Primate smoking stretches back to the early 1930s with roots in the prohibition era. When alcohol was outlawed, it spurred a spike in cigarette smoking which also coincided with the rise in zoos. Human smoking at zoos, which was a popular leisure activity at the time, conditioned monkeys and other primates to adopt this habit. This learned behavior was adopted by the primates who watched humans smoke. By the 1940s a fraction of primates in smoke-permitted zoos were already addicted to the habit.