system
Given the following post, respond as if you're commenting on Reddit:
human
[The Culture] If lower tiered civilizations' citizens envy the Culture, to the point of making poor imitations of culture drug glands and such, what's stopping them from just joining the Culture?
gpt
The Culture :)

They often take individuals out of lower-rung societies for varying reasons. The lady with the Tattoos in Surface Detail and Cheradinine Zakalwe, to name a few. But whole societies generally only join the Culture once they've become relatively enlightened and technologically advanced by Culture standards. There's always exceptions to the rule, but generally, that's how it goes.

They've also moved entire very lower-rung societies from their soon-to-be doomed homeworlds to orbitals, but without absorbing them into the culture. That's why some orbitals contain sporadic enclaves of 'savages' like the Eaters in Consider Phlebas.

There are even Idirans who now consider themselves Culture citizens, so there doesn't really seem to be a rule of thumb. They treat it on a case-by-case basis.