In Figure 13.6, the wage rate starts at £OW in both occupations. Assuming that demand increases in each market by the same amount (D to D1), the wage rate will increase to £OW1.For occupation (a), this will be higher than for occupation (b). A differential has opened up in wages. The qualifications and length of training needed to become a doctor have influenced the supply curve of labour such that this occupation commands a higher salary than unskilled occupations. This differential will not be competed away because of the barriers to mobility which exist and in fact this differential has an important role to play in persuading individuals to go through the long and demanding training process to become a doctor (see Section 13.4 below). Differentials also exist for jobs which are dirty or dangerous, as supply might not be forthcoming if wages did not reflect these factors.