If the marginal social benefits foregone of the wrong choice are large, relative to the private costs incurred, then quantitative restrictions are optimal. Why? Because fixing quantities to achieve pollution control, while letting prices vary, does not have large private costs. When the marginal social benefit curve is steeper than the marginal private cost curve, restrictions dominate. The results flip when the marginal cost/benefit trade-offs are reversed. If the private costs of the wrong choice are high, relative to the social benefits foregone, fixing these costs through taxation is likely to deliver the better welfare outcome. When the marginal social benefit curve is flatter than the marginal private cost curve, taxation dominates. So the choice of taxation versus prohibition in controlling pollution is ultimately an empirical issue. To illustrate the framework, consider the path of financial regulation in the US over the past century. The US announcements in January are in many respects redolent of US financial reforms enacted during the late 1920s and early 1930s. Then, restrictions were imposed on both bank size and scope, in the form of the McFadden (1927) and Glass-Steagall (1933) Acts. The history of both, viewed through Weitzman’s lens, is illuminating for today’s debate. 9  Basel Committee on Banking Supervision (2009). 10  For example, Brunnermeier et al (2009), NYU Stern School of Business (2009). 11  For example, Kay (2009), Kotlikoff (2010). 12  Stern (2006). 13  Weitzman (1974).

And add-ons to these capital and liquidity buffers for institutions posing the greatest systemic risk are being discussed to address the second. 9 In essence, this is a taxation solution to the systemic risk pollution problem. 10 There is a second approach. On 21 January 2010, US President Barack Obama proposed placing formal restrictions on the business activities and scale of US banks. Others have made complementary proposals for structural reform of banking. 11 Typically, these involve separation of bank activities, either across business lines or geographies. In essence, this is the prohibition solution to the systemic pollution problem. This sets the scene for a great debate. It is not a new one. The taxation versus prohibition question crops up repeatedly in public choice economics. For centuries it has been central to the international trade debate on the use of quotas versus subsidies. During this century, it has become central to the debate on appropriate policies to curtail carbon emissions. 12 In making these choices, economists have often drawn on Martin Weitzman’s classic public goods framework from the early 1970s. 13 Under this framework, the optimal amount of pollution control is found by equating the marginal social benefits of pollution-control and the marginal private costs of this control. With no uncertainty about either costs or benefits, a policymaker would be indifferent between taxation and restrictions when striking this cost/benefit balance. In the real world, there is considerable uncertainty about both costs and benefits. Weitzman’s framework tells us how to choose between pollution-control instruments in this setting.