导读：
美国经济学家阿瑟·拉弗是供给侧经济学最早的拥趸之一，在那个时期，其他大部分经济学家都开始弃用凯恩斯学说而纷纷倒向米尔顿·弗里德曼。1974年，拉弗曲线，一个阐释最优税率的理论，开始获得美国决策者的青睐。拉弗后来阅读全文
美国经济学家阿瑟·拉弗是供给侧经济学最早的拥趸之一，在那个时期，其他大部分经济学家都开始弃用凯恩斯学说而纷纷倒向米尔顿·弗里德曼。1974年，拉弗曲线，一个阐释最优税率的理论，开始获得美国决策者的青睐。拉弗后来任职于里根总统的经济政策顾问委员会。里根的供给侧经济试验为现任总统特朗普提出的经济方案—特朗普经济学提供了先行参考。
拉弗曲线很简单。当税率由零开始增长，税收也随之增长。根据拉弗曲线，税率上升到一定的点的时候，税收会达到最大值，这时候如果税率继续上升，税率就会开始下降，因为高税率抑制了人们的经济活动。零税率和100%的税率都会导致零税收的结果。只有最合适的税率才会产生最大化的税收。
美国高昂的企业税导致税收太少。根据牛津大学塞德商学院，2015年美国法定企业所得税为40.5%，居G20集团之首。爱尔兰最低，英国（20%）仅高于爱尔兰。然而OCED国家中，美国的税收收入却几乎处于末位。如果特朗普降低税率，税收很可能会升高。
国家间国税收竞争也同样重要。美国跨国公司降低国内纳税额的途径多种多样，比如：通过避税天堂、外包、进行离岸投资、将利润留置海外以避免资金汇回本国时征税。这就减少了美国的税收，并抑制了国内投资、增长和生产力，从而间接冲击了税基。
G20集团的其它国家，包括英国、印度尼西亚、意大利、印度、日本、法国，都有意在2020年之前降低企业所得税税率。美国之前一直只是站在一旁观望毫无动作，现在特朗普准备要回击。
特朗普的边境税计划就是贸易方面的一记回击。它暴露了世界贸易组织的规则缺陷：直接补贴出口商而对进口商征税是禁止的；但是间接征税，比如增值税和销售税，同样可以起到保护出口的作用，却是允许的。
在这种情况下，美国处于不利地位。公司税务竞争扭曲了整个税收系统。政府不断赤字提振需求，而大部分资金却被企业窖藏起来未得流通。需求疲软导致公司不愿意投资，而投资疲软又导致需求不足。不仅不投资，公司还回购股份，利用创纪录的低利率来提高资产负债表的杠杆。
1981年美国国会通过里根的经济复兴计划，大幅削减了个人所得税。财政刺激被紧缩的货币政策所抵消。保罗·沃尔克，时任美联储主席，将利率提到新高。美元大幅走强，导致财政刺激效益和税收流向海外，经常账户出现前所未有的赤字。里根为世界其他地区倒是做出了贡献。如果拉弗的理论没错，特朗普的企业减税计划将起到刚好相反的效果。美元或许会像80年代那样走强，但这只会促使美国贸易保护主义的进一步抬头。
原文如下：
The US economist Arthur Laffer was an early exponent of supply-side economics at a time when most others were discarding John Maynard Keynes for Milton Friedman. In 1974 his ‘Laffer curve’, an illustration of the theory on optimum tax rates, became popular with US policy-makers. Laffer later served on President Ronald Reagan’s economic policy advisory board. The history of Reagan’s supply-side experiment provides precedent for the incumbent president’s proposed economic policy – Trumponomics.
The Laffer curve is simple. As tax rates rise from zero, tax revenue increases. There comes a point when revenue is maximised but, according to Laffer, if tax rates continue to rise, revenue starts to fall because people are deterred from working by the high taxes. Both zero and 100% tax rates collect zero revenue. A single most-efficient tax rate produces the maximum revenue.
Punitive US corporate tax rates collect relatively little revenue. According to the University of Oxford’s Said Business School, in 2015 the US had the highest statutory corporate tax rate (40.5%) of the G20 member countries. After Ireland, the UK (20%) had the second lowest. Yet among members of the Organisation for Economic Co-operation and Development, the US comes close to the bottom of the revenue table. If Donald Trump lowers the high rates, this is likely to boost tax revenue.
Global tax competition is of equal importance. US multinationals reduce their domestic tax bills by locating in tax havens, outsourcing, investing in offshore activities and retaining profits abroad to avoid tax on repatriated earnings. This directly reduces US tax revenue and indirectly impacts the tax base by lowering domestic investment, growth and productivity.
Several G20 nations, including the UK, Indonesia, Italy, India, Japan and France, intend to cut corporate tax rates before the end of the decade. America has hitherto stood aside. Trump plans to retaliate.
On trade, Trump’s border adjustment tax proposal can be seen as a reprisal. It exposes a defect in World Trade Organisation rules. Direct subsidies to exporters and taxes on imports are banned. But indirect taxation, such as value added and sales taxes, have the same effect and are allowed.
In these cases, the US is the losing party. Corporate tax competition distorts tax systems. While government deficits support demand, too much leaks into corporate savings, causing cash to be hoarded. Companies won’t invest in the light of anaemic demand, and the failure to invest causes demand deficiency. Instead, companies buy back shares and exploit record low interest rates to leverage balance sheets.
In 1981 Congress slashed personal taxes through Reagan’s Economic Recovery Tax Act. The fiscal stimulus was offset by monetary stringency. Paul Volcker, then chair of the US Federal Reserve, raised interest rates to record levels. The dollar soared and much of the stimulus and tax revenue leaked abroad through a record current account deficit. Reagan did the rest of the world a favour. If Laffer is right, Trump’s corporate tax cuts can do the opposite. The dollar may again soar as it did in the 1980s – this would only encourage further US trade protectionism
[1] Brian Reading was an Economic Adviser to Prime Minister Edward Heath and is a Member of the OMFIF Advisory Board.