IMPORTANCE: Potential effects of breast feeding on children's behaviour remains an elusive debate given inherent methodological challenges. Propensity score matching affords benefits by ensuring greater equivalence on observable social and health determinants, helping to reduce bias between groups. OBJECTIVES: We examined whether the duration of breast feeding had an impact on children's externalising and internalising behaviours. STUDY DESIGN: A cohort study (Encuesta Longitudinal de la Primera Infancia cohort) that included 3037 Chilean families who were enrolled in 2010. Follow-up data was collected in 2012. SETTING: General community. PARTICIPANTS: Population-based sample. Eligibility criteria: children born full-term with complete data on matching variables. Matching variables included: healthcare system as a proxy of income, presence of a partner/spouse in the household, maternal age, educational level, IQ, working status, type of work, diagnosis of prenatal depression by a healthcare professional, smoking during pregnancy, delivery type, child sex, weight at birth, incubation following delivery, and child age. EXPOSURE: Duration of breast feeding. MAIN OUTCOMES AND MEASURES: Externalising and internalising problems assessed using the Child Behaviour Checklist. RESULTS: Matched results revealed benefits of any breast feeding, up to 6 months, on emotional reactivity and somatic complaints (mean difference of -1.00, 95% CI, -1.84 to -0.16 and -1.02, 95% CI, -1.76 to -0.28, respectively). Children breast fed between 7 and 12 months also had reduced scores on emotional reactivity, in addition to attention problems (mean difference of -0.86, 95% CI, -1.66 to -0.06 and -0.50, 95% CI, -0.93 to -0.07, respectively). No benefits were observed for children breast fed 13 months or more. CONCLUSION: Reduced internalising difficulties and inattention were found in children breast fed up to a year, suggesting that breast feeding may have beneficial impacts on these areas of development. The magnitude of effect was modest. Extended durations of breast feeding did not appear to offer any benefits.