Jason Furman of the Council of Economic Advisers gave an illuminating talk on the sources of weak business investment, largely aimed at refuting the “Ma! He’s looking at me funny!” school, which attributes US economic weakness to the way the Obama administration has created uncertainty, or hurt businessmen’s feelings, or something. As Furman shows, it’s a global slowdown, very much consistent with the “accelerator” model in which the level of investment demand depends on the rate of growth of overall demand.

It seems worth pointing out, or actually reiterating, several implications of this analysis that go beyond Obama-bashing and its discontents.

First, if weak demand leads to lower investment, which it does, and if fiscal austerity is contractionary, which it is, then in a depressed economy deficit spending doesn’t crowd investment out — it crowds investment in. Or to be more explicit, austerity policies don’t release resources for private investment — they lead to lower private investment, and reduce future capacity in addition to causing present pain. Conversely, stimulus in times of depression supports, not hinders, long-run growth.

Second, secular stagnation — persistent difficulties in achieving full employment — is a real concern if potential growth is slowing due to a combination of demography and weak technological progress, which seems to be happening. Lower growth means lower investment demand, so getting the private sector to spend enough gets harder.

Finally, an extreme case of this arises in China, where the exhaustion of the reserve of underemployed peasants plus, perhaps, a slowdown in the rate of technological catchup means that the very high investment rates of the past can’t be sustained. Look out below.