What is commonly known as “average life expectancy” is technically known as “life expectancy at birth” or “life expectancy at age 0” and refers to the average number of years that a newborn baby can expect to live in a given society at a given time. It is one of an infinite number of “life expectancies” that one can calculate, such as “life expectancy at 10” (the number of years that a 10-year-old can expect to live in a given society at a given time) and “life expectancy at 60” (the number of years that a 60-year-old can expect to live in a given society at a given time).

A major determinant of life expectancy at birth, especially in our ancestral past and in many developing nations today, is infant and child mortality rate. Life expectancy in such societies is so low because many infants and children die before they reach adulthood. For example, Professor Anthony A. Volk of Brock University in Canada estimates that as many as half the children during our evolutionary history, and as recently as the 18th century in Europe, may have died before the age of 12. The figures are comparable in many poor African nations today, where as many as a third of the children die before the age of 5.

Think about it. If half the children die before the age of 12 (let’s say, at the average age of death of 6), then the remaining half would have to live on average to be 74, for the life expectancy at birth to come out to be 40, with the implication that roughly half of the remaining half – a quarter of all babies born – live to be older than 74. In many contemporary hunter-gatherer tribes, the modal lifespan for adults (excluding those who die in ) is between 70 and 80 years; in other words, most adults die when they are around 70 or 80. This is essentially the same as that in contemporary industrialized societies. The difference is that, in the latter, virtually everybody survives childhood.

Another way of putting it is that, while life expectancy at birth is much lower in our evolutionary history and in many developing nations today than in contemporary industrialized nations, life expectancy at 15 and life expectancy at 30 in the former are not that different from what they are in the latter. Life expectancy at birth of 40 in a given society decidedly does not mean that a 20-year-old can expect to live only for 20 more years. More than likely, a 20-year-old in such a society can expect to live for another 50 years.

Adults everywhere and at all times, including our evolutionary past and in many developing nations today, live to be about the same age. Life expectancy at birth is very low in our evolutionary past and in many developing nations today because many infants and children die. For those who are lucky enough to survive childhood, the life is just as long as it is for everyone else and very few die in their 30s and 40s.

P.S. Professor Volk’s magnificent paper “Is child death the crucible of human evolution?”, which highlights infant and child mortality as a major (but often neglected) source of evolutionary selection pressure, is forthcoming in the Journal of Social, Evolutionary, and Cultural Psychology’s annual conference volume on the 2008 meeting of the NorthEastern Evolutionary Psychology Society (NEEPS). For more information, please contact Professor Volk (TVolk@brocku.ca).