It's exactly the same thing as doctors associated with doctors. At 19, 20, 21 years old, there simply isn't a broad enough base of experience to differentiate people. The major factor amongst all Americans is, "Did you pursue higher education?" Given that I went to a high school that had 100% enrollment in college (save for one kid who is currently driving NASCAR Nextel Cup but that's a different story), and we only associated with the other private schools who had similar enrollment statistics, socialized at the same country clubs, and attended the same society functions, that's was my peer group.

The next life stage began and we went to college. I don't associate much with the people who went to Ivy and near-Ivy schools (ironically, I elected not to), but I do associate with people from similar backgrounds in different cities who attend my school, as well as people from high school who attended different, similar tier institutions. There are certainly a handful of people I would have associated with in high school that I simply don't have anything to say to any longer. I mean, if you went to a glorified community college and commuted from home because you went to rehab, or dropped out of school to tour with your band, our paths have diverged.

I have friends who have graduated and gone on to med school. We stay in touch, but they associate themselves with other med students far more now. During residency, it won't be much different. I have friends that have gotten married and are beginning to have kids. They associate with the friends that are married and having kids. I, however, haven't really begun that dividing phase. For another couple months, anyways, I still divide people by education quality and their parents' socio-economic background.

And the point of the previous post was that it was just another way of removing outliers to move toward the center of people most similar to me. If you were in my peer group in high school and went to a non-national university, then you aren't in my peer group any longer. Because of how well I know my peers, I can also pretty safely assume that you're an idiot.