Recent studies, as summarized, for example, in E. Mortensen et al. (2002). The association between duration of breastfeeding and adult intelligence. Journal of the American Medical Association, 287, 2365–2371, suggest that breastfeeding of infants may increase their subsequent cognitive (IQ) development. Both experiments and observational studies are cited.

(a) What determines whether some of these studies are experiments?

(b) Name at least two potential confounding variables controlled by breastfeeding experiments.