Within developed countries, there are well-documented differences in mortality rates by race, income, education, occupation, or urban/rural status. There is a definite hierarchy to healthiness-the higher the socioeconomic status of a group, generally the lower the mortality rate. Some explanations for this include definite differences in access to medical care, in access to the resources needed to buy food and shelter, in health-related behaviors such as smoking, and in levels of "psychosocial stress.