In 1798, Thomas Malthus (1766-1834), a British clergyman and economist, published the "Essay on the Principle of Population," which argued that population multiplies geometrically and food arithmetically. Therefore, the population will outstrip the food supply. In addition to cutting the birth rate by sexual restraint and birth control, Malthus advocated that all people "defective" in any way, who look or behave or function differently than the rest of society, should be identified and eliminated. Therefore, only those who are "normal," those who can make the greatest contribution to society, would survive.
The idea that one must look "normal" or make a specific contribution to society in order to live is not new with Malthus. This idea existed in the ancient era and persists today. This "Return on Investment Syndrome" recurs throughout history, stating the ability to be productive and repay society for what one receives, rather than what one can contribute, determines that person's worth.
With the industrial revolution of the 18th century, more and more people flooded into cities, working for slave wages and living in squalid conditions. Children represented a large portion of the work force, performing grueling work for twelve to sixteen hours per day. Pauper children were often contracted to factory owners for cheap labor. To get rid of "imbecile" children, parish authorities often bargained with factory owners to take one "imbecile" with every twenty children.