Mexican immigration wave ended years ago

British Web engineer Simon Holliday operates a word association website. The site gives you a word and asks you to type in the first thing that comes to mind. The statistics for the word "immigrant" are revealing. The most common user entry is "illegal." The second is "Mexican."

This probably comes as no surprise to anyone paying attention to current debates over immigration policy. What might come as a surprise is that these words are rapidly losing their relevance.

Since 2007, we've seen a net increase of 2.3 million immigrants in the United States. That's the population equivalent of adding an extra Nebraska, with another half-million people on top of that.

And how many Mexicans were included in that 2.3 million increase? Exactly zero. In fact, the number of Mexican immigrants in the most recent estimates is slightly below the 2007 value.

There may well be some Mexican-born residents of the United States who arrived within the past five years, but for every arrival there has also been a departure.

Statistics on the illegal immigrant population are harder to come by, but it's safe to say that over the past five years, immigration has become overwhelmingly less Mexican. Since Mexico is the No. 1 source of undocumented migrants to the United States, the immigrant population has almost certainly become less illegal.

What does all this mean for the word association game? Where are these newcomers arriving from, if not Mexico? Why is this all happening? Does this mean that we don't need to worry about building a fence anymore?

All good questions. Let's start with the demographics -- a potentially dull subject, but the root of all things migratory. Since the Colonial era, immigration to the United States has tended to originate in one of two types of places: those experiencing war, and those experiencing population explosions. It's this second group that has concerned the nation's lawmakers.

Across the centuries, we've always been worried when we see massive inflows of migrants from countries with high birth rates. We worry that the flow will never stop, and America's cultural balance will tip. We worried about German-language newspapers 250 years ago; we worry about Spanish now.

Mexico had a population explosion, but it is over. The Mexican population doubled between 1970 and 2000, a product of high birth rates and declining death rates. Today, Mexico's birth rate is just high enough to keep the population steady.

The great wave of Mexican immigration that accompanied the population boom will soon recede, as has every prior immigration wave our nation has experienced. The recession, which had a particularly severe impact on the job prospects of unskilled migrants, sped the process along. That explains the reversal in immigration from Mexico.

The new arrivals are coming from a variety of places. The top source: Asia. In 2007, the number of Mexicans in the United States exceeded the number of Asian immigrants by 1.5 million. Today, the two groups have pulled even. Africa and the more distant parts of Latin America are increasingly important as well.

These trends can help us to think about what our "new" immigration policy should look like. Border control will be of declining importance in a world where most migrants arrive in the United States from a different continent. Spanish will begin to recede as the nation's de facto second language; Mandarin, Hindi and Tagalog will be among the most prominent replacements.

Guest worker programs might not work as well as they would have in the past; the vast networks of underemployed Latin Americans who might have once jumped at the chance at a temporary job in the United States are aging out of their prime years of physical labor, and there aren't as many younger adults around to take their place.

More than anything else, these trends signal that the days of fretting about the tide of illegal Mexican immigrants are, to a large extent, behind us. That tide has receded, and for demographic reasons it may not rise again.

As for the word association game, it's hard to say what the top responses will be 10 years from now. When someone says "immigrant," the most common response may well be, "What?"

Jacob L. Vigdor is a professor of public policy and economics. This commentary was originally published in The Washington Examiner.