The total number of people apprehended for illegally crossing the southern U.S. border has been steadily falling for almost two decades. It's a long-term trend that sociologists, economists and federal officials have been tracking for years.

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The trend apparently at odds with statements made this week by President Trump, the secretary of Homeland Security and the attorney general. They defended the administration's detention and prosecution policies by saying that the number of people crossing the southern border has increased.

Officials frequently refer to two numbers to illustrate the increase in unauthorized immigration. As Homeland Security Secretary Kirstjen Nielsen said in a speech on Monday:

"Since this time last year, there has been a 325 percent increase in Unaccompanied Alien Children and a 435 percent increase in family units entering the country illegally."

However, fiscal year 2017 was a very unusual one for illegal immigration. There was an enormous dip in the number of people apprehended at the southern border in the first part of last year compared with the previous four years, according to U.S. Customs and Border Protection. Sociologists are studying the possible reasons for that dip, but the numbers are clear.

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A more telling month-to-month comparison would be between this May and May of 2014, at the peak of the most recent surge in people migrating from Central America. This May, Customs and Border Protection took 6,405 unaccompanied children into custody, compared with 10,578 children in May 2014. The trend was similar for families, with 9,485 apprehensions in May 2018, compared with 12,772 in May 2014.

There are some sub-groups of people who are consistently crossing the border in greater numbers in recent years. The largest group is people from Central America seeking asylum in the United States because of violence in their country of origin. As Nielsen noted, "Over the last 10 years, there has been a 1,700 percent increase in asylum claims, resulting in an asylum backlog" — from about 5,000 claims in 2007, up to more than 90,000 in 2016.

The increase in the number of people seeking asylum is part of a larger shift in the reasons people give for entering the U.S. at the southern border.

For many years, the best predictor of how many people would attempt to cross into the U.S. from Mexico was the strength of the American economy. When the economy was strong and unemployment was low, more people crossed the border. When the economy was weaker and unemployment was higher, fewer people were apprehended while crossing.

A graph comparing border apprehensions with U.S. average unemployment shows how, between the early 1990s and the Great Recession, as one measure goes up, the other goes down.

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But that trend has changed since the end of the recession, around 2009.

As NPR's Hansi Lo Wang reported last year:

"The number of unauthorized immigrants from other countries has been on the rise since the end of the recession, hitting an estimated 5.7 million in 2016. Immigrants from Central America and Asia – largely from India and China — are two of the main drivers of that growth.

"These demographics shifts within the unauthorized immigrant population mark a transition that began after the recession, according to Jeffrey Passel, a demographer at [the Pew Research Center].

" 'A lot of Mexican unauthorized migration was related to family and friendship linkages between people in Mexico and immigrants in the U.S.,' he adds. But those connections may not be as strong as before, since fewer immigrants have been crossing the border.' "

For that reason, Passel and others who study unauthorized immigration say they expect the number of Mexicans crossing the border to continue dropping.

It appears that the post-recession dip in migration started a trend that's now feeding itself. With fewer people crossing for work, fewer family members are seeking to follow.

"You have forever lost those networks that would have been formed had migration continued as usual," explains Pia Orrenius,a labor economist with the Federal Reserve Bank of Dallas.

Studies confirm that today, the strongest pull for people crossing the border without authorization is the desire to be with family in the U.S. Stepped-up border security may make it more difficult and dangerous for people to successfully cross the border, but research has found that criminalizing unauthorized entry into the U.S. does not deter those fleeing violence, and may only delay those seeking to reunite with family.

And that means issues such as violence in Central America will likely become even more relevant drivers of unauthorized immigration, shaping who arrives at the U.S. border and how America responds.

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