But when Frum and Sullivan shift from politics to policy, they go awry. (Zakaria, while endorsing immigration enforcement in general, doesn’t endorse any specific method.) First, they call illegal immigration a crisis—not just a political crisis for Democrats because Trump is using it to rally support, but an actual crisis because undocumented migrants are deluging America at the border. Frum says America is experiencing an “accelerating surge of illegal immigration” and a “renewed mass movement from Central America.” Sullivan says America is facing a “wave of illegal immigrants.”

This is misleading. Over the last decade, illegal immigration has been going down. Between 1983 and 2006, according to the Border Patrol, the United States apprehended roughly one million—and sometimes as many as 1.5 million—undocumented immigrants per year along America’s southwest border (where the vast majority of undocumented migrants cross). That number steadily dwindled during Obama’s presidency. In fiscal year 2016 (which began in October 2015 and ended in September 2016), it was 408,000—less than half the number in 2009.

Then Donald Trump came along, and in fiscal year 2017, the figure plunged even lower: to 304,000. The apprehension numbers remained at historic lows for the first months of FY 2018 (which, remember, began in October 2017), until they rose this March. In February, the Border Patrol apprehended 36,000 would-be crossers. That number grew to 50,000 in March. It remained at 50,000 in April and reached 51,000 in May.

This is the “surge” and “wave” that Frum and Sullivan are talking about. Yes, apprehensions rose between February and March. But that may be seasonal: Apprehensions have risen between February and March for five of the last six years. And the numbers haven’t risen significantly since; they’ve held steady. To make the increases appear alarming, Attorney General Jeff Sessions has compared this year’s March and April numbers to March and April’s of last year. But last year saw the fewest apprehensions since 1971. Even with the recent rise, over the last three months, the U.S. has apprehended roughly as many people as it apprehended during the same stretch in fiscal years 2013, 2014 and 2016—and between one-half and one-quarter as many as it did before the Obama years.

By historical standards, this isn’t a “mass movement.” It’s the opposite. And illegal immigration is unlikely to return to the levels of the 1980s, 1990s, and 2000s anytime soon for one simple, and under appreciated, reason: Mexican women are having fewer children. Since the early 2000s, the number of Mexicans being caught at the border has collapsed. Even a strengthening U.S. economy hasn’t lifted the numbers, because the young Mexican men who in past decades crossed the border today don’t exist in the same numbers. That’s because, since 1960, the Mexican birthrate has dropped from almost seven children per mother to just over two. Which means the pool of potential migrants is far smaller.