One of the central arguments made against increasing public funds for preschool, like President Obama’s $75 billion Preschool for All proposal, rests on the “fade-out” effect. Researchers have repeatedly observed that the cognitive gains children make from attending preschool—whether measured by IQ, vocabulary, or math and reading tests—appear to fade out by the time they finish the primary grades, if not sooner. Some policymakers have used this observation to argue that if preschool has such negligible impact, public funds should not support it.

At the same time, educators have long wondered why disadvantaged children who attend pre-school initially outperform their peers in math and reading, but tend to lose their academic edge by the end of third grade. Research indicates three likely root causes—some of which may have little to do with preschool itself and more to do with what happens to young children once they enter elementary school.

“Catch up” Effect in Elementary

In 2011, Steven Barnett of Rutgers University published an article in Science that examined the effectiveness of preschool interventions in the U.S. and in two developing countries. The article suggested the fade-out effects early in children’s school careers could be associated with schools’ efforts to remediate early elementary students who had not attended preschool. Barnett suggested the decrease in the achievement gap between preschool graduates and children who did not attend preschool is less about the fading out of preschool students’ skills and more about the catching up other students are doing. International studies of preschool impact in developing countries showed no pattern of declining effects, which Barnett suggested could indicate their systems of primary education were less able to help students catch up than primary education in the United States.

Researcher Katherine A. Magnuson has tried to zero in on the aspects of classroom instruction that are likely to affect whether children who did not attend preschool catch up with preschool graduates in reading and math achievement. In a 2007 paper, she and colleagues Christopher Rhum and Jane Waldfogel showed that small class sizes (less than 21 students) and more than 90 minutes of daily reading instruction help children who did not attend preschool catch up to their peers, while larger classes with lower levels of instruction maintain the gap between preschool graduates and children who did not attend preschool. But more research is needed to observe the specific teaching strategies that help children who did not attend preschool catch up and could challenge preschool graduates to persist in achieving at high levels.

The Importance of Elementary School

The quality of children’s educational experiences in elementary school affects both whether children who missed preschool catch up and whether preschool graduates’ academic gains persist. In 2000, researchers Janet Currie and Duncan Thomas published a paper showing that while white children who attended Head Start showed gains in vocabulary and reading skills that persisted into adolescence, African American Head Start attendees’ gains faded out quickly. They were also able to show that African American Head Start graduates went on to attend lower-quality elementary schools, as measured by student test score performance. Their findings suggested that improving the quality of elementary schools attended by African American Head Start graduates could help maintain their initial academic gains.

Preschool Quality Matters

While the quality of a child’s later elementary school matters for measuring preschool’s long-term benefits, the quality of the preschool experience itself matters, too. In 2010, a group of researchers led by Gregory Camilli of the University of Colorado at Boulder analyzed 123 comparative studies of U.S. preschools and found that preschool programs provided significant, long-term benefits for children’s learning and social skills. However, the study also found that whatever effect an individual preschool program has on children’s cognitive skills—as measured by gains in standardized test scores—that effect declines over time in elementary school. The ultimate impact of preschool on children’s cognitive skills appears to level off to about half of its initial effect. Thus, in order to have greater staying power, preschool programs need to help children make very large up-front gains in their learning and academic skills. Although existing research lacks clear documentation of the relationship between preschool program design and children’s outcomes, teacher practices associated with greater positive effects on cognition included direct instruction, small-group teaching, and individual interaction between teacher and student. Camilli notes that the rapidly expanding universe of state pre-kindergarten programs offers an important opportunity to develop experimental research studies that would better illustrate the relationships between preschool program designs and their effectiveness.

Preschool advocates note that cognitive gains aren’t the only benefits of preschool, and may not be the most significant or long-lasting. It’s the social-emotional skills children learn—how to get along and play nicely with other children, how to focus and persist on a task—that may provide the greatest benefits. For example, while the well-studied Perry Preschool Program alumni showed no long-term differences in IQ compared to eligible children who did not attend, their life outcomes differed sharply. Perry graduates had higher graduation rates, higher earnings, and fewer arrests. It’s these benefits that drive social return on investment, perhaps even more than strictly cognitive gains, Nobel-prizewinning economist James Heckman has observed.

All of this means that while researchers design stronger studies to guide policy on preschool, policymakers and educators can still learn valuable lessons. Elementary schools can use smaller class sizes and spend more time on reading to level the playing field between children who have and have not attended preschool—and can consider accelerating learning for children who enter with the preschool advantage. Preschools can also take steps to give their children bigger, more sustainable cognitive boosts by promoting conversation and intellectual exchange with teachers, both one-on-one and in small groups.

Maureen Kelleher is a Chicago-based freelance writer. She has written about early childhood for Education Week.