You mean if somebody's searching for something, they will get more of the same? There is an algorithm that computes similarity between their search and the next one. This is usually useful because you are looking for information on XY, and you get that. They also use it for advertising to try to match the content with what you care about so that they don't give you totally irrelevant advertising.
You're right. If you look for some bad things, it could give you more bad things. It's hard to control. You could have some sort of measure of negativity, I guess.