorce is significantly more common for bipolar, paranoid, and schizophrenic people for example. So I think we need to look at widows or something to control for those kinds of issues.
is that really a significant factor? i'd think that the number of people suffering from mental issues is a minority, and even if in their group are more divorces, those should only be a fraction of the total number of divorces.