At least they tried to control for 1 variable. They controlled for happiness level prior to the marriage, which has nothing to do with happiness while married. But, this study is still rubbish due to survivorship bias. People who are happy with their marriage will stay in the marriage. People who are unhappy may not stay in the marriage, and are not part of the study. To really measure if marriage causes happiness, you must run a controlled experiment and randomly assign people to a control and treatment group. The results of an observational study are invalid and meaningless. The article should conclude that “People who are in great marriages and decide to stay married …are happier than single people”
Incidentally, the article headline implies “Marriage makes you happy” Meanwhile, they later state the effect of living together makes you just as happy as legal marriage. So, they show that marriage has nothing to do with happiness, yet the headline states the exact opposite. Another case of a headline that the masses just accept at face value.