星期五, 9月 16, 2016

[經言經語] "The Trouble With Macroeconomics" by Paul Romer

[經言經語] https://www.facebook.com/macroecontalk/

Paul Romer 為紐約大學商學院教授, 2016 年 7 月開始擔任世界銀行首席經濟學家, 為開創內生成長理論的重要人物 (有興趣的讀者可進一步閱讀 《知識與國富論--一個探索經濟成長的故事》http://www.readingtimes.com.tw/timeshtml/ad/BE0152/ ), 亦為近年來諾貝爾經濟學獎呼聲頗高的候選人之一。他近來對於總體經濟模型的發展多所評論, 在 2016 年 9 月 14 日發表的一篇名為 "The Trouble With Macroeconomics" 的論文中, 就對始於 Ed. Prescott 的實質景氣循環 (RBC) 模型, 現今集大成於動態隨機一般均衡 (DSGE) 模型的當代總體模型做出許多批評。他將現在的總體模型稱為"後現實"(Post-Real)模型, 原因在於, 當代總體理論太過仰賴於無可名狀的隨機衝擊作為景氣波動的解釋。

Paul Romer 的文字酸意十足, 實不下於廣大鄉民, 茲摘錄幾段:

"Macroeconomists got comfortable with the idea that fluctuations in macroeconomic aggregates are caused by imaginary shocks, instead of actions that people take, after Kydland and Prescott (1982) launched the real business cycle (RBC) model."

"Proponents of the RBC model cite its microeconomic foundation as one of its main advantages. This posed a problem because there is no microeconomic evidence for the negative phlogiston shocks that the model invokes nor any sensible theoretical interpretation of what a negative phlogiston shock would mean."

"Once macroeconomists concluded that it was reasonable to invoke an imaginary forcing variables, they added more. The resulting menagerie, together with my suggested names now includes:

• A general type of phlogiston that increases the quantity of consumption goods produced by given inputs

• An "investment-specific" type of phlogiston that increases the quantity of capital goods produced by given inputs

• A troll who makes random changes to the wages paid to all workers

• A gremlin who makes random changes to the price of output

• Aether, which increases the risk preference of investors

• Caloric, which makes people want less leisure"

"With the possible exception of phlogiston, the modelers assumed that there is no way to directly measure these forces. Phlogiston can in measured by growth accounting, at least in principle. In practice, the calculated residual is very sensitive to mismeasurement of the utilization rate of inputs, so even in this case, direct measurements are frequently ignored."
"To allow for the possibility that monetary policy could matter, empirical DSGE models put sticky-price lipstick on this RBC pig. The sticky-price extensions allow for the possibility that monetary policy can affect output, but the reported results from fitted or calibrated models never stray far from RBC dogma. If monetary policy matters at all, it matters very little."

稍微解釋一下。在實質景氣循環模型中, 所謂的技術性衝擊 (technology shocks) 被視為最重要的景氣波動來源。然而, 總體經濟學家無法對於技術性衝擊給予一個具說服力的解釋。尤其是難以解釋什麼是時常發生的負向的技術性衝擊 (總不能老是用天災跟戰爭來搪塞)。因此, Paul Romer 將這種總體經濟學家假設的外生衝擊比喻成燃素 (Phlogiston), 並稱之為 phlogiston shocks。

燃素理論(Phlogiston theory)是一個已被取代的化學理論, 起源於17世紀。 這個理論假設, 任何物質在燃燒時, 都會釋放出一種名叫燃素(Phlogiston) 的成分。因此, Paul Romer 是以 phlogiston shocks 的稱呼來諷刺總體經濟學家。