Want a sign that the economy is on the rebound? Illegal immigration from Mexico is starting to rise again, according to a new report.

Immigration from Mexico fell to historic lows during the worst years of the recession. After four decades that brought 12 million people from Mexico to the U.S., people started heading back home and continued doing so from 2007 to 2011.

It's impossible to pinpoint the exact number of people crossing the southwest border with Mexico, but the study by U.S. and Mexican researchers estimates that immigrants headed north in the first half of 2012 outnumbered those heading back for the first time since 2007.

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