Science provides two explanations for why the same conditions affect children differently. The first, between-group differences, categorizes people according to demographic factors (e.g. parent education, family income, race, ethnicity, and community context) and focuses on differences between groups in terms of access to opportunities and outcomes in health, educational achievement, and lifetime earnings. Within every demographic group, however, outcomes among individuals also vary widely. This is the concept of within-group variation. Many programs attempt to address between-group differences, but few account for variation within groups. Service providers often make adjustments for individual children, but lessons learned are not disseminated widely or built into policies and systems. Programs and policies that are designed, implemented, and evaluated to account for variation in their effectiveness are better positioned to achieve larger benefits for society than current best practices because improved outcomes for more children will boost overall impacts. (author abstract)