We know about the trajectories of happiness for German and Dutch people who get married and stay married. Longitudinal research (in which the same people are followed for years — in the German study, more than 20 years) has shown that when people marry, those who will stay married enjoy a “honeymoon effect.”
They become a bit happier around the time of the marriage, but then that happiness dissipates over time. On the average, the Germans who married and stayed married returned to the same level of happiness they experienced when they were single, and that happened within a few years. The increase in happiness lasted longer for the Dutch.
In my writings on marital status and happiness (in Singled Out and elsewhere), I’ve pointed out that those happiness studies don’t really tell us how happiness will change when you marry, because the honeymoon effect occurs only for those who stay married. Those who marry and then divorce actually become a bit less happy as their wedding day approaches and that decline continues until the year before the divorce becomes final.
Individual people approaching marriage do not know which group they will end up in – the one that stays married or the one that gets divorced. If we want to know the implications for happiness (or anything else) of getting married, we need to look at the results for everyone who marries, and not just those who stay married.