The 1890s were a good decade for Keokuk County, Iowa. The population there, in the southeast corner of the state, neared 25,000 residents by the 1900 census. That number has been declining ever since.

As with much of the Great Plains, Appalachia and the old Cotton Belt, rural Keokuk County has been depopulating for a very long time. While vast stretches of America are still moving toward their peak populations, these places have been emptying out. They peaked — population-wise, at least — in 1920, or 1890, or even earlier.

In fact, over the full sweep of U.S. history, more than half of the counties in the nation are in population decline, their peaks at some point behind them. The dates are a fascinating collection of historical footnotes: Manhattan, filled with tenements, peaked in 1910. Sioux County, N.D., peaked in 1930. Wayne County, Mich. (the home of Detroit), peaked in 1970.

These tipping points mirror broad changes in the country's economy and migration: The population in many West Virginia counties was never higher than when coal mining was king. In parts of the rural South, population peaked in the early 20th century, before the Great Migration of blacks to northern cities and before agriculture became less labor-intensive, demanding fewer bodies. The Great Plains, as the above GIF illustrates, began to empty out en masse before World War II.

Migration researcher and economist Lyman Stone cobbled together historical census data (which was harder than it sounds) to illustrate these patterns of population decline. Like thinking back on the best years of your life, consider this map to be a picture of the high points for each corner of the country:

On that map, 1,160 counties out of 3,143 are at their highest population so far — their best years are right now or ahead of them. About three-quarters of the U.S. population, according to Lyman, live in these counties, which include a lot of booming suburbs. Just a quarter of us live today in declining places.

Population isn't a perfect proxy for prosperity. And Lyman points out that some places that shed residents didn't necessarily grow poorer or suffer more in ways we can measure with median incomes and unemployment rates. But, arguably, they did grow less important. Atlantic City, to take one of his examples, peaked in 1930 at about 66,000 residents. By 2010, its population had dropped by about 40 percent. But its share of the U.S. population — which continued to grow over that time — fell by even more.

"So it’s not just that Atlantic City, or Appalachia, or Cleveland, has fewer people," Lyman writes. "It’s that even the people they have matter less in comparison to the rest of the nation."

Now, past population loss doesn't necessarily imply future population loss. And a place that peaked historically could still peak again. The District of Columbia, for instance, topped out at about 802,000 residents in the 1950 census. The nation's capital lost residents over the next half-century, as many cities did in an era when the suburbs were ascendant and urban neighborhoods were neglected.

But the District is now gaining about a thousand residents a month, with one of the strongest examples in the country of an urban renaissance. At this rate, the city could overcome its historical population high in little more than a decade (Washington had about 672,000 people as of 2015).

Many Northeast and Midwestern cities had population peaks between the 1950s and 1970s, with precipitous declines in the decades that followed (this county-level data, though, obscures some of the drop-off, pulling in nearby suburbs to which residents fled). Baltimore, St. Louis, Chicago and Philadelphia (or the counties around them) are all below their historical population peaks, with vacant lots and unused buildings occupying some of their empty spaces.

In this larger historical context, population decline in America has been both a rural and a very urban phenomenon. Many of the places with their peaks still ahead of them lie somewhere in between.

More from Wonkblog:

The rise of humankind, in one mesmerizing map

The most interesting states in AmericaWho's really moving back into American cities