Studied a sample of around 10,000 men & compared the occupational class of sons & fathers.Found that in the period after WW2, there had been considerable upward social mobility, though much less downward mobility.Suggested that the introduction of free secondary education was not as important as many people believed, he argued that chances in the mid-20th century, there was a reduction in the size of the working class & an increase in the size of the intermediate & service classes.This points to an increase in absolute social mobility.