保羅·梅森：主流經濟模型讓世界陷入危境

保羅·羅默（Paul Romer）曾憑一紙研究震驚經濟學界，此事至今已有26年。羅默那篇名為《內生性技術變革》（Endogenous Technological Change）的文章，揭示了信息技術是如何從根本上改變世界的——它把經濟學的關注點從土地、勞動力和資本，轉移至“人、思想（指技術創新）和事物”。（保羅·羅默到底有多牛，不熟悉的請戳）

近三十年之後，在2016年9月14日，羅默發表了一篇論文，不少人認為其影響力堪比他的成名作。羅默在文章批判説，宏觀經濟學像一門30年止步不前的科學，其理解闡釋現實的能力甚至發生了退化。

作為世行首席經濟學家，有人認為，保羅·羅默對西方經濟學的批判，是主流西方經濟學開始瓦解的標誌。

1970年代末，當凱恩斯主義舊有的確定性失靈時，新一代經濟學家們將這門學科變成了一堆超級抽象的數學方程式。他們假設，經濟系統將趨於均衡態，只有來自系統外部的不可預測的衝擊能干擾這種均衡。由於衝擊起於外部，經濟學家們為了讓數學模型看起來合理，不得不發揮想象，猜測這些衝擊到底是什麼。

而羅默在《宏觀經濟學的困境》（The Trouble with Macroeconomics，這篇論文長達24頁，中文版觀察者網近期將推出)中嘲諷了這些想象中的破壞力。他將由此產生的宏觀經濟理論，比作只有當“巨魔、小鬼和以太（物理學家假想的物質）”存在才解釋得通的物理學。

抨擊西方經濟學失效，這不是什麼新提法了。倫敦金斯頓大學經濟學家斯蒂夫·基恩（Steve Keen）就一直聲稱，2008年的金融海嘯之所以規模巨大，正是由於人們過於依賴有缺陷的經濟模型——這讓決策者們低估了風險，最終讓世界陷入危境。值得一提的是，基恩畢生都以反叛者形象出現，而羅默是學界元老、美國主流經濟學的核心人物，現任世界銀行首席經濟學家。他對那些備受推崇、影響力巨大的經濟學者提出批評，這可是件大事。

羅默挑起的話題牽涉到巨大的利益。即使金融風暴已過去8年，仍然有經濟理論在持續誤導決策者們。該理論認為，中央銀行的舉措無關輕重。為抵禦全球經濟蕭條，各國央行共印刷了總計12萬億美元（9.1萬億英鎊）的貨幣，卻並未完全消除經濟威脅。經濟停滯讓央行行長、政府官員及社會理論家們夜不能寐，可權宜之計總是這一招——施行貨幣寬鬆政策。

正統經濟理論一直鼓吹，即使央行撤回所有的救助措施，也不會壞到哪裏去——因為那些數學方程式告訴他們，貨幣政策與產出並無相關性。理論家認為，英格蘭銀行行長馬克·卡尼和歐洲央行行長馬里奧·德拉吉可以將利率調高一倍、削減量化寬鬆，而經濟仍會按照相同速度增長。

羅默不無諷刺地將這一經濟學觀點稱為“後現實”，併為它之所以如此流行提供了一個極其簡單的解釋：人性的弱點。羅默將經濟學精英與理論物理學家相對比，指出兩者存有相同問題：過於自信、“一個碩大無朋的共同體”、有着近乎宗教團體的虔誠、傾向於忽視與理論不相匹配的結果——以及對出錯可能性考慮得少之又少。

這問題不只存在於經濟學領域。羅默提到，將糟糕的物理學與糟糕的經濟學相對比，結果顯示，在所有過於依賴數學的學科中可能存在一種“通用失敗模式”。基本上，“榮譽”歸於那些處在數學模型設計前沿的人物，而非正確運用模型描述現實的人。

