Regular handwashing with soap is believed to have substantial impacts on child health in the developing world. Most handwashing campaigns have failed, however, to establish and maintain a regular practice of handwashing. Motivated by scholarship that suggests handwashing is habitual, This study designed, implemented and analyzed a randomized field experiment aimed to test the main predictions of the rational addiction model. To reliably measure handwashing, the researchers developed and produced a novel soap dispenser, within which a time-stamped sensor is embedded. They randomized distribution of these soap dispensers as well as provision of monitoring (feedback reports) or monitoring and incentives for daily handwashing. Relative to a control arm in which households receive no dispenser, the study found that all treatments generate substantial improvements in child health as measured by child weight and height. The key test of rational addiction is implemented by informing a subset of households about a future boost in monitoring or incentives. It was found that (1) both monitoring and incentives increase handwashing relative to receiving only a dispenser; (2) these effects persist after monitoring or incentives are removed; and (3) the anticipation of monitoring increases handwashing rates significantly, implying that individuals internalize the habitual nature of handwashing and accumulate habit stock accordingly. The results are consistent with the key predictions of the rational addiction model, expanding its relevance to settings beyond what are usually considered `addictive' behaviors.