It's not rocket science. This is the only country in the world, as far as I know (I've only visited 20 or so), where, at least for 5-10 major ethnicities, you can find a group of people with a similar background, and feel somewhat at home. Yes, sure, there will be plenty of little tit-for-tats based on some silly difference you have with others, but at least you can find a place and very likely do something productive for the little society you live in. So, all we have to do to grow GDP is to allow more immigrants into the country. If you've taken a serious roadtrip around the country, you'll notice we have lots of space. Some cities, I believe Dayton, Ohio is one, is making it easier for immigrants to move in and start businesses. New businesses will have new employees, with incomes to spend. Leaving your home country and starting fresh is a very motivating experience, and people will work very hard to make that work. After all, didn't your ancestors do that?
http://www.nytimes.com/2013/10/07/us/ailing-cities-extend-hand-to-immigrants.html
