Returning to live in the United States, Ayres was made head of the division of statistics in the Playground Association of America. Beginning in 1908, he was prominently identified with the work of the Russell Sage Foundation, especially as chairman of the committee in charge of the Backward Children Investigation. He co-authored a highly influential book on "Laggards in Our Schools" (1909) with Luther Halsey Gulick. They argued that the most important causes of retardation were environmental. He continued his studies of intelligence tests, and drafted widely adopted recommendations for yardsticks of student progress on intelligence tests for the elementary schools. Ayres found that curricula were typically fitted for the brightest rather than average student. He found that schools with low withdrawal rates were most efficient and economical. Historian Raymond Callahan has characterized as "mechanical" his methods of dealing with student retardation, elimination, and promotion. Furthermore he largely ignored social and educational factors, and he used non-comparable data.