The number of unauthorized immigrants in the United States has fallen or flattened after rapid growth in the late 1990s and early 2000s, according to estimates by think tanks like the Migration Policy Institute and Pew Research Center.

The big picture: The overall number is about 11 million. While the surge of Central Americans trying to cross the border has attracted recent headlines, there has been a steady decline in the number of unauthorized immigrants from Mexico.

• These fluctuations mean the current influx and another in 2019 are more likely to change the demographic makeup of the undocumented population, rather than the overall number, MPI's research director Randy Capps told Axios.

Between the lines: In recent years, roughly two-thirds of new, unauthorized immigrants came to the U.S. legally but overstayed their visas — as opposed to crossing the border unlawfully, Capps said.

• This is especially common among the rising number of unauthorized immigrants from Asian countries.

Of note: It's difficult to get an accurate count of the unauthorized immigrant population. Researchers can use slightly different models, which leads to the disparities seen in the chart above.