作者：克鲁格曼
马克托马斯说他考虑过这个问题，实际上我也在思考。不管怎样，一项有效的财政刺激方案，它会增加政府债务，但它也可以（也必须）比媒体宣传的成本要低廉，这是确信无疑的。
假定利率是固定的（现在这是一个好的假设，因为利率正面临零界点），政府支出增加，那么教科书就会说，若刺激方案是dG，GDP的增加幅度将会是1/(1-c(1-t))。在这里，c表示边际消费意向，t是指边际税率。假设c值为0.5，t为1/3，则乘数即为1.5。这或多或少体现了传统的经济思维。
但是，如果1000亿美元的刺激方案会拉动GDP增长1500亿美元，而且边际税率为1/3，那么，意味着有500亿美元会流入额外收入中。故低成本高收效的的财政刺激政策，其每一美元的负债增幅带来的GDP增长倍数是3，而非1.5。因为对刺激方案的主要担忧是其会增加政府负债，所以与之相关的是低成本高效率的刺激方案，而非乘数效应。3看起来也比1.5好多了。
进一步思考，1500亿美元相当于GDP的1%，而这，用罗默和伯恩斯坦的说法，意味着一百万个工作机会，即5万美元创造一个工作机会，这远比评论家大肆批评的要好得多（再者，我们指的是更多的全职工作，而不是兼职）。
低成本高效率也提升了有效刺激方案和无效刺激方案的对比高度。我们依然假定，c=5，t=1/3，然后看一下税率削减的效果：乘数为0.75，相当于社会公共投资的一半，但是低成本高收效值为1，对投资只有1/3。
所以考虑一下刺激方案如何通过收入回流是很重要的。
http://krugman.blogs.nytimes.com/
Mark Thomas says he was thinking about this; I was actually thinking about it. Anyway, it’s true: the cost of an effective fiscal stimulus, in terms of adding to the government’s debt, can (and should) be much less than the headline cost.
Consider an increase in government spending; assume that the interest rate is fixed (a good assumption right now, because interest rates are up against the zero lower bound). Then textbook analysis says that if the stimulus is dG, the increase in GDP is 1/(1 - c(1-t)) where c is the marginal propensity to consume out of income and t is the marginal tax rate. Suppose c is 0.5 and t is 1/3; then the multiplier is 1.5, which is more or less the conventional wisdom right now.
But if $100 billion in spending raises GDP by $150 billion, and the marginal tax rate is 1/3, $50 billion of the spending comes back in additional revenue. So bang for the buck — increase in GDP per dollar of added debt — is 3, not 1.5. Since the main concern about stimulus is that it will add to government debt, it’s this bang for the buck measure, rather than the multiplier, that’s relevant. And 3 sounds a lot better than 1.5.
Take this a bit further: $150 billion is about 1 percent of GDP, which Romer and Bernstein say means a million jobs; so this says $50,000 per job, which is a much better number than the critics have been throwing around (plus many more workers with full-time rather than part-time jobs).
Bang for the buck also heightens the contrast between effective and ineffective stimulus policies. Stay with c = 0.5, t = 1/3, and look at the effects of a tax cut; the multiplier is 0.75, half that for public investment, but bang for the buck is 1, only 1/3 that for investment.
So thinking about how stimulus comes back via revenues is important.