Last week, Ars Technica (and I'm sure other news sites) posted an article on a large-scale survey of health outcomes in Britain, under the headline, "Good news for unmarried couples — cohabitation is good for you" (subtitle: "Married partners tend to be healthy, but living with someone works just as well"). Link.
I'm actually hyper-critical about people who sling around the phrase "correlation does not imply causation" too much in improper cases, but here's a golden example
where it does apply; the headline "cohabitation is good for you", is totally
unwarranted. Now, the findings do say that married & cohabiting
people are healthier than people who live alone. But this could be
either X causes Y, or Y causes X, or other more complicated
interactions. One hypothesis is that "cohabitation is good for you [by
improving health]"; another hypothesis is that "being healthy is good
for your prospects of getting a partner", i.e., healthy people make for
more attractive marriage/cohabitation partners. If you think about it,
I'd say that the latter is actually the more common-sense direction of
the causation here.