纳瓦罗作为经济学家水平怎么样？ 第1页

当年Trump竞选的时候，我关注的一些经济学家博客都对Navarro穷追猛打。特别记忆犹新的一件事是，Trump当时提出：为了增加GDP，我们应该减少进口，增加出口；而为这个建议背书的就是Navarro。这一建议遭到了几乎所有写博客的经济学家的嘲笑，因为Trump和Navarro的理由是，根据恒等式

GDP=C+I+G+NX， 即GDP = 消费 + 投资 + 政府支出 + 净出口

NX = EX - IM， 即净出口 = 出口 - 进口

所以只要通过种种方式（比如关税）增加出口，减少进口，我们就能提高净出口，就能提高GDP。

但是问题在于这个恒等式说到底是一种会计恒等式，换言之我们只是可以用右边这些来计算左边，而不是说左边取决于右边。增加出口，就意味着要从国民消费的本国产品中拿走一部分卖给国外，C就减少了；减少进口，就意味着不让国民消费一部分外国产品，C也减少了，最终GDP并不会因此改变。

Update:

注意这里的问题不在于增加出口减少进口是否能够增加GDP，实际上我国在改革开放时期的“三来一补”政策，包括我国长期用较低人民币汇率、出口补贴换取出口优势的政策都有效地提高了我国的GDP。问题在于这么做的逻辑不能是一个会计恒等式……

Update(04/17):

看到有位知友 @尾班车 说我把特朗普和纳瓦罗当别人家的傻子，但我一开始就说了，这个观点是转述其它经济学家的，所以并不是我把他们当傻子，而是其它经济学家把他们当傻子……然后我刚才去翻了一下，找到了一些当时的资料：

首先原话是这样的：

Trump proposes eliminating America's $500 billion trade deficit through a combination of increased exports and reduced imports.

Marginal Revolution引用了这段话，评论区讨论激烈：

The New Yorker的一篇文章：

This is a tautology, equivalent to saying one plans to weigh less in the future through a combination of losing weight and not gaining weight.

Greg Mankiw的博客文章：

Their analysis of trade deficits, starting on page 18, boils down to the following: We know that GDP=C+I+G+NX. NX is negative (the trade deficit). Therefore, if we somehow renegotiate trade deals and make NX rise to zero, GDP goes up! They calculate this will bring in $1.74 trillion in tax revenue over a decade.
But of course you can't model an economy just using the national income accounts identity. Even a freshman at the end of ec 10 knows that trade deficits go hand in hand with capital inflows. So an end to the trade deficit means an end to the capital inflow, which would affect interest rates, which in turn influence consumption and investment.
I suppose that their calculations might make sense in the simplest Keynesian Cross model, in which investment is exogenously fixed and consumption only depends on income. But that is surely not the right model for analyzing the impact of trade policy over the course of a decade.