With so much information available to the public, the ability to sort out fact and fiction and to discern relevant and irrelevant facts is more important than ever. A good example is the difference between causation and correlation.
This week Corrie and I went to our first birthing class in preparation for the delivery of our first son (due July 26). Jen, our very kind and energetic instructor, was giving us some information about different decisions we would need to make. She strongly the nutritional value of breast milk over formula, stating that breast-fed babies are proven to be smarter than formula-fed babies.
Though I am in favor of breastfeeding, I am skeptical of her claim. First, it would be important that the surveys that substantiate the claim be conducted with a large enough sample–it’s simple enough to find one breastfed person who turns out to be smarter than one formula-fed person. Beyond sample size, it is also important to control for other factors (age, education and income of parents, nutrition beyond infancy, race, gender, heredity, etc.), in order to do an apples-to-apples comparison (not that infants are apples). Third, the difference in intelligence may not be statistically significant in light of other factors.
But even if it could be demonstrated that breastfed babies end up with higher IQs than formula-fed babies, this would not prove causation, only correlation. Perhaps causation runs the other direction: more intelligent parents tend to want to breastfeed their babies for some reason, and so the babies are breastfed because they are smart (or their parents are smart), rather than being smart because of breastfeeding.
Perhaps higher intelligence and breastfeeding are both results of another factor. This was my first thought when the subject was brought up. A stay-at-home mother who breastfeeds may be more likely to have a higher income, since her family can presumably sacrifice a second income at least for a little while. Income is connected with education. Alternatively, the benefit of breastfeeding may be non-nutritional: a mother who bonds with her child while breastfeeding may be more likely to continue to nurture the child in other ways as s/he grows up. Or, perhaps she was already so inclined, so that the breastfeeding and continued nurture have the same cause root cause.
Sorting out correlation and causation is complicated. There are also spurious correlations. My friend, Ann, is fond of a graph showing an inverse relationship between the number of pirates in the world and the global temperature. Maybe the solution to global warming is beefed-up batch of buccaneers.