“People are willing to adopt many health goods at a price of zero (or almost zero) but almost completely unwilling to adopt it at prices just slightly above zero,” they write. In India, a free bar of soap is snapped up; one costing a few cents languishes on shop shelves. The same tendency held true for studies involving deworming medicine, mosquito nets, and water disinfectants.

It’s not just that a $0 price tag made these services more affordable. The report authors argue that free, to consumers, implies that doing that health behavior—whether it’s putting up a mosquito net or getting a flu shot—is the social norm. Buying something is a choice, but getting something is like winning the lottery…

People were also less likely to over-think health decisions that were free—and thus less likely to change their minds about following through. And at no cost, they were more likely to provide them for family members. When anti-malarial mosquito nets were made free, for example, parents in developing nations were more likely to attach them to their children’s beds.