A 2016 paper from DIW Berlin unambigously states:
In a sample of preschool children we find that longer breastfeeding duration is associated with higher levels of patience and altruism as well as a lower willingness to take risk. Repeating the analysis on a sample of young adults indicates that the observed pattern is replicable and persists into adulthood. Importantly, in both data sets our findings are robust when controlling for cognitive ability and parental socio-economic status. We can further rule out that the results are purely driven by nutritional effects of breastfeeding. Altogether, our findings strongly suggest that early childhood environment as measured by breastfeeding duration systematically and persistently affects preference formation.
Now given that substantial neurocognitive effects (read: IQ) of breastfeeding have been touted but not quite replicated, I'm pretty skeptical about the DIW paper ... but this one is perhaps more plausible because it concludes about potentially simpler behaviors, so the outcomes might be mediated (at a biological level) by some hormone(s) or neurotransmitter(s) and/or circuitry in a much more circumscribed area of the brain... and at social level (as hinted at the end of the abstract) breastfeeding could just be a proxy for a more "peaceful" (my term) environment, which could well affect the child's brain development.
