… is from page 55 of the great economic historian T.S. Ashton’s 1951 paper “The Treatment of Capitalism by Historians,” which is chapter 1 of the 1954 volume edited by Hayek, Capitalism and the Historians:
Ignorance of the elements of economic theory led historians to give political interpretations to every favorable trend. In scores of books the improvement in conditions of labor in the nineteenth century has been attributed to factory legislation; in hardly any is it pointed out that rising productivity of male labor had something to do with the decline in the number of children exploited in the factories or the number of women degraded in the mines.