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118 Whether or not, as some economists believe, a too-rapid expansion of commercial and industrial plant or the fear of it, was a principal cause of the depression, there is apparently some support in these figures for the thesis that the ensuing decline in private construction activity was an umportant contributing factor. Certainly, however, there is no evidence in the trend of public construction, as shown in table 22, to indicate that an excessive expenditure for public works was in any degree responsible; because such expenditures expressed as a percentage of the national income rose scarcely at all in the periods from 1915 to 1926, and not sufficiently to offset the decline in private construction as the depression approached in the next 4 years.

The figures of table 22 may suggest that an expenditure for all classes of construction and maintenance work approaching 18 percent of the national income is somewhat excessive. A ratio of 15 percent, approximating the 28-year average of the period 1915–42, inclusive, and the 4-year average of the period 1939-42, might represent a safe and perhaps sustainable relation.

But if the maintenance of some such relation is assumed to be desirable, to be accomplished by an increase of public construction as private construction decreases, it will be seen from table 22 that the combined measures of Federal, State, and local governments, taken prior to, the outbreak of the war in Europe, failed signally to attain that end.

The slight increase in the average ratio of the normal public construction expenditure to the national income, which measured the effort of the 4 years from 1931 to 1934, was sufficient only to raise the ratio for all construction to 11.6 percent of the national income; and even with the further addition of the early Federal relief expenditure, the percentage was raised only to 11.9, an average lower than the lowest previous 4-year average, recorded in the period from 1915 to 1918, inclusive.