Why is life expectancy increasing?

Cafe Hayek has a nice, brief discussion of Robert Fogel's book The Escape from Hunger and Premature Death, 1700-2100 and Angus Deaton's review of the book in Journal of Economic Literature. I haven't read Deaton's review but have read Fogel's book and highly recommend it for anyone, even if you don't have a background in economics. The book can best be categorized as the intersection of economic history and nutritional science.

If nothing else, the book is full of fascinating statistics, graphs and facts about health and development. For instance:

1. The health of working class people in England during the Industrial Revolution likely worsened. Higher wages may have compensated workers for the fact that they were likely to die younger and live unhealthier lives.
   
2. Life expectancy is statistically correlated with birth weight. It turns out that we can calculate an "optimal birth weight" for different countries, meaning the birth weight that maximizes expected life span. That figure turns out to be much higher for the U.S. than for India. This suggests an average American child given to an average Indian family would be seriously undernourished because her metabolism would be much higher than the average Indian child's.
   
3. Income inequality is correlated with inequality in the heights of adults for a given country. Countries like the U.S. and U.K. which have become more equal over the past century have seen an equalization of heights among different social classes. In India or sub-Saharan Africa, there are still large differences in heights across social classes.
4. People who are shorter and lighter tend to be more likely to die young and are less able to fight off disease and infection. On the other hand, childhood illness is likely to reduce adult height and weight and therefore possibly shortening someone's life span by making that person more likely to succumb to disease. There are therefore feedback effects from eliminating childhood illness or by feeding children more nutritious food.
   

Fogel seems to put a lot of weight on public health improvements in the late 19th century as an important factor in increasing life expectancy (Deaton seems to think Fogel underplays this; I disagree). I think this is a question still very much open to debate, however. Is economic growth the primary cause of increasing life expectancy or are better sanitation, public health and medical technology more important?