Several studies suggest that breast-fed babies become more intelligent children than formula-fed babies. One such study (Mortensen, Michaelsen, Sanders, & Reinisch, 2002*) involved a sample of over 3,000 women and men born in Copenhagen, Denmark, between October 1959 and December 1961. The samples were divided into five categories based on duration of breast-feeding, as assessed by physician interview with mothers at a one-year examination. The child's intelligence was assessed using the Wechsler Adult Intelligence Scale (WAIS) at a mean age of 27.2 years. The results showed that duration of breast-feeding was associated with significantly higher scores on the verbal, performance, and full scale WAIS IAs. This difference was observed even after the researchers adjusted for differences in the social class and maternal education of the two groups. (This adjustment allowed the researchers to rule out any preexisting differences in the groups that might have independently contributed to IQ differences in their children.) The authors acknowledged that other differences between the groups, such as the children's genetic potential or their parents' caregiving skills or motivation to nurture, could describe the results. However, they believe that human milk contains various hormones and other factors that enhance brain growth and maturation.

1. State the research hypothesis in your own words. Identify the independent and dependent variables, and define all important concepts and terms as they are used in this study.
2. What evidence do the researchers offer as a test of their hypothesis? Is this evidence empirical (observable)? Is it valid?
3. What explanation do the researchers offer for their findings? Does this explanation make sense based on the evidence?
4. Given the results of this study, why can't the researchers draw a causal connection between type of food and later intelligence? What might be an alternative explanation for the results of this study? What could the researchers do in order to make a causal connection between the dependent and independent variables?
5. Are there any practical implications for this research?