What to do about illegal immigration? Too many people are paralyzed by the magnitude of the problem, and figure that since we can’t deport them all, we’ll have to bite the bullet and let them all stay legally — i.e., give them amnesty.

But this is a digital (on-or-off, one-or-zero) approach to an analog problem. Our goal should not be a magical solution that eliminates illegal immigration, but rather a real-world solution that reduces it over time.

This approach — which has come to be called “attrition through enforcement” — involves a program of consistent, comprehensive application of the immigration law (something we have never attempted), not only at the borders, but also at our consulates overseas and at worksites and elsewhere inside the country. The aim is to reduce the number of foreigners sneaking in to the country (or overstaying visas) and at the same time increase the number of illegal immigrants already here who go home — some forcibly through deportation, but most voluntarily, through what might be called self-deportation. By engineering a steady decrease in the total number of illegal aliens, instead of the continual annual increases we’ve permitted over the past two decades, we can back out of a problem that has taken many years to develop.

But can it work? In particular, can illegal immigrants be induced to pack up and go back?

The evidence is in and the answer is “yes.” The Bush Administration began with a deep hostility toward immigration enforcement and a commitment to amnesty. But as the drive for amnesty was stopped by public outrage, the Department of Homeland Security has been given the green light to actually do its job. There have been significant increases in detention capacity, Border Patrol agents, border fencing, deportations, and local jurisdictions cooperating with federal immigration authorities. Perhaps most important have been the efforts to turn off the jobs magnet that attracts illegal immigrants and keeps them here. Worksite arrests have grown five-fold since 2004 and the E-Verify program, a voluntary online system which enables employers to identify illegal workers, has been ramped up significantly and now vets more than 10 percent of all new hires. Arizona this year has started requiring use of E-Verify by all employers in the state, and soon its use will be a requirement for federal contractors as well.

The results of this stepped-up enforcement were reported by the media in story after story quoting illegal immigrants saying that they were packing up and leaving because of the new enforcement climate. But data was hard to come by, since the enforcement push was so new.

Now there is research showing that attrition through enforcement works. A new report from the Center for Immigration Studies (which I head) used Census Bureau surveys to estimate that the illegal-immigrant population has fallen from a peak of 12.5 million in August of last year down to 11.2 million this past May, a drop of 1.3 million or 11 percent. This decline is at least seven times larger than the number people removed from the country by the immigration authorities during that period, meaning that most of the drop was due to illegal immigrants deporting themselves. If that rate of decrease were to continue, the illegal population would be cut in half in five years.

So far, so good. But did enforcement contribute to the decline or was it driven just by the weakening economy? Though the slowdown in construction and other industries no doubt contributed to the decline, there are several reasons to think that enforcement was a major factor in the decision of illegal immigrants to leave. First of all, the decline in the number of illegal immigrants started before their unemployment rate increased; in the past, much smaller dips had been seen in the illegal population, but only after their unemployment rate increased — which stands to reason, of course. What’s more, only the illegal population declined; the number of legal immigrants continued to grow.

And the enforcement climate is determined not only by actions but also by words — especially the words of lawmakers debating immigration policy. It seems that the number of illegal immigrants actually spiked last summer as the Senate conducted a high-profile debate on the McCain-Kennedy amnesty bill. That debate was widely reported in the immigrant media, which presented amnesty as an inevitability, the culmination of several years of activism backed by all the major institutions of American society. When instead the legislation failed spectacularly in the Senate, as the result of an unprecedented public outcry, those amnesty expectations were dashed, casting the enforcement push in a whole new light. As a result, the illegal-immigrant population began to drop almost immediately.

The challenge will be to maintain this new enforcement climate under a new administration. After all, 90 percent of illegal aliens are still here, and the pressure will have to continue if the problem is to be shrunk down from today’s crisis to a more manageable nuisance. Unfortunately, both presidential candidates have an digital, all-or-nothing view of the problem, and have legalization as their chief priority.