Population trends are not always what they seem at first glance. About 75% of large metro counties experienced population growth in 2018; but in 77% of these counties, more Americans moved out of the county than into the county, according to research from Harvard University’s Joint Center for Housing Studies (JCHS).
Understanding how populations are growing or declining  can provide insight into what is going on in a particular community or housing market.
For example, it is notable that in four of the five large metros with the highest rates of cost-burdened renters , more Americans are moving out of the metro than into the metro. At the same time, some major metros are gaining new residents from abroad, pushing their populations up even as current residents move away.
“People are often surprised by the fact that many major US cities are growing but also losing people to domestic migration,” said Riordan Frost in a JCHS blog post.
Frost explained that population changes can be broken down into two categories: migration and natural population change.
Natural population change encompasses all births and deaths in an area. Migration is composed of people moving in or out of an area, either domestically or internationally.
Overall, 56% of counties in the nation experienced population increases in 2018.
When examining just the 68 counties considered “large central metros,” as defined by the National Center for Health Statistics Urban-Rural Classification Scheme, Frost found 75% experienced a growing population in 2018. However, 77% of them experienced negative domestic migration. Furthermore, only 52% experienced positive migration overall—comprising both domestic and international moves.
In some cases, natural population increases are making up for migration trends.
In 16 counties, including those that encompass Philadelphia; Cincinnati; Houston; Dallas; and San Diego, natural population growth more than made up for the fact that more people moved out of these counties than into these counties.
Using Dallas as an example, Frost explained that 22,000 people moved out of Dallas County between 2017 and 2018, but the county gained 14,000 residents from outside the United States. The net migration impact is 8,000.
At the same time, the county gained 23,000 new residents through natural population changes, meaning there were 23,000 more births than deaths in the county over the year.
On the other hand, 17 large counties charted a population decline despite natural population growth over the year. These counties included those that encompass Los Angeles; Chicago; Milwaukee; Cleveland and much of New York.
Frost pointed out that population data can provide insight into the decisions Americans are making about where they live, “which is important to understand, in light of near-record lows in residential mobility.”