Could we persuade more mothers to breastfeed if we told them that their children would not only be more intelligent, but also earn more money? A well-publicised study from the city of Pelotas, Brazil, suggests that this is true (Victora CG et al. Lancet Glob Health 2015;3:e199–205). They recruited over 5900 babies born in 1982 and were able to study nearly 3500 of them at the age of 30 years. They already had demographic data on their mothers, and at age 30 they tested their IQ and gathered data on duration of education …