People pass graffiti along the border structure in Tijuana, Mexico, on Jan. 25. (Julie Watson/AP)

President Trump is intent on completing a border wall with Mexico, even if it means instigating a trade war with the U.S.'s second-biggest export market. His spokesman said Wednesday that the U.S. might levy a blanket 20 percent tax on Mexican imports as one way of fulfilling Trump's campaign promise that Mexico would pay for the wall.

Trump promotes the notion that migrants have been flooding across an insecure border, and taking jobs that would otherwise go to Americans. To be sure, there are indeed millions of unauthorized immigrants working in the U.S., though economists are divided on whether the contributions to the economy they make ultimately create more job opportunities for U.S. citizens. The Pew Research Center estimates that “the U.S. civilian workforce included 8 million unauthorized immigrants in 2014, accounting for 5% of those who were working or were unemployed and looking for work.”

And the largest chunk of those unauthorized immigrants are indeed from Mexico, which is the clear target of the wall, along with other Central American countries.

But since a tipping point in 2012, the net flow of migration from Mexico, both legal and not, has actually decreased. From 2009 to 2014, Pew estimates that 1 million Mexicans and their families (including U.S.-born children) left the U.S., while 870,000 Mexicans arrived.

The numbers are estimates, of course, because the flow of unauthorized migration across the U.S.-Mexico border is, by its nature, surreptitious. Pew uses a modeling tool that draws from various sources, including Mexico’s national household survey, to make its best estimate.

Another measure of how many people are trying to cross the border illegally is how many get caught every year. And that number is going down for Mexicans, too, from more than 400,000 in fiscal 2010 to about 177,000 in the current one to date. People fleeing gang-related violence spiraling out of control in El Salvador, Honduras and Guatemala have added significantly to that number in the past year, but the overall trend remains one of net flow southward.

Mexico's own economy has improved, and growing opportunities there have have caused many who once migrated to return home. Simultaneously, the American economy now supports far fewer construction and manufacturing jobs than it once did, and those were a mainstay of the Mexican migrant community. Under President Barack Obama, the U.S. also began more strictly enforcing immigration laws and greatly expanded its use of deportations. Some called Obama the “deporter in chief.”

Meanwhile, migration from Asia to the United States has ramped up, and now outpaces overall arrivals from Mexico. This would've been unthinkable a decade ago, when more than 10 times as many people from Mexico came to the U.S. as from China.

In 2014, roughly 136,000 people came to the U.S. from India, 128,000 from China and 123,000 from Mexico, according to census data. Between 2009 and 2014, 45 percent of all immigrants to the U.S. were born in Asia.

Asian immigration is widespread geographically, too. A Wall Street Journal analysis found that across much of the heartland and New England, they far outpace arrivals from Mexico and Central America. Even California registered more Chinese immigrants than Mexican immigrants in 2014, with India not far behind. The study noted that “there were 31 states where more immigrants arrived from China than from Mexico that year, up from seven states in 2005. Newly arrived immigrants from India in 2014 outnumbered those from Mexico in 25 states, up from four states in 2005.”

By and large, recent Asian immigrants are well educated and as such compete with a different set of Americans for jobs, but also contribute to faster-growing sectors of the American economy.

In The Post's Wonkblog, Ana Swanson cites the research of Jed Kolko, the chief economist at Indeed, a job search engine, who found that “about half of immigrants 25 and older in 2015 who arrived in the U.S. in the last five years had a bachelor’s degree, up from roughly one-third of those who arrived between 2006 and 2010 and 27 percent of those who arrived in 2005 or earlier.”

For perspective, only 31 percent of American citizens 25 or older have a bachelor’s degree. As China and India's economies (and populations) have grown, they have also begun to send more of their students to U.S. universities than ever before.

Read more:

Fearing Trump’s wall, Central Americans rush to cross the U.S. border

5 challenges Trump may face building a border wall

Mexican president cancels visit to Washington as tensions with Trump administration intensify