According to Butler, overcrowding in mankind, as in all other animal species, increased hostility and aggression, which led to war, "the greatest crime of all". (TV: Invasion of the Dinosaurs)

The Cult of Skaro sought to acquire, amongst other things, aggression from humanity, which was felt by Dalek Sec when he transformed into a Human-Dalek. (TV: Evolution of the Daleks)

The Twelfth Doctor identified fifteen species which were known for aggression on the Trap Street, London. (TV: Face the Raven)