Progressive folk in the second half of the twentieth century thought that cohabitation would improve the prospects for marriage, and eventually significant segments of the public have come to agree (e.g., in 1984, 77% of Canadians were accepting of cohabitation for those couples that "want to make sure that their future marriage will last"). Practice makes perfect, after all, and through cohabitation a couple might find that they are not "a good fit", so such cohabitation, they thought, would be a good thing for marital quality and duration. (Of course these arguments have defeaters: practice at cohabitation is not the same thing as practice at marriage, and close proximity might blind one to whether someone is a good fit.)
Then in 1988, Booth and Johnson published a study showing a strong correlation between premarital cohabitation and divorce--over the three-year course of their study 5% of those who had not cohabited divorced while 9% of those who had cohabited divorced. Moreover, while controlling for demographic factors decreased the disparity, it did not remove the disparity. Since then, a stream of sociological publications confirmed the correlation between cohabitation and subsequent divorce, as well as between cohabitation and poorer marital interaction style (lest one think that this is all an artifact of the fact that duration is a poor measure of marital quality).
There is no controversy that correlation is a fact. But correlation does not show causation. Although there are some studies that suggest causation (e.g., apparently the strength of belief in the permanence of marriage decreases with the length of cohabitation), and although several mechanisms have been proposed (e.g., the mechanism of drifting into a marriage with people whom one would not have married had one not cohabitated with them), we are not in a position to say that cohabitation causes poorer marital interaction or divorce.
However, we are in a position to see that there is no reason to believe the view that cohabitation in general improves marital prospects. That is just not what the data show.
In my Philosophy of Love and Sex class at Georgetown, I would have my students read some of the sociological papers on the correlation between cohabitation and marital problems. Now, here is something interesting. Even after we have read and discussed all of that research, I would still hear a student saying that they couldn't marry someone they hadn't lived with, because it would be too risky.
Now, I could understand not being convinced by the case for cohabitation causing these problems (I am not completely convinced myself). But that one would continue to think that cohabitation helps with marriage after having seen the data is rather disappointing.
I think one thing this shows is just how resilient deeply ingrained social beliefs, especially ones supported by plausible-seeming arguments (the practice and test-for-fit arguments), are in even quite intelligent people (my students were generally very smart). No surprise there.
Another potential mechanism could be a dismissal of the idea that statistical data on behavioral patterns has a bearing on one's own decisions. We have free will, after all, so we might think that the fact that the statistics do or do not show something about patterns of behavior is irrelevant--we can, with our own free will, choose to be exceptions to the statistics. Now, I believe in incompatibilistic free will, but I also accept the indubitable fact that our behavior is influenced by all kinds of factors, some of them amenable to statistical study--the free-will argument just isn't very good here. Moreover, the free-will argument would equally undercut the idea of cohabiting for the sake of improving future marital success--for if our behavior is all really up to us, with no external influence, then whether the couple cohabits or not, marital success is in the hands of the couple.