The U.S. population grew by 0.79% last year, just slightly faster than the previous year’s growth rate but still among the slowest rates in decades.

According to the U.S. Census Bureau, the number of Americans living in metropolitan areas rose to 275.3 million in 2015, an increase of 2.5 million. While urban areas on the whole are growing, the population of several metro areas shrank last year. Over the last five years, the populations of 23 metros declined by more than 2%. By contrast, the U.S. population increased by 3.9% over that time.

William Frey, senior fellow and demographer at public policy research think tank Brookings Institution, explained, the areas gaining the most in population are “gaining more than they have in the previous few years, the losing areas losing more than they have — many of them — in the previous three years.” Frey noted that this migration trend is a continuation of the one that has been going on since before the recession.

People need to leave a metropolitan area in order for its population to decline. However, these areas are shrinking also because they are failing to attract new migrants. With only a few exceptions, all of the fastest shrinking metro populations declined primarily as a result of net migration.

70. Hartford-West Hartford-East Hartford, CT

> Population growth (2010-2015): -0.23%
> Total population: 1,211,324
> Per capita income: $56,078
> Unemployment rate: 6.00

Jobs are perhaps the single most important driver of urban expansion. The relatively weak job markets and low income levels in these 20 shrinking areas mean there are likely fewer economic opportunities, which would attract new residents and encourage current residents to stay. The unemployment rate in every one of these areas exceeds the national jobless of 5% in January 2016. The average income is also lower than the national per capita income of $47,615 in all 20 metros.

Of the 20 fastest shrinking metro areas, half are in the rust belt — the central U.S. region characterized by its decaying industrial base and population loss. Many of these urban areas are still heavily dependent on manufacturing. The percentage of the workforce employed in manufacturing is greater than the national proportion of 10.3% in 12 of the 20 areas.

These cities also tend to have older populations. Migrants tend to be younger in general, so this could mean these areas are not as attractive to Americans most likely to relocate. The birth rate also tends to be lower in places with older populations.

In all but two of these 20 areas, the percentage of residents 65 and older exceeds the national proportion of 14.5%. Nationwide and in most U.S. areas, births are far more likely to outpace deaths. In nine of the 20 fastest shrinking metro areas, however, the population decrease from deaths was greater than the population increase from births over the last five years.

Based on recently released U.S. Census Bureau estimates, 24/7 Wall St. reviewed population changes in the 381 U.S. metropolitan statistical areas from July 2010 through July 2015. Poverty rates, educational attainment rates, and workforce composition came from the Census Bureau’s 2014 American Community Survey. Data on incomes and price levels as of 2014 and 2010, respectively, are from the Bureau of Economic Analysis. Unemployment rates are for January 2016, and annual unemployment rates for 2010 and 2014 are from the Bureau of Labor Statistics. Metro area GDP per capita income are from the Bureau of Economic Analysis and are in 2010 dollars.

Click here to see America’s fastest shrinking cities.

Click here to see America’s fastest growing cities.