In political psychology, it's known anger is the strongest emotion for increasing participation.
If you can make your audience mad about something, levels of participation increase.
The problem is you can't control anger. People stay mad. They get madder. They transfer their anger from one issue to the next. Once you press that anger switch, you don't know where you end up.
Yet 'happy' communities aren't the most active neither. Pleasant, polite, uplifting, optimistic discussions don't significantly increase participation in communities. In fact, polite communities are often considered boring. This deters participation.
Communities are stronger when they oscillate at the same emotional frequency. They're stronger when members see others reacting in the same way as they are. But which emotion should it be?
Which discussions will you decide to initiate, facilitate, highlight, and otherwise draw attention to? Do you look for discussions tinged with fear, disgust, sadness, joy, ecstasy, terror, or amazement?
More importantly, which emotion will you use when you create content, highlight relevant issues? What tone of voice will you embrace? What words will you use? Too formal/polite and you deter participation, too angry you might pull the pin on a loose grenade
We do know that more intense emotions lead to higher levels of participation. The answer therefore lies in the deeper levels of the Plutchik emotional wheel. Test different emotions. Use your subtle influence to test surprise, anxiety, anticipation, sadness, and joy.
Measure which lead to higher levels of participation and a stronger sense of community.