Could their work ethic have anything to do with their immigration status? If you're working in the U.S. illegally, you tend to keep your head down at work, you don't cause problems, you don't complain, you don't goof off, and in general you try not to draw attention to yourself because you know that you're just one phone call away from being picked up by Immigration officials and tossed back over the border. I've worked with Mexican-Americans and their work ethic was no different than their white co-workers. They goofed off, they took longer than allowed breaks and lunches, they complained a lot, they did sloppy work, etc., just like their white co-workers.