若羅默所言正確，他的理論將對政府官員和央行行長們的決策方式產生重大影響。比起那些抽象的模型，你更需要貼近現實——隨着電腦模擬技術的發展，這一切都會變得唾手可得。

個體為本模型（agent-based model）嘗試着儘可能詳盡地仿造現實及模擬其隨機性，而非將現實簡化為若干個變量。這類模型被普遍使用在天氣預報和城市交通規劃上：可將它們想成電腦遊戲《模擬城市》專業版。在個體為本模型中，你不必通過整體運算來判斷百萬民眾購買麪包的數量會增加還是減少。你只需創建100萬個數字化的“人”，併為他們提供數字化的麪包和金錢。

牛津大學教授J·多恩·法默（J Doyne Farmer）長期倡導經濟學界採用個體為本的建模方式；英格蘭銀行首席經濟學家安迪·霍爾丹（Andy Haldane）也是這一觀念的支持者。霍爾丹表示，現實比基於數學的經濟想象得更復雜，而且是非理性的。從上班路上買的咖啡和麪包，到我們存放養老金的基金，我們每天做出的買賣決策絕不像主流經濟學者們設想的那樣，受理性驅使。因此，當老一輩信仰數學的經濟學家期待穩定，並假定經濟混亂是由“小鬼”引起的時候，非主流經濟學家正等着大且不可預測的危機的到來。

在羅默發表這篇論文之後，若你站在海格特公墓的馬克思墓前，或許能聽見墓石之下傳來一陣德國人的笑聲。馬克思也曾是抽象理論的追隨者——在1870年代，他害怕自己因為不懂基於數學的經濟學而掉隊，專門做了1000頁的學習筆記，試圖掌握微積分。

但是，最後，馬克思想用數學為一些重大的不可預見性建立一個模型，過去十年這些問題已經日漸清晰：專家們認為不可能出現的繁榮-崩潰週期，如今重返現實；學術泰斗們數十年前保證終結的經濟蕭條，如今再次浮現。馬克思從未達成甚至接近這個建模目標——此後，經濟學便每隔25年在穩定和不穩定的理論中搖擺一次。

羅默代表主流經濟學所做的自我檢討，意味着，十來年裏學術界一直嘗試將危機的起因歸結於“巨魔和小鬼”，而從今以後不得不將視角轉向體系內部，試圖從中探求造成不穩定的因素。個人預感，答案會出自個體為本的大規模電腦模擬，在這一過程中，數百萬虛構的人物會基於性、利他主義等非理性衝動做出隨意的決定，而非僅為了追逐財富。

左派們將模型設計引向對內在的觀察，而精英陣營仍對這一視角懷有敵意：階級、性別和種族作為經濟事實存在；1%的富人比其他99%的人掌握更多的信息資源；危機無法避免，但如能坦然接受這個事實，可以降低負面影響。

以及，最為重要的是：如果你是一座國庫、一個主要政黨或一家中央銀行的決策者，對持有“資本主義不穩定”觀點的人員予以解僱或排斥，絕不是個好主意。

（本文翻譯自英國《衞報》9月19日文章，原文標題：It’s time to junk the flawed economic models that make the world a dangerous place）觀察者網李玲譯、楊晗軼校。翻頁請看英文原文。）

It’s time to junk the flawed economic models that make the world a dangerous place

Paul Mason

It’s 26 years since Paul Romer shook the discipline of economics with a single research paper. Entitled Endogenous Technological Change, Romer’s article showed how information technology changed something fundamental about our world – moving the focus of economics away from land, labour and capital towards “people, ideas and things”. Last week, a generation later, Romer published what many see as an equally significant intervention. Macroeconomics, he argues, is like a science that has not only stalled for three decades, but has actually gone backwards in its ability to understand reality.

In the late 1970s, as the old certainties of Keynesianism collapsed, a new generation of economists moved the discipline on to the terrain of super-abstract equations. Their assumption was that the economy tends towards equilibrium, and that only unpredictable shocks from outside the system can disturb it. Since the shocks come from outside, for the purposes of these mathematical models, the economist has to imagine what they might be. In The Trouble With Macroeconomics, Romer mocks these imaginary disruptions. He compares the result to a kind of physics that only works if there are “trolls, gremlins and aether”.

