My favorite measure of socioeconomic progress – I might in fact argue it’s the only one that matters – is increases in life expectancy. The gap between life expectancy in the United States and Southern Africa is staggering, and the same can be said of the difference between the US today and in 1900. In both cases the difference is roughly 30 years. For all its importance, however, there’s little consensus on where life expectancy gains actually came from. We’ve known for decades that life expectancy follows something called the “Preston curve” with respect to income:
The relationship is usually described as log-linear, but it’s pretty close to a steep initial curve joined to a flatter one at a kink point. Getting over that initial hump near $6000 (2005 PPP dollars) is pretty crucial. Based on the curve, economists have often argued that the real action in life expectancy is driven by income gains – give people more money and they take better care of themselves, moving right along the curve.
Anybody who’s read the stuff I post here, or just talked to me about traveling to developing countries, knows I’m fairly skeptical of that approach. Clean water clearly has a huge impact on people’s daily lives, and I’m convinced very few people have any idea how important it is. Americans who travel to poor regions commonly talk about “traveler’s diarrhea” and relate it to eating street food or just a strange diet. Their afflictions are in fact caused by intestinal parasites in the drinking water, and eating any kind of cooked food is fairly low-risk relative to drinking from the tap, brushing your teeth with tapwater, or even eating (rinsed) fresh vegetables. I’m singling out my own countrymen here just as an example – outside of a select group of middle-class people in the developing world, very few people anywhere understand how dangerous dirty water is. My instinct is that major health gains, at least with respect to oral-fecal disease transmission, have come from public policies that imposed higher standards from the outside, rather than people learning how to protect themselves from contaminated water and then gaining the income to afford it.
So it’s no surprise to me that David Cutler and Grant Miller were able to attribute over half of the drop in mortality in the early 20th-center US to better water treatment. But I’d argue their approach might understate the benefits of better sanitation. As I’ve pointed out repeatedly, better health, especially early in life, lead to outsized gains in income and wellbeing down the road. If that holds for waterborne illnesses as well then in addition to “shifting the Preston Curve up” – improving mortality at all income levels – water quality improvements can give people more income so that they can move along the curve as well, and ideally past that crucial $6000 hump.
I’ve seen a lot of encouraging efforts to promote better water quality in poor countries. My fellow UM graduate student Jess Hoel is working on a project to scale up home chlorination, for example. But most seem focused on individual behavioral change, and I’m doubtful that can succeed. In the US, at least, we sidestepped the tricky problem of public health education and compliance entirely, and imposed clean water from the top down. As I speculated in the title of this post, I think that’s the kind of intervention that can make a huge difference.