I've always thought of humor as a way of gaining trust. A joke that succeeds proves that you share a common understanding about some aspect of life. This is why explaining a joke never works (if you have to explain it, it proves you didn't understand each other), and why it's much easier to get a friend to laugh than a stranger; if someone already knows and trusts you, any little thing will remind them of that trust. It's also why insider jokes are only funny to insiders; they are a way of proving shared knowledge and cannot work without it.

It sounds like these researchers showed a special case of this: if you want be the leader in a situation then you need to be able to get people to trust you, so naturally would-be leaders will tell jokes in an attempt to create and reinforce trust. Also, if you're not a leader and you tell a joke then it could be considered a challenge by an insecure leader, so this will be avoided by people who don't want to rock the boat. But to say that jokes are only about aggression is too much. If everyone in the group is comfortable telling jokes then that's a sign that there's a lot of mutual trust.