“It’s not just the level of serotonin,” Paul said emphatically, jabbing his fingers in the air. The New York air was clear and bitingly cold, and his breath left a drifting trail of mist behind him. “That’s way too simple. It’s what serotonin does to the neuron. The way it changes the neuron’s chemistry, and its metabolism,” he said. “And that might vary from one individual to the next.”
