One of the consequences of Malthus' theory of population was to promote the idea of a national census to measure population dynamics. Many countries have adopted the practice of taking a census, usually at intervals of five or ten years. The accumulated data allowed Warren Thompson and F. W. Notestein to develop the Demographic Transition Model (DTM). By 1945 the model described four stages of economic development.

1--pre-industrial society with high, fluctuating birth and death rates
2--developing countries with lower death rates and expanding population
3--birth rates fall and population begins to level off
4--birth rates drop further causing population to stabilize or even decline

        There has been a general decline in fertility in many parts of the world. In a few countries, like Russia, Ukraine, and Japan, sustained sub-replacement fertility has been severe enough to cause populations to decline (Wales et al., 2006c). In Japan, which has an incredibly high population density of 300 people per square kilometer, you might think that a decline in the population would be viewed with relief. After all, this country has three times the population density of China, a country that has instituted a One-Child Policy to stem its own population growth. The subways in Japan are so crowded during the morning rush hour that cities, such as Tokyo and Nagoya, feel the need to have special women-only cars. It is not only universities and other sectors of a depressed economy that cater to young people that are worried about the repercussions. Government tax offices are worried, too.

        Let's examine three simple population models to see if these tax concerns might be justified. Remember that our models assume that all production comes from the parents. We can further postulate that (a) they raise their own children--their parental burden and (b) pay taxes to support the entire group of grandparents (all of whom are retired)--their tax burden. Observe the shifting burden as fertility drops from the 4-Child (expanding) Population Model to the 2-Child (stable) Population Model. In an expanding population each set of parents is supporting 5 people, four children and one grandparent, while in a stable population each set is supporting 4 people, two children and two grandparents. If, as we have previously assumed, everyone consumes at the same level, then the combined parental and tax burden is less. That's the good news, but if fertility drops below replacement levels to 1-child per couple the burden continues to shift from parental responsibilities to taxes. In the 1-Child (contracting) Population Model each couple supports one child and four grandparents. The burden returns to the level of five people per couple. In so doing, the burden shifts from a voluntary parental burden to an involuntary tax burden. The involuntary character of this burden might make it psychologically harder to accept. Unlike taxes, the responsibilities and burdens of parenthood can be avoided by not becoming parents in the first place. That, in fact, may well explain why the fertility rate is dropping in countries like Japan.
        The United Nations has announced three possible projections for global population growth--none of them as radical as our 4-child and 1-child models, of course. Please keep in mind that because of child mortality in the real world, a total fertility rate of 2.2 is actually needed to maintain a stable population with 4 people being supported by each couple. Although the high U.N. projection of 2.35 children per couple represents a slowing of current birth rates and a lowered burden on the middle generation (parents) of 4.009 people per couple, exponential growth would continue to push population up with no end in sight. The U.N.'s medium projection of 1.85 allows the population to peak at 9 billion in the year 2075, while burdening each couple with 4.060 dependents. The low projection of 1.35 would have us reach the peak of 7.4 billion by 2050 with the burden increased to 4.487 dependents per couple. Cobb (2006) predicts an even steeper decline in birth rates with a peak of 7 billion in 2040. If he is right, this uncomfortably steep decline will undoubtedly create considerable stress.

        Disease and death have always kept plant and animal populations in check (see Coutts, n.d.). It's all a part of the growing-aging process. Famine, war, and epidemics simply accelerate that very natural process. It's a matter of degree and timing. Ehrlich's predictions of catastrophe have all been realized in less dramatic form than he envisioned.

        Conditions of hunger and malnutrition seem always to be present in developing countries, where populations have reached a saturation level for an agricultural way of life. Then natural or socio-political disturbances can trigger large-scale famine--as they did in Bangladesh in 1974, in Ethiopia in 1984 and 1985, and in North Korea from 1997 to 1999. Luckily famines have remained both regional and sporadic with death tolls contained to the order of one million. On a global scale there seems to be enough food for our most basic needs, though it is not distributed equally by any means.
        Demand for a limited supply of scarce resources, goods, and services leads to such fierce competition within the human species that it frequently ignites violent behavior between individuals, groups, and governments. Urban centers with high-density populations seem to generate the lion's share of violent crime. At the international level large conventional wars have, to a large extent, been replaced by civil wars accompanied often by logistic support and occasionally by the intervention of major powers. Four million people perished in the Second Congo War from 1998 to 2004; as many as 2 million in Afghanistan between 1979 when the Soviets intervened and 2001 before the American invasion. By comparison the more conventional Iran-Iraq War (1980-1988) produced about one million casualties. Fighting breaks out between religious and ethnic groups as often as it does between ideologies. This has led to massacres such as those that took place in September 1982 at Sabra and Shatila and genocide like that in Bosnia (1992-1995) and Rwanda in 1994, with death tolls in the hundreds of thousands. The development of weapons of mass destruction, their spread to unstable countries, and the export of suicide terrorism from the Middle East to other parts of the globe has greatly increased the chances of a man-made catastrophe.
        While groups of Homo sapiens are locked in mortal combat with members of their own species, all of mankind is engaged in interspecies competition at both the micro and macro levels. Microscopic organisms and viruses can take advantage of high human population density and mobility. The Spanish flu epidemic of 1918-1920 killed at least three, perhaps six, times as many people as did the combat of World War I 1914-1918. Almost 100 years later a similar strain of avian flu threatens a larger, denser world population. Attempts to eradicate our micro enemies, unfortunately, have only been successful against one virus. Now with only a small portion of young people vaccinated against smallpox, the worry is that a man-made virus could become a devastatingly powerful biological weapon. For a while tuberculosis seemed to be retreating, then in the 1990s it made a comeback, causing the World Health Organization to declare a global health emergency. Meanwhile AIDS continues to make steady progress. All the while Richard Dawkins' Blind Watchmaker keeps churning out new challenges to our antibiotic arsenal--new drug resistant strains and new diseases such as SARS and Ebola.
        On a macroscopic scale, humans have been perhaps too successful at dominating other species, sometimes to the point of extinction. Human activity has tended to destroy any species that we cannot domesticate. Wildlife is disappearing so fast that it has already been named the Holocene extinction event. Not only are we ruining the habitats of countless animals and plants, we are destroying our own habitat with contaminated soil and polluted water. Chemicals we pour into the atmosphere are destroying the protective ozone layer and raising temperatures to a point that threatens our climates. Clearly we need to reduce our consumption of natural resources and disposal of waste. In order to do that we need to limit our population.