Introduction

During the latter half of the 20th century the average life expectancy throughout the developed world began to increase rapidly. The term “life expectancy” is often confused with “lifespan”, which measures a subtly different parameter. Life expectancy represents the likelihood of surviving to a given age and is often an indicator of the overall health of a country. Increases in life expectancy in developed countries are mainly due to improvements in medicine, public health and nutrition. The primary cause of death in such countries is age-related diseases such as, cardiovascular disease, strokes, infection as result of impaired immune function and numerous cancers. Conversely, life expectancy can fall due to problems such as famine, war, disease and poor health. For example, in regions of Africa and Asia where AIDS is a predominant cause of death, life expectancy is dramatically decreased compared to developed countries and is expected to decrease further if medical intervention is not effective (Logie, 1999). The average life expectancy of a person living in a developed country is about 75 years compared with that of 43 years in some regions of Africa. Lifespan on the other hand is the maximum survival potential as defined by the longest surviving member of a population. For humans, it has been estimated to be between 115 and 120 years. Increases in life expectancy have not resulted in any significant increase in the maximum human lifespan.