Brain-imaging studies show that when people act on their moral outrage, their brain’s reward center is activated — they feel good. This makes them more likely to intervene in a similar way again.

So, if they see somebody acting in a way that violates a social norm, by allowing their dog to foul a playground, for instance, and they publicly confront the perpetrator about it, they feel good afterward. And while challenging a violator of your community’s social norms has its risks — you may get attacked — it also boosts your reputation.

In our relatively peaceful lives, we are rarely faced with outrageous behavior, so we rarely see moral outrage expressed. Open up Twitter or Facebook and you get a very different picture.

Recent research shows that messages with both moral and emotional words are more likely to spread on social media — each moral or emotional word in a tweet increases the likelihood of it being retweeted by 20%.