At a couple of the campuses I know best, there is actually a stigma attached to STAYING with a sorority after sophomore year. At one, the sororities would take pledge classes of 50+ freshmen each year, but they might only graduate 8 seniors. Rush is a huge deal for freshmen, but it's like you must be a loser if you're still relying on your sorority as an upperclassman. The attitude was, don't you have anything better to do? You're dressing up to go to a MIXER? To a large extent, this applied to the "cool" sororities too.
This didn't apply to fraternities; it was considered legit for guys to be living in the house as seniors, etc.
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NIGHTDREAMS