It’s quite an interesting concept, and considering the tremendous health differences that exist between babies that are breastfed and those that are formula-fed, it is very possibly a causative effect. The article said that the researchers asked parents about breastfeeding duration and also noted subsequent mental health problems, and found that children who were breastfed as infants were less likely to be diagnosed with some sort of mental health problem, and that the longer they nursed, the lower the likelihood of such a diagnosis.
However, it may be an association, not a causation — women who breastfeed, especially those who breastfeed longer, may treat their children differently from women who formula-feed. I remember reading several months ago about a study released that showed that women who talked on the phone a lot had children with more behavioral problems. Some people immediately suggested that there wasn’t a physical reason for that — i.e., that cell phone signals messed up their child’s brain in some way (either in utero or after birth) — but rather that women who talk on the phone constantly may ignore their children, and without proper parental direction, these children may have problems. A woman who nurses must hold her child in a cuddling and nurturing position; a woman who bottle-feeds can leave the baby alone with the bottle in the crib, car seat, etc., and never or rarely hold the child. (I’m not suggesting that all bottle-feeding women are heartless, just pointing out an obvious possibility.) It may be that there is more of a causation with the cuddling and nurturing during a feeding (which can be done with bottle-feeding) than with the actual makeup of breastmilk.
Still, whatever the reason, with all of the benefits of breastfeeding to both mother and baby, this is just one more reason for promoting breastfeeding, and reducing the reasons why women may not breastfeed.