Population features may well be at work, poverty, diet, living conditions, co-infections, parity, nutritional status of mothers at conception and through pregnancy, environmental factors affecting the population, etc. Even in the case of smoking, the relative risk of lung cancer varies from 4.0 (400% increase in risk) to only 2.0 (200% increase) in America versus Japan due to differences in the other factors at work on the two populations. Different incidence rates in different populations exposed to the same biologic or chemical agent is not unusual.