There are firstly what may be called the direct results. Prime costs of production do not increase as rapidly as prices, and supplementary costs rise even less rapidly than prime costs. Prices rise faster than wages and interest charges, and rents tend to remain fixed by leases and other arrangements. Especially in the first year or two of rising prices, the rise in wages tends to be slow; in the later stages it ordinarily becomes more rapid.[41] Thus Mitchell in his study of wage and price movements during the Greenback Period in the United States (1860-80) writes that "... The table shows an almost universal rise of wages during the war--though a rise far from equal to the advance of wholesale or retail price."[42] And in his study of price and wage movements from 1890-1910 in the United States he writes, "The figures indicate that the prices of labor are influenced by changes in business conditions, but in less measure than the price of commodities, even at retail. The general average declines after the panic of 1893, recovers in 1896, advances in 1898-1903, makes very little gain in the dull year of 1904, and then rises rapidly again in 1904-7. But the degree of rise and fall is considerably less than that of commodities at wholesale and just about the same as that of food at retail."[43] The lag of wages behind prices varies in degree in different industries and occupations, for neither prices nor wages go up uniformly. The general direction of wage change is marked, but there is nevertheless considerable variation in the amount of wage change.[44] These variations in wage change are to be explained by the fact that the wage earners tend to fall into groups whose economic fortunes are in some measure independent of each other. They therefore are only slowly affected by changes in each other's position. On the other hand, since the increase in expenses of production in most industries tends to lag behind the rise in the price obtainable for products, profit returns increase during such periods, especially in industries in which the wages bill is an important part of the expenses of production. To quote Mitchell again, "The net resultant of these processes is to increase profits. Of chief importance is the fact that supplementary costs rise slowly in comparison with the physical volume of business.... In many instances prime costs also lag behind selling prices on the rise...."[45]