When Keynes wrote about slavery to the idea of some defunct economist, he may well have been referring to Malthus. The idea of population growth outstripping food supply has had a long life, retaining great vigor even in its advanced age.
In its modern version, the Malthusian idea is that population growth is driven by high fertility (and low mortality), eventually outstripping the carrying capacity of the environment and natural resources. Physical scarcity will eventually force a stable population outcome, while producing plenty of human misery by war, disease, and famine. (Ever watch Soylent Green?) The most oft-cited essay on modern Malthusianism has got to be "The Tragedy of the Commons" by Garret Hardin, published in Science. Hardin's argument is that the world is an open access resource ("commons"), and population growth forces an inexorable pressure upon it. Individually rational behavior from the growing population causes a depletion of global resources. Solutions offered - appeals to conscience in restraining demands on resources, or privatizing the commons - are all impractical. Hardin's recommendation? Do away with the idea that families have the fundamental human right to determine their own size. Implement population controls by "mutual coercion, mutually agreed upon."
Written as it was in 1968, the essay is now very seriously dated. What really happens when we allow households to determine their own size voluntarily? It depends. In wealthier nations, households tend to be smaller. In fact in many countries the fertility rate is below 2.0 (the absolute minimum for population replacement), hence without migration, these populations will shrink! These countries include: Canada, Japan, Korea, France, Germany, Greece, Norway, Portugal, Sweden, Spain, Italy, Singapore, Germany, and the United Kingdom. Why? A host of reasons - educated, working, socially assertive women, access to modern contraceptive technologies, all correlated with developing country status, are the key reasons. Growth to a developed country status would be the long term cure for overpopulation, averting the dire Malthus-Hardin prognostications.
In the medium term though, I don't deny that rapid population growth is itself a drag on per capita income growth. One reason may be through diminished household asset formation: the more children, the lower the savings (Orbeta, 2006) and the lower is schooling per child.
So there is an argument to providing incentives to limiting household size. Tax exemptions based on number of dependents have the perverse effect of encouraging larger household size (simply by lower the cost of bigger families.) Subsidies on contraceptives and information drives on their responsible use will also play a role. Now if your religion doesn't allow you to avail of these technologies, then of course you are not to be coerced. But neither should the state allow its population policies to be hijacked by the values of one or two religions, however widely held.