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.

Historian Raymond Callahan has characterized as "mechanical" his methods of dealing with student retardation, elimination, and promotion.

On behalf of the American Statistical Association in 1915 Ayres became secretary of the Joint Committee on Standards for Graphic Presentation, which was chaired by Willard C.

[6] On behalf of the Russell Sage Foundation, in April 1917 Ayres organized the Division of Statistics of the Council of National Defense.

It had no statistical office until early 1918, when Ayres's section came under military auspices and he was made a lieutenant colonel with a staff of fifty.

After making pessimistic analyses of the state of the economy in the late 1920s, for example, he was one of the few economists to argue the October 1929 stock market crash foreshadowed a great depression.

In the 1930s he argued in favor of public regulation of banking, minimized the negative side of abandoning the gold standard, and criticized the National Recovery Act, urging instead legislation to stimulate business to price and profit competition.