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Overpopulation is the state of any species exceeding, in numbers, the ecological carrying capacity of its ecosystem. This is primarily expressed in terms of habitat and food supply available. In conventional predator-prey relationships (most commonly illustrated with rabbits and foxes), increased predator numbers mean that food supply drops and the population subsequently starves. The cycle completes as the population dwindles, allowing the prey population to recover.
This principle can be applied to human population growth just as much as it can to hypothetical rabbits and foxes. Ecological carrying capacity is established scientific fact, but often denied, or more usually, ignored in the pursuit of endless economic growth.
The main factors in the problem are:
- Total world population, expected by demographs to settle at 10-12 billion
- The consumption per inhabitant, direct or indirect (mainly through livestock)
- The total world food production
- The repartition of this food among the human population
Denialism of overpopulation is common within fundamentalist Christianity, as well as the Catholic Church, who see any discussion of the issue as a violation of the Biblical commandment to "be fruitful and multiply, and replenish the earth" (Genesis 1:28).
There are also various conspiracy theories that overpopulation is a cover issue for a plot to reduce the human population through genocide; this conspiracy theory is sometimes advanced, for example, by AIDS conspiracy theorists (who actually believe AIDS was created in a CIA lab to depopulate Africa), as well as perennial wingnut/moonbat candidate Lyndon LaRouche. The most common and popular form of overpopulation denialism these days, however, comes from the influence of laissez-faire economists, such as the Wall Street Journal, and the late Julian Simon (who is popular among libertarians). This form of denialism teaches there are "no limits to growth," and is associated with the view that economic growth can and should continue indefinitely, and that continued economic growth depends on a perpetually growing human population. It is also closely tied to excessive optimism over globalization and technology, as well as economic deregulation. These views are collectively sometimes referred to as "cornucopian" — in that they believe there is an endless supply of matter to support an ever-growing population and economy. It denies the fact that at some point, the consumption demand will run up against natural limits in supply.
Another form of denialism comes from those on the other extreme of the issue, who believe carrying capacity is set in stone and deny that the carrying capacity can be increased due to science and advances in technology increasing food output, and from easier access to energy; see the "failed predictions" section below.
Rash and inaccurate predictions of impending world famine which erred on the side of sensationalism over sober analysis have characterized a great deal of past interest in the subject matter, including by respected scientists. Thomas Malthus, one of the earliest to theorize about overpopulation, wrote in 1798 that the population would grow exponentially while the food supply would only grow arithmetically, failing to take into account technological developments that would provide greater agricultural yield. (Not to mention that food itself is an exponentially growing population.) During the 1960s, Paul Ehrlich predicted massive famine by the 1970s, which didn't happen due to the Green Revolution (which was indeed a revolution in agriculture but is anything but 'green'). His prediction was also based on the assumption that the birth rate in the United States and other Western countries would continue at high Baby Boom levels, and may well have been postponed by the birth rate dropping to near-replacement levels due to the sexual revolution and the related widespread availability of birth control.
In all cases these predictions appear to "fail" because they did not take into account technological and medical advances that delay what seems to be inevitable. One cannot blame them for what they could not know.
In 1679, Antoni van Leeuwenhoek (1632 - 1723) estimated that the maximum number of people Earth can support is 13.4 billion. Many more estimates of how many people Earth could support followed. The estimates have varied from 1 billion to 1000 billion. Estimates published in 1994 alone ranged from 3 billion to 44 billion.
Since 1679, there has been no clear increasing or decreasing trend in the estimated upper bounds. The scatter among the estimates increased with the passage of time. This growing divergence is the opposite of the progressive convergence that would ideally occur when a constant of nature is measured. Such estimates deserve the same profound skepticism as population projections. They depend sensitively on assumptions about future natural constraints and human choices. Many authors gave both a low estimate and a high estimate. Considering only the highest number given when an author stated a range, and including all single or point estimates, the median of 65 upper bounds on human population was 12 billion. If the lowest number given is used when an author stated a range of estimates, and all point estimates are included otherwise, the median of 65 estimated bounds on human population was 7.7 billion. This range of low to high medians, 7.7 to 12 billion, is very close to the range of low and high UN projections for 2050: 7.8 to 12.5 billion. A historical survey of estimated limits is no proof that limits lie in this range. It is merely a warning that the human population is entering a zone where limits on the human carrying capacity of Earth have been anticipated and may be encountered. 
Finding a solution to human overpopulation is difficult in that it essentially requires people to reproduce in fewer numbers, something they may be unwilling to do, and any success is likely to be a very gradual process. Greater awareness of and access to contraception around the world would help promote population control, although this is opposed by some cultures and religions, particularly the Roman Catholic Church. There is an international childfree movement that may help curb the problem of overpopulation. More aggressive, though less realistic, is the Voluntary Human Extinction Movement, which encourages complete stop of human reproduction.
