Massive study analyzes 23 million headlines from 47 news media outlets in the US from 2000-2019. They find that over this time period, headlines became more likely to denote anger, fear, disgust and sadness and less likely to be emotionally neutral. Headlines from right-leaning news media were consistently more negative than headlines from left-leaning outlets as well.
https://journals.plos.org/plosone/article