Studies that look at how different methods of feeding affect children have consistently attested the positive effects of breastfeeding, which was found to correlate with better physical and mental health later in life. These findings have led to many national governments, inter- and non-governmental organizations recommending breastfeeding as the preferred way to nourish an infant. However, when studying the differences between children that were breastfed and not breastfed within the same family, almost all of these positive correlations disappear. This finding is explained by selection bias, i.e. the fact that studies do not randomly assign which mother breastfeeds and which doesn't and therefore are vulnerable to the fact that - at least in the context of the United States and other Western nations - those richer and with more socioeconomic capital choose to breastfeed in higher numbers.
A study published in 2005 in Health Services Research compared U.S. sibling pairs as well as unrelated U.S. children for the positive effects breastfeeding might have on them in adolescence. Out of 13 indicators, there was a positive correlation for seven significant factors in the between-family sample, including a positive correlation between breastfeeding and a better GPA, vocabulary and academic performance as well as better weight and depression outcomes. When comparing the effect of breastfeeding among children of the same family, only a positive correlation with better vocabulary scores remained. No correlation was found for either group for positive outcomes concerning diseases like asthma, allergies or diabetes as well as other indicators asking about good family relations.
Breast milk does have some benefits, namely that it is the ideal food for babies in its composition and also contains small quantities of hormones and other bioactive ingredients that formula cannot mimic. However, as many babies end up not being breastfed for a variety of reasons, the availability of these components doesn't seem crucial and is potentially overlaid by the larger socio-economic context a baby is born into, at least when observing the average child in a large study group. It is sometimes also pointed out that breast milk is superior as it is free, but critics have pointed out that the time invested in breastfeeding is actually costing families if the mother works, once more indicating that breastfeeding can be seen as a privilege in some contexts.
While the studied group of unrelated children was much larger at more than 20,000, the makers of the study also employed their around 5,400-strong sibling sample to calculate correlations independent of family status. Here, they also found that breastfeeding correlated positively with much of the same factors as in the larger study group, showing a variety of positive outcomes seemingly tied to breastfeeding.
