Nell restricts cruelty to hominids, although good evidence suggests that secondary aggression in rodents and particularly primates may be considered cruel. A considerable literature shows that glucocorticoid secretion stimulated by stress facilitates learning, memory, arousal, and aggressive behavior. Either secondary aggression (to a conspecific) or increased affiliative behavior reduces stressor-induced activity, suggesting the reward system can be satisfied by other behaviors than cruelty.