保罗·梅森：主流经济模型让世界陷入危境

保罗·罗默（Paul Romer）曾凭一纸研究震惊经济学界，此事至今已有26年。罗默那篇名为《内生性技术变革》（Endogenous Technological Change）的文章，揭示了信息技术是如何从根本上改变世界的——它把经济学的关注点从土地、劳动力和资本，转移至“人、思想（指技术创新）和事物”。（保罗·罗默到底有多牛，不熟悉的请戳）

近三十年之后，在2016年9月14日，罗默发表了一篇论文，不少人认为其影响力堪比他的成名作。罗默在文章批判说，宏观经济学像一门30年止步不前的科学，其理解阐释现实的能力甚至发生了退化。

作为世行首席经济学家，有人认为，保罗·罗默对西方经济学的批判，是主流西方经济学开始瓦解的标志。

1970年代末，当凯恩斯主义旧有的确定性失灵时，新一代经济学家们将这门学科变成了一堆超级抽象的数学方程式。他们假设，经济系统将趋于均衡态，只有来自系统外部的不可预测的冲击能干扰这种均衡。由于冲击起于外部，经济学家们为了让数学模型看起来合理，不得不发挥想象，猜测这些冲击到底是什么。

而罗默在《宏观经济学的困境》（The Trouble with Macroeconomics，这篇论文长达24页，中文版观察者网近期将推出)中嘲讽了这些想象中的破坏力。他将由此产生的宏观经济理论，比作只有当“巨魔、小鬼和以太（物理学家假想的物质）”存在才解释得通的物理学。

抨击西方经济学失效，这不是什么新提法了。伦敦金斯顿大学经济学家斯蒂夫·基恩（Steve Keen）就一直声称，2008年的金融海啸之所以规模巨大，正是由于人们过于依赖有缺陷的经济模型——这让决策者们低估了风险，最终让世界陷入危境。值得一提的是，基恩毕生都以反叛者形象出现，而罗默是学界元老、美国主流经济学的核心人物，现任世界银行首席经济学家。他对那些备受推崇、影响力巨大的经济学者提出批评，这可是件大事。

罗默挑起的话题牵涉到巨大的利益。即使金融风暴已过去8年，仍然有经济理论在持续误导决策者们。该理论认为，中央银行的举措无关轻重。为抵御全球经济萧条，各国央行共印刷了总计12万亿美元（9.1万亿英镑）的货币，却并未完全消除经济威胁。经济停滞让央行行长、政府官员及社会理论家们夜不能寐，可权宜之计总是这一招——施行货币宽松政策。

正统经济理论一直鼓吹，即使央行撤回所有的救助措施，也不会坏到哪里去——因为那些数学方程式告诉他们，货币政策与产出并无相关性。理论家认为，英格兰银行行长马克·卡尼和欧洲央行行长马里奥·德拉吉可以将利率调高一倍、削减量化宽松，而经济仍会按照相同速度增长。

罗默不无讽刺地将这一经济学观点称为“后现实”，并为它之所以如此流行提供了一个极其简单的解释：人性的弱点。罗默将经济学精英与理论物理学家相对比，指出两者存有相同问题：过于自信、“一个硕大无朋的共同体”、有着近乎宗教团体的虔诚、倾向于忽视与理论不相匹配的结果——以及对出错可能性考虑得少之又少。

这问题不只存在于经济学领域。罗默提到，将糟糕的物理学与糟糕的经济学相对比，结果显示，在所有过于依赖数学的学科中可能存在一种“通用失败模式”。基本上，“荣誉”归于那些处在数学模型设计前沿的人物，而非正确运用模型描述现实的人。

若罗默所言正确，他的理论将对政府官员和央行行长们的决策方式产生重大影响。比起那些抽象的模型，你更需要贴近现实——随着电脑模拟技术的发展，这一切都会变得唾手可得。

个体为本模型（agent-based model）尝试着尽可能详尽地仿造现实及模拟其随机性，而非将现实简化为若干个变量。这类模型被普遍使用在天气预报和城市交通规划上：可将它们想成电脑游戏《模拟城市》专业版。在个体为本模型中，你不必通过整体运算来判断百万民众购买面包的数量会增加还是减少。你只需创建100万个数字化的“人”，并为他们提供数字化的面包和金钱。

牛津大学教授J·多恩·法默（J Doyne Farmer）长期倡导经济学界采用个体为本的建模方式；英格兰银行首席经济学家安迪·霍尔丹（Andy Haldane）也是这一观念的支持者。霍尔丹表示，现实比基于数学的经济想象得更复杂，而且是非理性的。从上班路上买的咖啡和面包，到我们存放养老金的基金，我们每天做出的买卖决策绝不像主流经济学者们设想的那样，受理性驱使。因此，当老一辈信仰数学的经济学家期待稳定，并假定经济混乱是由“小鬼”引起的时候，非主流经济学家正等着大且不可预测的危机的到来。

在罗默发表这篇论文之后，若你站在海格特公墓的马克思墓前，或许能听见墓石之下传来一阵德国人的笑声。马克思也曾是抽象理论的追随者——在1870年代，他害怕自己因为不懂基于数学的经济学而掉队，专门做了1000页的学习笔记，试图掌握微积分。

但是，最后，马克思想用数学为一些重大的不可预见性建立一个模型，过去十年这些问题已经日渐清晰：专家们认为不可能出现的繁荣-崩溃周期，如今重返现实；学术泰斗们数十年前保证终结的经济萧条，如今再次浮现。马克思从未达成甚至接近这个建模目标——此后，经济学便每隔25年在稳定和不稳定的理论中摇摆一次。

罗默代表主流经济学所做的自我检讨，意味着，十来年里学术界一直尝试将危机的起因归结于“巨魔和小鬼”，而从今以后不得不将视角转向体系内部，试图从中探求造成不稳定的因素。个人预感，答案会出自个体为本的大规模电脑模拟，在这一过程中，数百万虚构的人物会基于性、利他主义等非理性冲动做出随意的决定，而非仅为了追逐财富。

左派们将模型设计引向对内在的观察，而精英阵营仍对这一视角怀有敌意：阶级、性别和种族作为经济事实存在；1%的富人比其他99%的人掌握更多的信息资源；危机无法避免，但如能坦然接受这个事实，可以降低负面影响。

以及，最为重要的是：如果你是一座国库、一个主要政党或一家中央银行的决策者，对持有“资本主义不稳定”观点的人员予以解雇或排斥，绝不是个好主意。

（本文翻译自英国《卫报》9月19日文章，原文标题：It’s time to junk the flawed economic models that make the world a dangerous place）观察者网李玲译、杨晗轶校。翻页请看英文原文。）

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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