persistently poor (Duncan et al., 1994). On average, family incomes increase as children age, but average patterns conceal a great deal of year-to-year volatility, making it important to consider how economic resources at different points during the childhood years affect development. The malleability of young children's development and the overwhelming importance of the family (rather than school or peer) context suggest that economic conditions in early childhood may be far more important for shaping children's ability, behavior, and achievement than conditions later in childhood.

Efforts to understand the developmental effects of poverty have relied on both experimental and nonexperimental studies. Experimental designs involving manipulation of family incomes are extremely rare. In four income maintenance experiments in the 1960s and 1970s, experimental treatment families received a guaranteed minimum income. Impacts on pre-school children, however, were not assessed. School performance and attendance were affected positively in some sites for school-age children, but not for high school adolescents. In two sites reporting high school completion and advanced education, these were higher for the experimental