There are both positive and negative influences on mortality and life expectancy, such as improving socioeconomic conditions (positive) or increases in a certain cause of death (negative). In many developing countries, increasing child survival and infectious disease control is leading to increasing life expectancies. However, in some sub-Saharan countries severely affected by the HIV/AIDS pandemic, life expectancy has decreased in the past two decades due to increases in premature death (www.unaids.org).
In most developed countries, life expectancy has been increasing steadily since the middle of the 20th Century, due mainly to the near-eradication of infectious disease and high standards of living (which includes diet, sanitation and healthcare). However, even in developed countries these positive influences on life expectancy may change when looking at population sub-groups. For example, life expectancy among African-Americans decreased throughout the late 1980s, due in part to increasing rates of HIV infection and homicide which offset other positive influences (Kochanek, K.D. et al. 1994. Am. J. Pub. Health 84(6): 938-44).
Public health campaigns and cultural change may also have a measurable influence on life expectancy. In Australia, the rise in cigarette smoking in the middle of the 20th century resulted in large increases in mortality from lung cancer, cardiovascular disease, respiratory and other conditions. These increases in mortality had a retarding effect on life expectancy, especially in the 1960s (see Australian trends in life expectancy). Public health campaigns and changes in public health regulation began to reduce smoking rates. The effect of legislation, rises in tobacco taxes and other health promotion activities is now starting to become evident in the mortality rates and other measures. A sharp decline in the proportion of males who are smoking has been followed by a decline in the incidence of male lung cancer. A rise in smoking prevalence among females in the latter part of the 20th century has been followed by a rise in the incidence of female lung cancer (although female smoking rates are also now in decline).
Increasing rates of chronic disease may now have a growing negative influence on life expectancy in both developed and developing countries. This is also the case for chronic disease risk factors, such as obesity and overweight. Indeed, recent research in the United States suggests that high obesity levels may lead to decreasing life expectancy in that country during the 21st Century (Olshansky, S. et al. 2005. Obstetrical and Gynecological Survey 60(7): 450-452).