One thing that drives people to troll is, unsurprisingly, their mood. Studies have shown that people’s moods, as revealed by the tone of their posts on Twitter, follow a remarkably predictable pattern: Relatively positive in the morning, and more negative as the day wears on. You can guess the weekly pattern: Mondays are the worst, and people seem to feel better on the weekend.

There’s an almost identical circadian rhythm for trolling, according to the research team.

The researchers also uncovered a pile-on effect. Being trolled in another comment thread, or seeing trolling further up in a thread, makes people more likely to join in, regardless of the article’s content. Based on this, it’s easy to see how committed trolls could inspire—or really infect—others. It’s not so different from a real-world mob, only on the internet it’s much easier to find yourself in the midst of one.

In a controlled experiment, the Stanford and Cornell researchers established that together, mood and exposure to trolling can make a person twice as likely to troll.