What is a word or phrase for a sort of "de facto bias"?

Question

I'm struggling to find the words to convey the concept even now, but perhaps it would make more sense to illustrate by example.

Certain communities like Reddit and Quora tend to be liberal-leaning and have other attitudes and opinions not entirely reflective of the population as a whole, in large part simply because they are populated by the kind of people who can find these communities. Your average computer engineer is much more likely to be familiar with Reddit and use it regularly than your average janitor or priest, and especially more than someone without reliable Internet access.

Similarly, Wikipedia articles or tumblr blogs are unlikely to be written by your grandparents, and thus reflect a certain subset of content and attitudes by virtue of the people who use them most.

I thought about implicit bias, but it doesn't seem to quite reflect the unintentional barrier of entry in these cases, although it's the closest I could find off-hand.

Answer 1

I did some more research, and it seems as if self-selection bias is appropriate here, though maybe it's not the best answer. — Wikipedia

In statistics, self-selection bias arises in any situation in which individuals select themselves into a group, causing a biased sample with nonprobability sampling.

In this case, it fits in the sense that particular occupations, attitudes, and beliefs tend to group people together, especially on Internet communities, resulting in the biased sample I was talking about. However, since not all of these attributes or selections are entirely by choice, there might be a more specific term.

I welcome any further refinements or suggestions!