The number of people living in Bulgaria is expected to drop by nearly a quarter by the year 2050—and its neighbors aren't going to fare much better demographically, per UN population projections cited by Quartz. Over the next 30-plus years, Bulgaria will see its population fall from around 7 million to about 5.4 million, a 23% drop. Other Eastern European countries will see similar declines, with all of the nations that top the UN's list experiencing decreases of at least 15% in the same time period. What one researcher says is to blame: residents migrating to other countries, high death rates, and lower fertility rates. Specifics on the five nations at the top of list follow.