Some have argued that reducing capital gains tax rates would increase short-run and long-run economic growth. The long-run level of output depends on the amount of saving and investment. Saving and investment increase the amount of capital in the economy and hence, aggregate supply (i.e., the amount of goods and services available in the economy). Many economists note that capital gains tax reductions appear to have little or even a negative effect on saving and investment (see above).
Consequently, capital gains tax rate reductions are unlikely to have much effect on the long-term level of output or the path to the long-run level of output (i.e., economic growth). Furthermore, it is argued that a temporary or permanent capital gains tax reduction is an effective economic stimulus measure. An effective short-term economic stimulus, however, will have to increase aggregate demand, which requires additional spending. A tax reduction on capital gains would mostly benefit very high income taxpayers who are likely to save most of any tax reduction.
Economists note that a temporary capital gains tax reduction possibly could have a negative impact on short-term economic growth. A temporary tax cut could induce investors to sell stock (i.e., realize capital gains by reducing the lock-in effect), but provides no incentive to invest since investors know they will face higher tax rates in the future. To the extent that the resulting sell-off depresses stock prices, consumer confidence, already low during recessions, could be further undermined thus reducing consumer spending.
http://www.fas.org/sgp/crs/misc/R40411.pdf