This is old news I thought. Haven't there been studies concluding pre-marriage co-habitation correlates with increased divorce rates for at least ten years?
Yep...
Indeed there have, although it's very difficult to determine cause and effect from such studies.
If two people live together for years before getting married, and basically act the same after they got married as they did before, I could see why divorce would be likely. In many such situations, the marriage is largely an afterthought to the relationship--a sort of "oh why not". Cohabitation without marriage is an arrangement of convenience: "I'll love, honor, and cherish you as long as things don't get too rough". Getting married after a few years, if not accompanied by real changes in the relationship, could be closer to "Well it doesn't look as though things are going to get too rough" than to "I am going to be faithfully yours as long as we both shall live, especially when things get rough as they are almost certain to do".
It should hardly be surprising that an attitude of "Things don't seem too rough" would lead to divorce. On the other hand, I don't know any good way to predict how people will perform in hard situations; if they naturally occur and a couple survives, that's a good sign, but I wouldn't think it wise to create them artificially.
My dad used to say, "there's on free lunch", and "try it before you buy it has nothing to do with dating." I know he was right on the money.