MEXICAN immigrants were said to be holding down wages and taking jobs that could go to honest Americans. The poorest natives were supposed to be suffering most grievously. “We cannot afford to disregard it,” intoned the president. “We do not condone it.” The immigrants were soon sent home and not allowed to return.

All that happened in the early 1960s. The president was John F. Kennedy; the Mexicans were participating in the bracero programme, which allowed almost half a million people a year to take seasonal work on America’s farms. But the parallels with the present are plain. Donald Trump has also complained that immigrants are keeping Americans from good jobs and has promised to do something about it (another parallel: not since Kennedy has America seen such an astonishing presidential coiffure). So it is a good moment for a bracing new assessment of the bracero scheme and its demise.

Michael Clemens and Hannah Postel of the Centre for Global Development, and Ethan Lewis of Dartmouth College, have used archived records of American agricultural jobs and wages to test whether Kennedy was right. Did ending the bracero scheme in 1964 in fact lead to higher wages and more work for Americans in the fields?

Muchachos de campo The answer is a firm no. In states where farmers had relied heavily on foreign labour—a group that includes California and Texas—American natives found a few more farm jobs in the mid 1960s. But the rise was small and temporary; within a few years the long decline in agricultural jobs had resumed. And the trend was almost identical in states where there had been no braceros. Similarly, farm wages rose in states where there had been lots of migrant workers, states where there had been few migrant workers and states where there had been almost none (see chart). Ending the bracero scheme seems to have affected American workers not a bit. This would seem, as a contemporary put it, to repeal the laws of supply and demand. And the authors rule out two obvious explanations for why the change was so ineffective. Above-board Mexican migrants were not replaced by illegal immigrants: the surge in illicit workers began only in the 1970s. Nor were they replaced by legal immigrants from elsewhere. The explanation is, rather, that farmers swapped Mexicans for machines.

Some farm jobs, like tomato picking, could be automated fairly easily in the 1960s. And ending the bracero scheme seems to have accelerated mechanisation in the tomato fields of California. Much the same happened with cotton and sugar beet. Other crops, like lettuces and asparagus, still required human pickers. Production of some such crops simply declined.

These days America has a more direct method of raising labourers’ wages: it forces farmers to pay them more. In California, America’s most important farming state, politicians have ensured that workers will receive at least $15 an hour by 2023. And Manuel Cunha, a citrus grower who is president of the Nisei Farmers’ League, complains about other costly reforms, such as mandatory overtime pay for people who work more than eight hours a day.

In response, he says, farmers are moving from crops that require careful handling, like apricots—“just look at an apricot and it will turn brown”—to crops that can be harvested by machine. Almond trees are spreading across California. In spring the fields are white with their blossom. In September great machines shake the nuts to the ground and sweep them up.