Generally, it's a mixture of things. Namely: 1. The topic. Obviously a tech site will tend towards being a bit more respectful/sophisticated than a political o

Generally, it's a mixture of things. Namely: 1. The topic. Obviously a tech site will tend towards being a bit more respectful/sophisticated than a political one. 2. The initial group of users. As you may have guessed, your first few users set the tone of the community to follow, so if they're particularly intelligent/knowledgeable about the topic/whatever then your community will likely keep at least a bit of that culture going forward. 3. Moderation. Generally, a forum or community has to strike a balance between being too moderated and not moderated enough, with the former leading to a stifling atmosphere filled with yes men and the latter becoming anarchy/troll heaven. Something like Ask Historians is far towards the heavily moderated site, whereas something like 4chan and competitors are towards the anarchist side. 4. Barriers to entry. Invite only or paid services tend to have higher quality content than free ones where anyone can join. 5. Site layout has some effect, generally the harder it is to use, the more determined people will need to be to participate, which can (sometimes) push up quality. The minimalist/old school design here drives away a certain percentage of the 'mainstream' audience. 6. Anonymity is a mixed bag. A small, paid/invite only community of professionals in a certain field that's not anonymous can do really well, but it won't make a lick of difference on the scale of Facebook/Twitter/Instagram. Either way, you can certainly engineer forums to encourage a certain culture/type of community, though it's still kinda luck based whether you actually get that.