Following each copulation with the same female his reward circuitry squirts less and less dopamine. Consider the above graph. The fifth time a rat copulates with the
same female it takes him 17 minutes to get off. Ejaculation time increases as dopamine released
decreases. But if he keeps switching to novel females, he can do his duty very quickly all five times. His brain renews his virility with strong squirts of dopamine in response to each new partner. *
Unlike rats,
humans are pair bonders. We're wired, on average, to raise offspring together—and to find a fair amount of contentment in our unions (potentially). But the Coolidge Effect lurks in us, too, and awakens when duty trumpets loudly enough. I once had a conversation with a man who had grown up in Los Angeles. "I quit counting at 350 lovers," he confessed, "and I guess there must be something terribly wrong with me because I always lost interest in them sexually so quickly. Some of those women are really beautiful, too."
At the time of our chat his third wife had just left him for a Frenchman and he was discouraged. She had lost interest in him.