Which comes first?

I may have fallen into my own trap of confusing cause and effect - the so-called post hoc ergo propter hoc fallacy.  I posted back in 2008 about the fact that married couples seem to prosper better than couples who simply co-habit, and suggested that this was sufficient reason for the government to encourage couples to marry through tax breaks and such.  But now listening to what someone was saying the other day, it may not be as simple as that - the suggestion is that it is not being married that makes the difference so much as the characteristics of those people who tend to get married.  Such people, goes the argument, are more likely to be self-assured, more likely to have a clear vision of the direction they see their life taking, and more likely to be resilient in the face of difficulties, and these are all traits which will benefit them in life generally, so any success they have may come from the sort of people they are rather than simply the fact that they are married. Interesting.