The use of chemicals injected into the brain
Carefully timed visual cues
Watching a violent movie.
Going for a run.
Those who received no further reward for punching the clown.
Those who received marbles on half the trials and a disturbing buzzer sound following the other half.
Those who received consistent punishment, i.e. the disturbing buzzer sound, each time they punched the clown.
More imitation of specific aggressive acts
More aggressive responses of all kinds
Here's an interesting quiz for you.