The People's Republic of China's response to its populations problems is the one-child policy, instituted and enforced since 1979. Fearing its economic and societal consequences, however, China started to formally phase out this rule in 2015. With standards of living in China rising rapidly due to miraculous economic growth, it is doubtful that China's birth rate will rise significantly, if at all.
Isaac Asimov took an interest in overpopulation and proposed that homosexuality should be considered a "moral right" since homosexual sex doesn't lead to reproduction, which would help curb overpopulation. While it is occasionally claimed that homosexuality is a response to overpopulation, there's no reason to believe this is true. Also, given the upsurge in surrogate pregnancies, artificial insemination, and various other alternative methods of child production, both present and projected, homosexuality may cease to prevent reproduction.
The demographic transition model holds that as nations transition from low-income, undeveloped nations to high-income, developed nations, their birth rate and death rate both drop. This model posits five stages, and in the fifth and last stage of demographic transition, population levels off and reaches a state of zero or negative population growth. With increased prosperity comes greater autonomy for women in society, better access to family planning and birth control, women entering the workplace and limiting their number of children, and more free flow of information (with the effect of secularizing the society and lessening the influence of religions which promote large families); conversely however, during the middle stages of the demographic transition the death rate drops faster than the birth rate, causing a temporary increase in population. Countries like Japan, much of Europe, and the native inhabitants of the United States are at or near the final stage, and have achieved a state of zero-to-negative population growth without any need for coercive policies such as China's.
Abuse of term from the extreme Left and Right
Left-leaning critics of the arguments supporting overpopulation note that they tend to be levied against the lower-class people of Third World countries, who are stigmatized as having more children than they can afford, even though this is generally true. They argue that it is classist because some hard green activists who support the stereotypes fail to consider the excesses and byproducts of capitalism in developed nations, which is incredibly vague and meaningless; xenophobic because it specifically targets people in developing nations, which is stupid because they are the ones reproducing the most; and sexist because it shames women in particular for having too many babies, though men can use contraception as well.
On the other extreme of the political spectrum, the "overpopulation" scare is sometimes invoked by cranks and white supremacists to justify raping other women as part of a race war to prevent white genocide.
(Native) Population growth is falling in almost all First World countries. In extreme cases, the birth rate is well below 2.1, the replacement rate in the case of Japan. The UN has projected that we will likely never reach 11 billion people on Earth. Third World countries are the main source of unsustainable population growth, but as standards of living rise, birth rates tend to drop. Whether the human population growth is curbed enough remains to be seen.
Long seen as the overcrowded filthy third world, South and Southeast Asia have actually ceased their exponential growth. All those tens of millions of jobs that have been going overseas? Well, they've gone to Asia, and more importantly, a lot of those jobs have been going to women. It turns out that a lot of women would rather work for a meager salary for herself than be forced into marriage as a baby-factory. Both India and Bangladesh now have birth rates barely above 2 per woman, and it's still dropping. Vietnam now has a birth rate below 2 per woman, and Thailand a mere 1.5, effectively making the population growth negative longterm. Same with Malaysia, and especially Singapore.
Not just Asia, but Latin America as well. Mexico's birth rate has plummeted to just above 2 per woman and still dropping, and Brazil is well below 2 per woman. Colombia, Argentina, Chile, also below 2. Even in Africa birth rates are falling, although much more slowly.
A lot can happen in 40 years.
- Cornucopian vs. Malthusian debate
- Demographic crisis
- Depopulation conspiracy theory
- Georgia Guidestones
- Some statistics by the BBC
- Why environmentalists often ignore overpopulation
- Zehner, Ozzie. “Population/Overpopulation.” In Green Culture, edited by Paul Robbins, Kevin Wehr and J. Geoffrey Golson, 366-69. London: Sage, 2011.
- I-didn't-reproduce Day. A segment by Bill Maher.
- I don't want children — stop telling me I'll change my mind. TED talk by Christen Reighter.
- Calhoun JB. Death Squared: The Explosive Growth and Demise of a Mouse Population. Proceedings of the Royal Society of Medicine. 1973;66(1 Pt 2):80-88.
- Calhoun JB. Population Density and Social Pathology. California Medicine. 1970;113(5):54.
- Gibert, Helen. Lyndon LaRouche: "Fascism Restyled for the New Millennium"
- http://www.juliansimon.com/writings/Norton/NORTON02.txt — Chapter 2: "Population Growth is Not Bad for Humanity"
- Asimov, Isaac (1996). Yours, Isaac Asimov, edited by Stanley Asimov. ISBN 0-385-47624-8.
- Fertility rates throughout the world, for Our World in Data. Lot's of other fun stuff in this link re birth rates