In the 1730s and 1740s he began taking notes on population growth, finding that the American population had the fastest growth rates on earth. Emphasizing that population growth depended on food supplies—a line of thought later developed by Thomas Malthus—Franklin emphasized the abundance of food and available farmland in America. He calculated that America's population was doubling every twenty years and would surpass that of England in a century. In 1751, he drafted "Observations concerning the Increase of Mankind, Peopling of Countries, &c." Four years later, it was anonymously printed in Boston, and it was quickly reproduced in Britain, where it influenced economists Adam Smith and later Thomas Malthus. Franklin's predictions also alarmed British leaders who did not want to be surpasse