ZPG is the name of a non-profit organization dedicated to informing people about overpopulation and overconsumption. ZPG stands for zero population growth (q.v.).
Founded in 1968, the same year as Dr. Paul Erlich's book The Population Bomb, ZPG's original message was somewhat simple: it encouraged American families (who, at the time averaged 3.4 children per female) to limit their families to just two children.
However, the late 60s were a rather revolutionary time, and ZPG's message became entwined not only with population control but also environmental and ecological concerns. ZPG's influence helped shape some legislative changes, as well, paving the way for the revision of the Comstock laws to remove the obscenity label from contraceptives in 1970, as well as helping to give contraceptive rights to the unmarried (Supreme Court Eisenstadt v. Baird 1973). In addition, the group was a strong proponent of adoption instead of procreation. They also funded a number of vasectomy clinics.
ZPG also became active in abortion legislation, and in the 70s, the fertility rate in the USA dropped to 1.8 children per female. However, the 80s and 90s saw a huge increase in the number of anti-abortion laws hit the books, and the organization has come to realize that its original message has become a bit dated and perhaps too simple for the times.
My mother founded the Houston chapter of ZPG in 1970. She adopted both my brother and I, and this information was drawn from a phone conversation with her. Thanks, Mom! :-)