The study by the Center for Immigration Studies, a think tank that advocates stricter limits on immigration, based its findings on Census data indicating that the number of less-educated, working-aged Hispanic immigrants - defined as 18- to 40-year-olds with a high school diploma or less - has dropped by more than 10 percent, or about 830,000 people since last August. In addition to enforcement, the study?EUR??,,????'???s authors say the economy is also likely playing a role in the reduction of illegal immigrant numbers.

Previous research suggests that a large share of less-educated foreigners are in the country illegally, and that they make up the bulk of the illegal immigrant population. Furthermore, while earlier declines in the number of these Hispanic immigrants have been linked to a rise in their unemployment rate, the current drop-off began last year almost immediately after the Congress abandoned legislation to legalize undocumented immigrants and six months before there was any significant rise in their unemployment rate.

During the same period, the number of foreigners who were more educated or non-Hispanic-and therefore far less likely to be illegal immigrants-continued to rise or hold steady.

?EUR??,,????'??The evidence is consistent with the idea that at least initially more robust enforcement caused the number of illegal immigrants to decline significantly,?EUR??,,????'?? said Steven A. Camarota, one of the study?EUR??,,????'???s authors.

The study found that, according to the Center?EUR??,,????'???s best estimate, the illegal immigrant population has declined by 11 percent through May 2008 after hitting a peak in August 2007. The implied decline in the illegal population is 1.3 million since last summer, from 12.5 million to 11.2 million today.

According to the study, the estimated decline of the illegal population is at least seven times larger than the number of illegal aliens removed by the government in the last 10 months, so most of the decline is due to illegal immigrants leaving the country on their own.

One indication that stepped-up enforcement is responsible for the decline is that only the illegal immigrant population seems to be affected; the legal immigrant population continues to grow.

Finally, the study states that there is good evidence that the illegal population grew last summer while Congress was considering legalizing illegal immigrants. When that legislation failed to pass, the illegal population began to fall almost immediately.