'Very few people are bright enough to get a place at Oxbridge (or even LSE, UCL, Imperial, Durham, St Andrew's etc) so when those that do graduate they have already proven themselves to be far above average so can get jobs that pay more than average.'
See i'dargue about that (well about most things really but that right now LOL )
I've had terrible jobs; receptionist,carer, etc etc. right now I live on carers benefit (H working but looking at redundancy, just offered place at Uni).
A few years back I went to college to do an Access. Whilst I was there Bristol invited me to apply for a place on the Social Policy course and when I chose to do something else, offered me that palce (Theology) without an interview, on references from my Tutor.
now, my understanding of Psych is that IQ is alrgely stable so I didnt gain IQ, I just had the oppourtunity to use it. Most of the people on my course as mature students (not at bristol in the end, but only because rent there for a family was too high / schools relatively poor compared to where we ended up- my sacrifice in status being the right decision to boost boys chances) were very bright indeed, but life in its wonderful ways had relegated them to crap jobs and incomes: childhood poverty being a major factor.
Your argument would work if we had a level playing field from birth but we dont and never will have. Plenty of people underachieve for a great many reasons.