Mexicans and Puerto Ricans are the two largest Hispanic populations in the United States. Mexicans tend to live farther west, while Puerto Ricans tend to live in the Northeast. That knowledge pointed him in the right direction, he writes, and he soon found that the most important factor in asthma drug response is not age, sex, or disease severity, but ethnic background. I.e. The drugs that often worked for Mexicans and African Americans did not work the same for Puerto Ricans.