The secret, he pointed out to me, was that the soap was more than soap. It was a behavior-change delivery vehicle. The researchers hadn’t just handed out Safeguard, after all. They also gave out instructions—on leaflets and in person—explaining the six situations in which people should use it. This was essential to the difference they made. When one looks closely at the details of the Karachi study, one finds a striking statistic about the households in both the test and the control neighborhoods: At the start of the study, the average number of bars of soap households used was not zero. It
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