brain concentrations of substances like serotonin are not immutable. They are not simply genetic givens—experience affects them. Certain kinds of stressors can decrease brain serotonin levels and thereby change behavior. For example, if you isolate animals at crucial developmental stages, if you keep them caged all alone, their serotonin drops. What is more, when you then release them and put them in contact with other animals, they are fiercely aggressive. Pain and fear also reduce serotonin levels and promote aggression. That’s how pit bulls are trained to fight. Heat, crowding, discomfort,
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