It's a shame that in dividing their cohort into 'ever breastfed' and 'never breastfed', the authors seem unaware of previous research indicating the 'dose-related' effect of breastfeeding. They also don't acknowledge research suggesting that genetic variation in the ability to metabolise certain fatty acids may be significant.

While it would be very interesting to examine the neuro-developmental effect of breastfeeding, from toddlerhood to adolescence, a study where the average period of breastfeeding in the 'ever breastfed' group is only four months, and where we have no information on the predominance or exclusivity of breastfeeding, is unlikely to reveal anything useful. Including in the 'ever breastfed' group babies whose breastfeeding experience is likely to have been only days (or even less), may well mask any effects that might have emerged in babies whose breastfeeding lasted a lot longer.

A better conclusion would be 'short, undetermined periods of breastfeeding show no long-lasting impact on cognitive development in twins'.