<P> Barber says, while there is "no clear evidence" of a decline in "the rate of growth of productivity" during the 1920s, there is "clear evidence" the population growth rate began to decline during that same period . He argues the decline in population growth rate may have caused a decline in "the natural rate of growth" which was significant enough to cause a serious depression . </P> <P> Barber says a decline in the population growth rate is likely to affect the demand for housing, and claims this is apparently what happened during the 1920s . He concludes: </P> <P> the rapid and very large decline in the rate of growth of non-farm households was clearly the major reason for the decline that occurred in residential construction in the United States from 1926 on . And this decline, as Bolch and Pilgrim have claimed, may well have been the most important single factor in turning the 1929 downturn into a major depression . </P> <P> The decline in housing construction that can be attributed to demographics has been estimated to range from 28% in 1933 to 38% in 1940 . </P>

What caused the start of the great depression