One of the classic exceptions to Rossi’s Iron Law of Policy Evaluation- that the expected impact of a rigorously evaluated public policy is zero- is sanitation. Large pre-modern cities generally had higher mortality than smaller cities or rural areas in the same countries, a distinction that is generally and plausibly linked to worse sanitation as well as greater pollution. Once sanitation improved, mortality fell. For example, one 2005 review of falling U.S. mortality in the late 19th century found that:
Mortality rates in the US fell more rapidly during the late 19th and early 20th
Centuries than any other period in American history. This decline coincided with an
epidemiological transition and the disappearance of a mortality “penalty” associated with living in urban areas. There is little empirical evidence and much unresolved debate about what caused these improvements, however. This paper investigates the causal influence of clean water technologies – filtration and chlorination – on mortality in major cities during the early 20th Century…We find that clean water was responsible for nearly half of the total mortality reduction in major cities, three-quarters of the infant mortality reduction, and nearly two-thirds of the child mortality reduction.
In data geeks’ favorite example, the pioneering doctor-meets-geomapping-analyst John Snow used a map of water sources and incidences of cholera to link an 1854 London outbreak to wells contaminated by waste, leading to belated improvements in London’s water system and an eventual end to large-scale cholera epidemics in Europe.
Infectious disease is the big exception to the law that no intervention makes that big a difference (which is why it is curious that vaccines have routinely come in for so much more criticism than far less effective elements of the medical system.) Sanitation is one of the most effective methods of controlling childhood exposure to infectious disease agents. Height in European countries began climbing at around the same time as sanitation was brought under control:
So I don’t think that even very skeptical people found it hard to believe evidence that poor sanitation practices such as open-air defecation across India and other low-income countries was largely responsible for childhood stunting in these countries.
Even as a tourist to India fifteen years ago, I saw many more visibly stunted kids in cities in Uttar Pradesh with poor sanitation than in cleaner parts of the country.
A recent large-scale set of multi-year randomized trials in Kenya and Bangladesh suggests, however, that, even if poor sanitation is involved in the processes that produce childhood stunting, stunting isn’t immediately amenable even to quite intense intervention. You can’t just expose households to sanitation interventions targeted to them and expect the next generation of kids from those households immediately to be taller, and even the combination of nutrition with sanitation produces small effects, not distinguishable from nutritional supplementation alone. As one quite clear write-up in an Indian newspaper notes,
“The studies, which targeted over 13,000 families, showed that water purification, sanitary latrines and hand-washing (WASH) interventions in select households were not enough to prevent stunting in those households…Three groups received the three individual WASH interventions, while a fourth received nutritional counselling and dietary supplements for children. The fifth group received all three WASH interventions, the sixth received WASH as well as nutrition, while a seventh served as a control. Once the pregnant women gave birth, stunting, diarrhoea and mortality rates were tracked among their children for two years…After two years, the Bangladeshi study found children in the WASH groups to be no taller than controls. Improved diet did not make a big difference either – it corrected only a sixth of the height deficit in the nutrition groups. The Kenyan study reported similar findings.”
In Bangladesh, it seems as though the researchers are possibly observing genuine benefits- a reduction in diarrhea incidence though not stunting for the groups exposed to water purification, sanitary latrines, and hand-washing, although this reduction was not present for the Kenyan children.
One author suggests that “We have forthcoming evidence that illustrates that although we did reduce exposure to faecal organisms somewhat, there was a large reservoir in the environment that children continue to be exposed to.” This would seem to be plausible but, depending on how broadly distributed that reservoir of disease organisms is and how difficult they are to eradicate, potentially disheartening, if merely, for example, playing in the dirt exposes kids to sufficient numbers of bacteria to negate the effects of improved sanitation. An optimistic alternative is that city-wide improvements in sanitation will have immediate effects not possible from household-level interventions. Another idea, not mentioned, is the idea that improved sanitation and nutrition could in fact have large effects on height, but only over multiple generations, through selection for greater height. This seems to have occurred and to be still occurring in Europe.
IQ and height have a lot in common- similar heritability, for example, and a similar tendency to rise secularly during periods of economic development while being resistant to any individual intervention aimed at changing them. Sanitation is among the very few One Weird Tricks that really work – a true exception to the Iron Law. I’m guessing that nobody is going to find that improved sanitation doesn’t help reduce cholera epidemics, no matter how far the Replication Crisis extends. But even the best tricks can’t solve everything.