Most of the correlational and experimental studies that have tested the hypothesis that mild-to-moderate protein-energy malnutrition (PEM) has an adverse effect on cognitive development disregarded the potential confounder effect of micronutrients. This omission may have been a critical flaw in study design because it is now recognized that iron deficiency increases the probability of deviations in the trajectory of children's motor and mental development from a normal developmental path. This paper discusses two frequently cited studies on the effects of PEM on neurointegrative and cognitive development and proposes that neither study can discard the hypothesis that effects attributed to protein and energy deficiency are, instead, determined by iron deficiency.