It's not really explained in TFA, but it has been considered at least:
> Prof Gill Livingston [said] “It is not just that more educated people do more cognitively stimulating jobs – they do – but cognitive stimulation in work through problem solving and new situations has an effect by itself.
> Prof Gill Livingston [said] “It is not just that more educated people do more cognitively stimulating jobs – they do – but cognitive stimulation in work through problem solving and new situations has an effect by itself.