Tropical diseases cause a skilled worker, who completes professional training by age thirty, to look forward to, on the average, just ten years of economic productivity in Zambia before dying at an average life span of around forty, but to be economically productive for thirty-five years until retiring at age sixty-five in the US, Europe, and Japan (average life span around eighty).This isn't right because there is a difference between average life expectancy and conditional life expectancy. It is easy, maybe even natural, to assume that if life expectancy is 40 and you are 30 you can expect to live 10 more years. But that isn't right, if you are 45 do you expect to live -5 years?
It turns out that this isn't an innocuous mistake. The low life expectancy in Zambia is driven by high child/infant mortality and a high AIDS prevalence. If you survived your childhood and don't have AIDS, then you will live into your 70s, on average, and probably die from cancer or heart disease like most Americans. A priest in Sierra Leone told me that they having a saying to that effect (paraphrased): "If you live to 18, you will live to 80."