Most of the people in LA were born here. They may have only been here for one generation, but they were born here. A friend of mine from Kent, Ohio is a big Browns fan. His teenage son who was born here is a big Rams fan.
Actually, 35% of LA county is actually foreign born. As in from another country. You're not going to suggests that <15% of LA county is actually transplants from other states. No way. Southern Cal is mostly transplants. Unless you've got a census report that suggests otherwise.