It’s not a new line of attack. The Kingston University economist Steve Keen has long argued that reliance on flawed models contributed to the scale of the 2008 crash – by encouraging decision-makers to underestimate risks, economic theory has the power to make the world more dangerous. But Keen is a lifelong rebel; Romer is a doyen of the profession, and from the heart of the US academic mainstream. His attack on some of the most esteemed and influential economists of our time is a big thing.

And the stakes are big, too. One of the theories that, even now, eight years after the crash, continues to disorient policymakers is the assumption that actions by central banks are irrelevant. A total of $12tn (£9.1tn) has been printed by central banks to stave off global depression, yet the threat remains real. Stagnation is a threat that keeps central bankers, governments and social theorists awake at night – with the palliative always being looser monetary policy.

Yet orthodox economic theory insists it would have no real effect if the central banks pulled all this support – since the equations tell them there is no correlation between monetary policy and output. Mark Carney or Mario Draghi could double interest rates and slash quantitative easing and the economy should grow at just the same rate, says the theory.

Romer, scathingly, calls this “post-real” economics, and suggests a horribly simple explanation for its popularity: human frailty. Comparing the economics elite with its equivalent in theoretical physics, Romer notes the same problems: over-confidence, “an unusually monolithic community”, near-religious group loyalties, a tendency to disregard results that don’t match the theory – and too little consideration of the risks of being wrong.

This is not just a problem for economics. Romer says the parallels between bad physics and bad economics suggest there might be a “general failure mode” in any discipline that becomes over-reliant on maths. Basically, the kudos goes to people at the cutting edge of designing mathematical models, not to those whose models match reality. If Romer is right, there are big implications for the way governments and central banks make policy. Instead of abstract models, you would need something much closer to reality – and, with the rise of computer simulation technologies, that is close at hand.

The agent-based model, instead of reducing reality to a few variables, tries to replicate reality – and its randomness – in detail. Such models are common in weather prediction, or city transport planning: think of them as a professional version of the computer game Sim City. In an agent-based model, you don’t try to work out whether a million people will, on aggregate, buy more bread or less bread. You create a million digital “people” and unleash them in world with digital bread and digital money.

Oxford professor J Doyne Farmer has long advocated the adoption of agent-based modelling in economics; the Bank of England’s chief economist, Andy Haldane, is a convert. Reality, says Haldane, is not only more complex than the maths-based economics imagines, it is also not rational. The sum of buying and selling decisions we take each day – from the cappuccino and croissant on the way to work, to the fund we keep our pension in – are driven by something other than the rationality that mainstream economists assume. As a result, while the old, maths-based economist expects stability and assumes a “gremlin” where it is disrupted, the heterodox economist expects big and unpredictable shocks.

If you stood close enough to Marx’s grave in Highgate, since Romer’s paper got released last week, you might hear a deep Germanic chuckle coming from beneath the stones. Marx, too, was a fan of abstraction – and worried so much that he was out of the loop of maths-based economics in the 1870s that he produced a 1,000-page notebook documenting his attempts to learn differential calculus.

But, in the end,Marx wanted to use maths to model the core unpredictabilities that have become so obvious to us in the past 10 years: boom-bust cycles that the professionals told us were impossible; depressions that the giants of modern academia assured us had been solved decades ago. Marx never even came close to achieving this – and since then, economics has veered between stability and instability theories, about every 25 years.

Romer’s huge mea culpa on behalf of mainstream economics is a sign that, after a decade-long hunt for trolls and gremlins as the cause of crisis, academia now has to begin the search for the cause of instablity inside the system, not outside it. My hunch is that the answer lies in large, agent-based simulations, in which millions of virtual people take random decisions driven by irrational urges – such as sex and altruism – not just the pursuit of wealth.

What the left can bring to the design of these models are the insights that still draw lines of enmity through elite campuses: that class, gender and race exist as economic facts; that the 1% always acts with more information than the 99%; that crises are unavoidable but can be mitigated by accepting they might happen.

And above all: that sacking or excluding people who insist “capitalism is unstable” is a bad idea if you are running, say, a treasury, a major political party or a central bank.

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