Angela Duckworth and Martin Seligman estimate that for every ten articles on intelligence and academic achievement, there has been fewer than one about self–discipline. Even so, the small body of research on self–discipline suggests that it has a significant impact on achievement. Walter Mischel and colleagues found in the 1980s that 4–year–olds' ability to delay gratification (for example, to wait a few minutes for two cookies instead of taking one cookie right away) was predictive of academic achievement a decade later.