In mainstream economic theories, the labour supply is the total hours (adjusted for intensity of effort) that workers wish to work at a given real wage rate. It is frequently represented graphically by a labour supply curve, which shows hypothetical wage rates plotted vertically and the amount of labour that an individual or group of individuals is willing to supply at that wage rate plotted horizontally.
Labour supply curves derive from the 'labour-leisure' trade-off. More hours worked earn higher incomes, but necessitate a cut in the amount of leisure that workers enjoy. Consequently, there are two effects on the amount of labour desired to be supplied due to a change in the real wage rate. As, for example, the real wage rate rises the opportunity cost of leisure increases. This tends to make workers supply more labour (the "substitution effect"). However, also as the real wage rate rises, workers earn a higher income for a given number of hours. If leisure is a normal good—the demand for it increases as income increases—this increase in income tends to make workers supply less labour so they can "spend" the higher income on leisure (the "income effect"). If the substitution effect is stronger than the income effect then the labour supply slopes upward. If, beyond a certain wage rate, the income effect is stronger than the substitution effect, then the labour supply curve bends backward.