After the terrorist attacks of Sept. 11, 2001, we heard stories about families moving from New York and New Jersey to Iowa because they wanted their children to grow up in a safer environment.

Similar stories surfaced after Hurricane Katrina in 2005 about people moving to Iowa from New Orleans to start new lives.

In coming weeks, I expect to see reports of Houston residents coming to Iowa to start over after Hurricane Harvey.

All of which points out something that most of us know but rarely think about — namely that Iowa has always been a state of migrants.

During the mid- and late 19th century, many, including my great-grandfathers, came to Iowa from Germany, Ireland and other locations throughout Europe. They arrived in such numbers that Iowa became, for a time, the fastest-growing state. Between the Civil War and 1900, Iowa’s population tripled from roughly 700,000 to 2.2 million.

Iowa State University’s population expert, Willis Goudy, traced the history of that growth in his 2005 publication “Iowa’s Numbers: 150 Years of Decennial Census Data With a Glance to the Future.”

Among other things, he noted that after such rapid growth during the 19th century, Iowa’s population stalled during the 20th century.

Iowa had the lowest population growth of any of the 50 states between 1900 and 2000. The increase amounted to a mere 31 percent from 2.2 million to 2.9 million.

By contrast, the population of the entire country grew a whopping 269 percent between 1900 and 2000. A comparable increase in Iowa would have boosted our current population to more than 6 million, or roughly double what it is today.

What happened to Iowa during those no-growth years? Why didn’t our growth keep up with the rest of the country, or even come close?

Goudy distills the numbers to account for “natural change” — the difference between births and deaths — and “net migration,” which is the difference between people moving into and out of Iowa.

Between 1900 and 2000, there were 1.8 million more births than deaths in Iowa, he noted. At the same time, 1.1 million more people left the state than came here. 

The result was that by the year 2000 Iowa’s population had only increased by about 700,000, or 31 percent, to 2.9 million. 

Goudy broke the migration numbers down by decades and discovered that the two decades with the biggest losses were the 1950s, when the booming postwar economy resulted in a net migration loss of 230,000 Iowans, and the farm crisis years of the 1980s when the net loss was 283,000.

During the 1990s, when significant numbers of Vietnam War refugees began arriving in Iowa, along with refugees from Eastern Europe and Africa, Iowa posted its first net migration gain — roughly 50,000 — since the 1910s. 

Goudy’s data ended with the 2000 census. 

But there are reasons to believe the immigration influx that began in the 1990s with Southeast Asians and others has continued with new arrivals from Central and South America during the 21st century.

But there has also been something else going on in recent decades. Virtually all of the net migration to Iowa from other states and other nations has been to the metropolitan areas around Des Moines and Cedar Rapids or to large university towns, like Ames and Iowa City.

And that’s on top of increased migration from rural Iowa to urban areas.

The bottom line is that while we think of Iowa’s population as homogeneous, it is constantly changing and evolving.

So when Texans start arriving in coming weeks, it will be to a different Iowa than existed even 16 years ago when post-9/11 families began arriving from New York and New Jersey.