The Georgia Guidestones were built in 1979. The first line says " Maintain humanity under 500,000,000 in perpetual balance with nature.". but the estimated population in 1979 was 4,378,000,000 people, are were expected to kill 3 and a half billion people?
Only if you listen to lunatics. This is a number that isn't supported by environmental sciences. Contemporary political ecologists are incredibly critical of neo-malthusianism, and have quality arguments. Huzzah for inter-disciplinary study!