SODIS is a cheap method of disinfecting water by putting it in the sun. Like many things, it works better in physics than society, where its effects were not significant, according to a study in PLoS medicine recently. The technical barrier is that people don’t do it much. About thirty two percent of participants in the study used the system on a given day. If you’re familiar with how little things work in reality, this is still surprising. Cheaply disinfecting water seems like it would be a hit with poor people whose children get diarrhea all the time and regularly die. Rural Bolivia, where the study was done, is a good candidate. The children studied usually get diarrhoea four times a year, which causes about fifteen percent of deaths of children under five. For the poorest quintile in Bolivia the under five death rate is about one in ten of those born alive.
The leader of the study, Daniel Mausezahl, suspects a big reason for this is that lining up water bottles on your roof shows your neighbors that you aren’t rich enough to have more expensive methods of disinfecting water. It’s hard to see from a distance the difference between chlorination and coliform-infested jerry cans, so drinking excrement can make you look better than drinking cheap clean water.
Fascinating as signaling explanations are, this seems incredible. Having live descendents is even more evolutionarily handy than impressing associates. What other explanations could there be? Perhaps adults are skeptical about effectiveness? There is apparently good evidence it works though, and there were intensive promotional campaigns during the study. What’s more, lack of evidence doesn’t usually stop humans investing in just about anything that isn’t obviously lethal in the absence of effective means to control their wellbeing. And parents are known for obsessive interest in their children’s safety. What’s going on?