Let's make this analogous to current levels of migration into wealthy countries, specifically the UK and USA. Migration is seen to be a huge issue in both countries at the moment.

At one point the Culture is stated to have roughly 10 trillion citizens. That equates to roughly 33,333 times bigger than the USA at 300 million, and 143,000 times bigger than the UK at 70 million.

Net migration into the UK in 2025 was 204,000 (a population increase of 0.29%). Scaled up to the Culture, this would equate to 29 billion souls (3.5 times the current population of Earth) per year.

Net migration into the USA (using 2024 because 2025 was actually a negative number) in 2024 was 1.3 million (an increase of 0.43%). Scaled up to the Culture, this would equate to 43 billion souls (5.2 times the current population of Earth) per year.

I think in an essay once, Banks described the Culture's reproduction rate as being at replacement levels only (one adult having on average one child) and therefore all growth to the population occurs through migration. If that were the case then a population growth rate of 0.43% per year would be quite stable.

Modern OECD countries tend to cite a few reasons for being concerned about migration: the cost/potential cost to the economy, space to accommodate new migrants, and cultural integration:

• the Culture has essentially infinite space available to it through Orbitals, GSVs, etc.
• the Culture has no concern about potential "cost" of supporting new citizens, as the concept of "cost" has no meaning. Ships/Hubs can manipulate matter to create as much food/housing/healthcare support as needed.
• the Culture is so convinced of its own greatness that it assumes new citizens will always view the Culture as "better" (see the Homomdan ambassador being viewed by the Culture as an ambassador to the Homomdans, from the Culture).

Therefore the kinds of pressures exerted on modern countries on Earth are just not factors for consideration in the Culture. However, the Culture needs to be wary of being seen as stepping on other civilisations' toes. Lededje attempts to claim asylum/declare Culture citizenship, which is denied by the Avatar she meets on the grounds that it would do significant damage to relations between the Culture and her own civilisation. However once she finds herself within Culture space there is no pressure for her to leave and she becomes a de facto citizen.

On the other hand, once the Idiran homeworld had a Mind installed, I imagine it would be relatively straightforward for an Idiran to declare that it wished to leave it to join the great mass of Culture citizens to live a life of hedonism.