about depopulation returned. Population planning movement In the 20th century, population planning proponents have drawn from the insights of Thomas Malthus, a British clergyman and economist who published An Essay on the Principle of Population in 1798. Malthus argued that, "Population, when unchecked, increases in a geometrical ratio. Subsistence only increases in an arithmetical ratio." He also outlined the idea of "positive checks" and "preventative checks." "Positive checks", such as diseases, wars, disasters, famines, and genocides are factors which Malthus believed could increase the death rate. "Preventative checks" were factors which Malthus believed could affect the birth rate such as moral restraint, abstinence and birth control. He predicted that "positive checks" on exponential population growth would ultimately save humanity from itself and he also believed that human misery was an "absolute necessary consequence". Malthus went on to explain why he believed that this misery affected the poor in a disproportionate manner. Finally, Malthus advocated for the education of the lower class about the use of "moral restraint" or voluntary abstinence, which he believed would slow the growth rate. Paul R. Ehrlich, a US biologist and environmentalist, published The Population Bomb in 1968, advocating stringent population planning policies. His central argument on population is as follows: In his concluding chapter, Ehrlich offered a partial solution to the "population problem", "[We need] compulsory birth regulation... [through] the addition of temporary sterilants to water supplies or staple food. Doses of the antidote would be carefully rationed by the government to produce the desired family size". Ehrlich's views came to be accepted by many population planning advocates in the United States and Europe in the 1960s and 1970s. Since Ehrlich introduced his idea of the "population bomb", overpopulation has been blamed for a variety of issues, including increasing poverty, high unemployment rates, environmental degradation, famine and genocide. In a 2004 interview, Ehrlich reviewed the predictions in his book and found that while the specific dates within his predictions may have been wrong, his predictions about climate change and disease were valid. Ehrlich continued to advocate for population planning and co-authored the book The Population Explosion, released in 1990 with his wife Anne Ehrlich. However, it is controversial as to whether human population stabilization will avert environmental risks. A 2014 study published in the Proceedings of the National Academy of Sciences of the United States of America found that given the "inexorable demographic momentum of