Some 1.3 million illegal immigrants have left the United States since Congress failed to pass comprehensive immigration reform in the summer of 2007. If the trend continues, according to a new study, the nation's illegal population will drop by half in the next five years.

Moreover, reports the Center for Immigration Studies, young Hispanic immigrants began heading south before the nation's economy did – a clue that what's driving the new outmigration is a stepped-up border and workplace enforcement, not a souring US job market. The source of the report – a think tank with a record of opposing illegal and even some legal immigration – is controversial in immigrant communities. But its findings could help frame the debate in a new Congress and a new administration.

The key conclusion is that enforcement, not the economy, is driving the decision to self-deport. ...