Last summer, Brexit shocked the world. Few global experts saw it coming. In their defense, most economic indicators didn’t point to a political upheaval. Gross domestic product in the U.K. was growing at about 2%, and unemployment had dropped to 4.9%. From a data perspective, things seemed OK.

Another metric, however, showed something different happening in the U.K. — “happiness.” In the two years leading up to Brexit, Gallup found that the percentage of people who were “happy” (or “thriving”) was in dramatic decline. In fact, the 15-percentage-point decline in the percentage of people rating their lives positively enough to be considered thriving was so dramatic that it remains among the largest two-year drops in Gallup’s history of global tracking.