When I was a child in elementary / junior high in the early / mid 80s, membership in the enrichment classes was largely dependent on IQ tests. I have very fond memories of these classes! We did all kinds of crazy advanced stuff, and had wonderful discussions. The membership was also quite diverse. However, in the late 80s they started to base membership on academic performance – as you might imagine, this started to skew the membership towards students from more affluent families who weren't really more insightful than the general population, just more diligent.