Is there actually a case for the Wall?

Donald Trump’s boast to build a “big, beautiful” wall along the southern border clearly provided a lift to his candidacy, arguably delivering him the Republican presidential nomination. Along with his promise to deport millions of immigrants who are living in the United States without legal authorization, it remains the leitmotif of his campaign, despite occasional bursts of softer rhetoric.

Mr. Trump is not wrong that immigration from Mexico and other countries in the poorer south over the last quarter-century has injured some American workers who competed with immigrants in the job market. It is not his concern alone; similar fears are shared by organized labor and others on the left of the political spectrum. Improbable as this may sound, the question he raises is legitimate.

But even looking at a best-case situation, the answer is still straightforward: No. Even if you care only about the workers most hurt by new immigrant labor, Mr. Trump’s proposals simply aren’t worth the cost.

In an article in the Journal of Economic Perspectives scheduled to be published this fall, Gordon H. Hanson and Craig McIntosh of the University of California, San Diego, lay out the most obvious reason walling off Mexico would be pointless: Mexicans aren’t coming anymore.