Mainstream economists largely acknowledge the problem of externalities but believe they can be addressed either through Coasian bargaining or the use of Pigovian taxes—extra taxes on goods that have externalities. According to economic theory, if Pigovian taxes are set so that the after-tax cost of the goods is equal to the social cost of the goods, the direct cost of production plus cost of externalities, then quantities produced will tend toward a socially optimal level. Hahnel observes, "more and more economists outside the mainstream are challenging this assumption, and a growing number of skeptics now dare to suggest that externalities are prevalent, and often substantial". Or, as E.K. Hunt put it: externalities are the rule rather than the exception, and therefore markets often work as if they were guided by a "malevolent invisible foot" that keeps kicking us to produce more of some things, and less of others than is socially efficient."