It used to seem common-sense that switch to comprehensive schools from the mid-1960s, and the big expansion of higher education, must have increased people's ability to move into a better-off social layer than their parents.
Even if inequality remained, the individual's chances of moving from one level in the scale to another must have improved. The number of students getting university first degrees increased by a factor of 16 between 1960 and 2011; the number getting second or third degrees increased much more, by a factor of 60. Overall participation in higher education increased from 3.4% in 1950, to 8.4% in 1970, 19.3% in 1990, and 33% in 2000.
Then researchers found that social mobility wasn't increasing.
32% of MPs, 51% of top medics, 54% of big-company bosses, 54% of top journalists, and 70% of High Court judges went to private school, though only 7% of the population do.
New research by veteran sociologist John Goldthorpe and others, published in the British Journal of Sociology (bit.ly/moblty), adds a further twist to the tale.
Social mobility, say the sociologists, hasn't in fact decreased. Instead: "there is a clear change in the direction of mobility. Over the past four decades, the experience of upward mobility has become less common, and going down the social ladder has become more common".
They analyse in terms of seven social "classes" (layers), and find that at age 27, 40% of men born in 1970 had arrived in a higher layer than their father, and 29% in a lower layer. Among those born in 1980-4, by age 27, 35% were in a higher layer, and 35% in a lower.
The shift among women is similar but less marked: in the 1970 cohort, 38% moved up, 33% down; in the 1980-4 cohort, 37% up, and 38% down.
Partly this is because of the expansion of white-collar work, counted as "higher" than blue-collar. In the 1980-4 cohort, more people are counted as having higher-layer parents, and therefore at risk of moving down, than in the 1970 cohort.
But that's not all. Another way of putting it is that the insecurity and chanciness of employment, always acute in the worse-off layers in the working class, has now spread to many occupations which used to be relatively "safe".
Back in 1970, if you had a university degree and you were modestly careful, you could be sure of a relatively good job. Even if you had no university degree, effort and talent at work gave you a reasonable chance of moving "up". The path "up" might be hard to climb, but it was relatively free of holes and clefts where you'd tumble down.
Now there is a "parchment ceiling" blocking off a whole swathe of jobs which even the most talented and diligent can never get without a university degree (of some sort). And those who have degrees, unless they have the connections and resources to find and sustain internships, often end up with insecure and poor jobs.