is the Dialy mail trying to redeem itself reporting this research here
but in the article is it breast milk that itself has the effect or "However, they said they cannot be sure that the effect is due to breastfeeding itself. It may be that women who breastfeed are more likely to have the sort of nurturing personality that leads to their children becoming less stressed than others."
Apparently it held true as a stress buster regardless of otherr socio economic factors which is interesting. It is a swedish study.