Health and nutritional risks co-occur in the lives of children under the age of 2 years who live in developing countries. We review evidence showing how these risks, in addition to inadequate psychosocial stimulation, prevent children from developing expected cognitive and language abilities. A systematic review and meta-analysis of 21 interventions aimed at enhancing stimulation and 18 interventions that provided better nutrition--all conducted since 2000--revealed that stimulation had a medium effect size of 0.42 and 0.47 on cognitive and language development, respectively, whereas nutrition by itself had a small effect size of 0.09. The implementation processes of these interventions are described and compared. A number of unresolved issues are outlined and discussed, including ways to maximize parental health behavior change, assess mediators that account for intervention effects, and expand the assessment of young children's brain functions that underlie language and cognition and are affected by nutrition and stimulation.
Keywords: behavior change; child development; developing countries; low- and middle-income countries; meta-analysis and systematic review; nutrition; psychosocial stimulation.