In July 1820, Godwin published "Of Population: An Enquiry Concerning the Power of Increase in the Numbers of Mankind" as a rebuttal to Malthus' essays. Godwin's main argument was against Malthus' notion that population tends to grow exponentially. Godwin believed that for population to double every twenty-five years (as Malthus had asserted had occurred in the United States, due to the expanse of resources available there), every married couple would have to have at least eight children, given the rate of childhood deaths. Godwin himself was one of thirteen children, but he did not observe the majority of couples in his day having eight children. He therefore concluded:In reality, if I had not taken up the pen with the express purpose of confuting all the errors of Mr Malthus's book, and of endeavouring to introduce other principles, more cheering, more favourable to the best interests of mankind, and better prepared to resist the inroads of vice and misery, I might close my argument here, and lay down the pen with this brief remark, that, when this author shall have produced from any country, the United States of North America not excepted, a register of marriages and births, from which it shall appear that there are on an average eight births to a marriage, then, and not till then, can I have any just reason to admit his doctrine of the geometrical ratio.