This paper assumes that workers can move from a market with high unemployment to one with low unemployment at a cost. In principle. equilibrium mobility can be greater or less than the social optimum. For most plausible parameter values. however. mobility is too low. Intuitively. mobility has a...

Climate change is a global "free rider" problem because significant abatement of greenhouse gases is an expensive public good requiring international cooperation to apportion compliance among states. But it is also a global "free driver" problem because geoengineering the stratosphere with...

This paper develops a simple model of a polluting industry and an innovating firm. The polluting industry is faced with regulation and costly abatement. Regulation may be taxes or marketable permits. The innovating firm invests in R&D and develops technologies which reduce the cost of pollution...