The issue of a border wall between the U.S. and Mexico has been one of President Donald Trump's signature issues. But this isn't the first time the U.S. has talked about a border wall. Back in 2006, President George W. Bush passed the Secure Fence Act. It ordered the building of around 600 miles of wall on the border between the U.S. and Mexico. Economist Melanie Morten and two colleagues examined the economic effects of that wall.