A study of more than 6,000 Australian mothers found that those who breastfed for at least four months were likely to have a stronger bond, while babies that were bottle fed were 2.5 times more likely to be maltreated by their moms, according to this USA Today report.
Lead researcher Lane Strathearn of the Baylor College of Medicine thinks this may be because the oxytocin released during breastfeeding strengthens the mother's bond with the baby, making her likely to treat it better. And after all, you could see the adaptive advantage of love as making it more likely a baby would survive, because the parents wanted to be together and cooperate in taking care of it.
However, as the article points out, it's also possible that this study's result is due to the mothers who were perseverant and dedicated enough to breastfeed would have been likely to take better care of their infants anyway.
The article also takes care to -- politically correctly -- emphasize that bottle-feeding moms can be just as good mothers as the breastfeeders.