Ideas of this kind did not merely reflect American experience in the postwar period. M. M. Postan, in his Economic History of Western Europe, 1945-64, has observed that the ascent of the European economy after 1945 was steeper and smoother than in any other period of modern history. Between 1918-38, he calculates, productivity in Western Europe increased at a rate of 1-7 per cent per annum, whereas between 1945-63 it increased at more than twice that rate, 3-5 per cent per annum. The aggregate G N P of Western Europe (at constant prices) was 2\ times greater in 1963 than in 1938. At the same time this postwar boom was much less interrupted by recurrent crises than was the case in any earlier period. The causes of this vigorous and more or less continuous growth are not entirely clear, but two important factors, according to Postan, were the very high rate of investment (much of which was government or government-sponsored investment), and the rapid and widespread advances in technology. Postan also suggests how an 'ideology of growth' emerged, not simply as a reflection of actual growth, but also arising out of earlier preoccupations with full employment which had their source in the controversies of the 1930s. 'Full employment', he writes, 'eventually developed into a policy and an economic philosophy much wider in its implications . . . into a policy of economic growth.'