2010-07-24

假大空經濟學?

大家可能知道,經濟學分微觀和宏觀經濟學。前者(即Microeconomics)研究的是個人的經濟行為,後者(即Macroeconomics)看的是總體的經濟表現。

經濟學雖然好像也理應跟民生民眾息息相關,但由於種種原因,這門“科學”發展至今,一般人(即使是其他領域的學者)幾乎一定讀不懂經濟學者的學術文章。

近日,美國眾議院科技委員會底下的一個小組為經濟學舉行了一場聽證會,找來五位學者為“建構一門與現實世界攸關的經濟科學”(Building a Science of Economics for the Real World)發表意見。聽證會的由來,是因為政客覺得在當前經濟危機下,理該審視現代宏觀經濟學的期許與缺失(examine the promise and limits of modern macroeconomic theory in light of the current economic crisis)。

這幾篇發言,雖不算上乘之作,但也不無洞見。再者,篇篇易懂易明,美國政客跟我們這些小市民讀來應有得著。其中特別推薦Robert Solow精簡發言。不要看Solow已80多歲,從發言看,這位為現代經濟學的建立與發展立下汗馬之功的元老級學者思路仍很清晰。

宏觀經濟學=假大空經濟學?宏觀經濟計量模型=脫離現實自說自話的數學遊戲?自己下結論吧。

Economic theory is always and inevitably too simple; that can not be helped. But it is all the more important to keep pointing out foolishness wherever it appears. ... I do not think that the currently popular DSGE models pass the smell test. They take it for granted that the whole economy can be thought about as if it were a single, consistent person or dynasty carrying out a rationally designed, long-term plan, occasionally disturbed by unexpected shocks, but adapting to them in a rational, consistent way. I do not think that this picture passes the smell test. The protagonists of this idea make a claim to respectability by asserting that it is founded on what we know about microeconomic behavior, but I think that this claim is generally phony.

The point I am making is that the DSGE model has nothing useful to say about anti-recession policy because it has built into its essentially implausible assumptions the “conclusion” that there is nothing for macroeconomic policy to do. I think we have just seen how untrue this is for an economy attached to a highly-leveraged, weakly-regulated financial system. But I think it was just as visibly false in earlier recessions (and in episodes of inflationary overheating) that followed quite different patterns. There are other traditions with better ways to do macroeconomics.

One can find other, more narrowly statistical, reasons for believing that the DSGE approach is not a good way to understand macroeconomic behavior, but this is not the time to go into them. An interesting question remains as to why the macroeconomics profession led itself down this particular garden path. Perhaps we can come to that later.