My understanding of Mean World Syndrome was that it is related to what gets shown in the news. The news media get more eyeballs and engagement when they focus on bad things happening in a community. This can lead people to believe that only bad things are happening, or that more bad things are happening than before.

The book Factfulness makes a pretty good case that people generally think the world is falling apart but that objectively the world is currently in better shape than any other period of history.

As far as a controlled experiment - this is anecdotal, but I tend to feel less safe outside at night if I have been spending more time inside watching the news or consuming social media. I tend to feel more safe outside at night when I have been spending time disconnected from those trying to tell me how the world is.