It is widely accepted that investing in early childhood helps build the foundations of a healthy, productive, and equitable society. Guided by that knowledge, a range of broad-based programs and targeted services make a significant difference for millions of children, yet a closer look at outcome data shows that some benefit greatly, some benefit less, and some not at all. Within this variation lies opportunity. Increasing effects for all children--especially those who currently benefit the least--may be the key that unlocks greater impacts at scale. Building upon science presented in the two previous working papers from the National Scientific Council on the Developing Child ("Connecting the Brain to the Rest of the Body: Early Childhood Development" and "Lifelong Health Are Deeply Intertwined and Place Matters: The Environment We Create Shapes the Foundations of Healthy Development"), this paper examines two types of variation. The most commonly addressed refers to differences between demographic groups (e.g., income, parent education, race/ethnicity), which are heavily influenced by structural inequities. The second, which science tells us needs increased attention, refers to individual variation within and across groups that is explained by how the effects of a variety of experiences are influenced by each child's unique genetic makeup and the timing of the exposure, beginning prenatally. This paper calls for a significant shift in how we design, implement, and evaluate policies and programs. It explains how expecting individual variation in effectiveness, and planning for flexible implementation, avoids false stereotypes based on income or race and is likely to achieve larger impacts for all children and thus generate greater returns on investment for society.