The MRP theory suggests that wage differentials result from differences in labour productivity. Measuring MRP can be difficult in reality, with a lot of work carried out in teams, which makes it difficult to isolate the contribution to output made by an individual worker. Also those who work in the tertiary and service sector; the output of one worker is often dependent on other workers.
The supply curve for labour slopes upwards – at higher wage rates more people will work. How many more people make themselves available at higher wage rates is influenced by elasticity of demand for labour. Influences on the supply of labour can be categorised; with financial, which involve: Overtime (if overtime is available it helps to increase supply), wage rates in substitute jobs (if wage rates are higher than in substitute jobs more labour tends to be supplied) and barriers to entry (these can lead to a higher wage rate being paid). The other category is non monetary influences: Increased mobility of labour (this could be occupational or geographical) and net labour migration (this is an increasing factor with more countries joining the EU and migrating