c © 2002 The Review of Economic Studies Limited Directed Technical Change

For many problems in macroeconomics, development economics, labour economics, and international trade, whether technical change is biased towards particular factors is of central importance. This paper develops a simple framework to analyse the forces that shape these biases. There are two major forces affecting equilibrium bias: the price effect and the market size effect. While the former encourages innovations directed at scarce factors, the latter leads to technical change favouring abundant factors. The elasticity of substitution between different factors regulates how powerful these effects are, determining how technical change and factor prices respond to changes in relative supplies. If the elasticity of substitution is sufficiently large, the long run relative demand for a factor can slope up. I apply this framework to develop possible explanations to the following questions: why technical change over the past 60 years was skill biased, and why the skill bias may have accelerated over the past 25 years? Why new technologies introduced during the late eighteenth and early nineteenth centuries were unskill biased? What is the effect of biased technical change on the income gap between rich and poor countries? Does international trade affect the skill bias of technical change? What are the implications of wage push for technical change? Why is technical change generally labour augmenting rather than capital augmenting