Expanding an educational level reduces class inequality in access to it only once advantaged groups have approached saturation there.
Raftery and Hout proposed MMI in 1993 from Irish data: expansion is absorbed first by the advantaged, so inequality in transition rates persists until their participation nears a ceiling and there is nowhere left for them to expand into. Widening access at a given level therefore does very little until that level is nearly universal for the group already using it.
In practice
- A university expansion that doubles places leaves the participation gap unchanged for a decade, then closes it quickly.
- A training scheme opened to all grades is taken up first by the grades already best served.