Google recently posted "The mail you want, not the spam you don’t":
http://gmailblog.blogspot.de/2015/07/the-mail-you-want-not-s...
http://gmailblog.blogspot.de/2015/07/the-mail-you-want-not-s...
I really wonder how these preferences are reflected if someone hasn't been using spam filters much. I get maybe at most a dozen spam emails escaped a year, and the rest of the emails go through (all mailing lists... I am on a lot of mailing lists) so I get about 50+ emails per day from just DL. So I barely ever need to mark something as spam, or even more something to another folder, so how does Google know what's my preference? It sounds like people who don't actively mark spams are less likely affected...