A long-term study has pointed to a link between breastfeeding and intelligence.
The research in Brazil traced nearly 3,500 babies, from all walks of life, and found those who had been breastfed for longer went on to score higher on IQ tests as adults.
Experts say the results, while not conclusive, appear to back current advice that babies should be exclusively breastfed for six months.
But they say mothers should still have a choice about whether or not to do it.
Regarding the findings - published in The Lancet Global Health - they stress there are many different factors other than breastfeeding that could have an impact on intelligence, although the researchers did try to rule out the main confounders, such as mother's education, family income and birth weight.
Dr Bernardo Lessa Horta, from the Federal University of Pelotas in Brazil, said his study offers a unique insight because in the population he studied, breastfeeding was evenly distributed across social class - not something just practised by the rich and educated.
Most of the babies, irrespective of social class, were breastfed - some for less than a month and others for more than a year.
Those who were breastfed for longer scored higher on measures of intelligence as adults.
They were also more likely to earn a higher wage and to have completed more schooling.