Why aren't developed countries saving?

National saving rates differ enormously across developed countries. But these differences obscure a common trend, namely a dramatic decline over time. France and Italy, for example, saved over 23% and 19% of national income in 1970, but only 9% and 4% respectively in 2008. Japan saved almost 33% in 1970, but only 7% in 2008. And the U.S. saved around 11% in 1970, but only 1% in 2008. What explains these international and intertemporal differences? Is it demographics, government spending, productivity growth or preferences?