In Washington, politicians are trying to reform America's immigration system, again. Both President Barack Obama and Republican Sen. Marco Rubio, R-Fla., are proposing "paths to citizenship" for an estimated 11 million illegal immigrants. Other proposals abound, including finishing the border fence, creating a better E-Verify system for employers and passing the last Congress' Dream Act.

All of these ideas, however, fundamentally misunderstand immigration in America: Future immigration is probably going to be governed not by U.S. domestic policy choices but by global demographics.

For the last 30 years, a massive number of immigrants have poured into the United States from south of the border. Today there are 38 million people living in the United States who were born somewhere else. That's an average of more than 1 million immigrants a year for three decades, a sustained influx unlike any we've seen before in U.S. history. And regardless of what policies Washington decides on, that supply is likely to dry up soon.

Like much else in the world, immigration is influenced by the fertility rate: that is, the number of children the average woman has during her lifetime. If the total fertility rate of a country is above 2.1 - what demographers called the replacement level - then that country's population grows. If the rate is below 2.1, then eventually its population contracts.

When it comes to immigration, demographers have a general rule of thumb: Countries with fertility rates below the replacement level tend to attract immigrants, not send them. And so, when a country's fertility rate collapses, it often ceases to be a source of immigration.

Consider Puerto Rico. In the 1920s, Puerto Ricans began to trickle into the United States. By the mid-1940s, 30,000 Puerto Ricans were arriving every year. In 1955, 80,000 Puerto Ricans came to the United States.

But from 1955 to 2010, the number plummeted. Even though the population of Puerto Rico had nearly doubled in that time, the total number of Puerto Ricans moving to the United States in 2010 was only 4,283.

Why? After all, migrating to the United States from Puerto Rico had become easier, not harder. And while economic conditions in Puerto Rico brightened somewhat, the opportunities and standard of living in the United States are still superior.

What happened is that Puerto Rico's fertility rate imploded. In 1955, Puerto Rico's total fertility rate was 4.97, well above replacement. By 2012, it had fallen to 1.64 - even further below the replacement line than the United States' rate.

And what has happened in Puerto Rico is happening all across Latin America, where fertility rates have been dropping on an express elevator for 20 years.

It's not a coincidence that sub-replacement countries - such as Uruguay, Chile, Brazil and Costa Rica - send the U.S. barely any immigrants at all. The vast majority of our immigrants come from above-replacement countries, such as Honduras, El Salvador, Colombia, Guatemala and Mexico. But even those countries are witnessing epic fertility declines too.

Over the last 30 years, Mexico has sent roughly two-thirds of all the immigrants - legal and illegal - who came to the United States. In 1970, the Mexican fertility rate was 6.72. Today, it's hovering at the 2.1 mark - a drop of nearly 70 percent in just two generations. And it's still falling. The result is that from 2005 to 2010, the U.S. received a net of zero immigrants from Mexico.

Certainly some of that change can be attributed to the Great Recession, particularly the slowdown in construction and the housing industry. But we may also be witnessing the beginning of a structural change in our immigration relationship.

It's important to understand that Mexico's experience is not unique but rather is part of a global phenomenon. Today, 97 percent of the world's population lives in countries where the fertility rate is falling. And once a country's fertility rate starts dropping, it nearly always settles far below the replacement level. And because of this, most demographic models project that the world's population will peak sometime before the end of this century, and then begin contracting - for the first time since the Black Death ravaged Europe.

The U.S. immigration debate is largely driven by politics. And the truth is that both liberals and conservatives have valid concerns about immigration's effect. But this political debate is shortsighted, and whatever policy choices we make will probably only matter in the near term. In the end, demography always wins.

Jonathan V. Last is a senior writer at the Weekly Standard and the author of "What to Expect When No One's Expecting: America's Coming Demographic Disaster."

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