Paul R. Ehrlich & Anne H. Ehrlich, The Population Explosion, 1990.

Population and Public Health

The role of population size in public health is often very subtle and not as easily measured as it is for ecosystem health. Twice as many people clearly add twice as much CO2 to the atmosphere, everything else being equal; twice as many people in a city will not double the chances of an epidemic. Of course, environmental degradation tied to population growth poses quite direct threats to human health; the well-known conse­quences of urban air pollution and contamination of food and water are obvious cases in point.1


If there are only a few thousand automobiles in a city, natural air movements may carry away the cars' noxious effluents and rainfall may cleanse the air so that there is little or no health hazard. A few hundred thousand motor vehicles, however, may easily overwhelm these natural dispersal and cleaning functions and produce life-threatening smog. [137]

A few thousand people living along a river may be able to put their sewage into it without causing it to become generally polluted. Dilution and the action of sunlight and microorga­nisms may keep the water pure enough to drink. A few hundred thousand people dumping their sewage into the same river may create a stew of disease-causing bacteria that only a fool would imbibe.

A few thousand people residing in a state may all be able to live on self-sufficient organic farms and enjoy fresh, uncon-taminated produce. A few million in the same state will have to leave the farming to a relatively few large-scale, "efficient" industrial agricultural operations. Their food will often be leavened by pesticide residues and preservatives.

The more people there are in an area, all else being equal, the more effort they will have to expend to avoid various types of "pollution." Once a certain threshold is passed, each additional person added will create disproportionately more. The ores mined to supply the new person will, on average, be of lower grade and have to be hauled farther. The oil that is refined into gasoline for his or her car will come from deeper and farther away. Water will have to be moved farther to slake his or her thirst. The garbage he or she produces will have to be moved to a more distant point or be treated more exten­sively.2

This situation is simply an example of the economists' "law of diminishing returns."3 Beyond a certain population size, if the per-capita affluence (A) of a population is held constant while the population grows, the environmental impact per unit of affluence (that arising from the technology factor, T) will nevertheless inevitably increase.4 More people require more resources, and the richest and closest supplies will be consumed first. Then it becomes necessary to drill deeper for groundwater or oil, to separate metals from poorer ores, and to extend supply networks. These activities all require the use of more energy per person, and that generates more pollution per person (as well as other kinds of environmental disruption). Diminishing returns hit poor countries even harder, since those countries often lack the capital required to apply technological fixes to pollution problems. There are no catalytic [138] converters on the multitudes of cars in Mexico City, for example, and that contributes to one of the worst air-pollution prob­lems on the planet.5 Perhaps the most poignant case of diminishing returns in such nations, however, has no immediate connection to pollution. As fuelwood near expanding villages is exhausted, women must walk farther and farther for it, sometimes spending most of every day at the task.6

And the experience of both rich and poor countries indicates that pollution-abatement efforts are doomed to some degree of failure, especially in the absence of population control.

The United States, for example, is famous for suffering a glut of automobiles. Since 1970, while the population of the nation has increased by 25 percent, the number of passenger cars sold in the United States increased by 50 percent.7 One result of that is continuing, unacceptably high levels of air pollution, causing as many as thirty thousand premature deaths annually, especially among those individuals with asthma and other respiratory problems. Perhaps 75 million Americans live where national air-quality standards for ozone, particulates, and carbon monoxide are not met.

All of this is in spite of the success of the Clean Air Act of 1970, which by 1989 did manage to reduce air pollution to two thirds of its 1970 level, even in the face of substantial population growth. If the population had not grown, however, air pollution would now be only a little more than half the 1970 level.8

The Office of Technology Assessment is not sanguine about the prospects for clean air by 2000, as projected in 1989 by the Bush administration in connection with its proposed new Clean Air Act.9 The Bush proposals deliberately avoided tightening controls on vehicles, saying that the American love affair with cars made further restrictions on automobiles unacceptable to the public! Of course, the further clogging of freeways and the increased smog might dim the luster of that love affair.

What no one is suggesting is any effort to reduce the P factor to help with the struggle to give the nation healthy air. We are not, of course, arguing that population size should be reduced so far that everyone can "go back to nature" and live [139] free of pollutants. Many benefits have accrued to those who live with some pollution. Cars provide the convenience of personal transportation (although, with increasing population densities, this is more and more an inconvenience). To many that convenience may be an ample benefit to exchange for the cost of a higher risk of dying young of heart disease or lung cancer. Having lots of people living along a river can provide the opportunity for lively cities with universities, concerts, a wide variety of industries to provide high-paying jobs, and a diversity of stores and restaurants in which to spend the money. Water-treatment plants, or even a slightly higher chance of getting cancer, may seem a small cost to pay for such benefits. Modern agriculture and global transport have provided an abundance and variety of food unknown in the past, and there is little evidence that, overall, the various residues that may be present in the food available in the United States have caused serious health problems.10

What we are arguing here is that pollution generally increases with population density, and that there is every reason to believe that after the population is dense enough to provide all the benefits one could wish, additional growth simply exacerbates the pollution problems and makes their solution much more difficult and expensive. Further, we would contend that in many cases pollution could be greatly abated by organizing ourselves differently -- by reducing the A factor as well as the T factor. For example, smaller cars, more car-pooling, and good mass transport could greatly reduce smog problems (as well as making commuting more pleasant). Better yet would be reorganizing our cities so more people could walk to work. Catalytic converters and methanol are not the only ways to skin the smog cat. But if our focus is only on A and T, the P factor will always get us in the end.

The population connection to pollution, like that to ecosystem health, is thus pretty clear-cut. But what is the effect of doubling the number of people on the chances of an epidemic propagating? How does doubling the size of a city affect its livability? It is to such issues that we now turn. [140]


Few people realize that epidemic disease and population size have interacted in important ways in human history. Indeed, William McNeill has argued that epidemics have played a key role in the rise and fall of many civilizations.11 For example, it is very difficult to explain the conquest of some 100 million Native Americans, many with advanced, well-organized societies, by a handful of Spaniards -- except by invoking the incredible impact of the European diseases they brought to the New World. Native Americans had essentially no resistance to the common diseases of the Old World, diseases that largely afflicted children in European populations which had been regularly exposed to them long enough to evolve natural immunity.

Less than fifty years after Cortez landed, the population of central Mexico had been reduced to about one-tenth the size it had been before contact. The viruses of smallpox, measles, influenza, and mumps, and the bacteria that cause whooping cough, diphtheria, scarlet fever, and a variety of other nasty ills, introduced by the invaders, were much more effective than the primitive guns of the Spaniards. Indeed, only a raging smallpox epidemic prevented the Aztecs from destroying the last of the Spaniards after forcing them out of the Aztec capital of Tenochtitlan (the predecessor of Mexico City). The epidemic was traceable to a soldier in the Spaniards' ranks, one of the few invaders not immune, who contracted the disease and spread it to the Aztecs. Many Aztecs were killed, including their leaders, making possible the Spanish reconquest of the city (which had some sixty thousand homes and a population estimated to be a few hundred thousand). Overall, pestilence brought by the Conquistadors may have killed something on the order of a third to half of all the Aztecs and Incas,12 and imported diseases continued to kill Native Americans in similar proportions into the mid-nineteenth century.

The destruction of Native American civilizations was apparently aided by the psychological impact of pestilence that differentially struck down the natives and allowed the Spaniards to escape virtually unscathed. Both the newcomers and [141] the Aztecs assumed that this was a sign of heavenly displeasure, and it broke the spirit of the Aztecs. Whatever the details, there can be no doubt that diseases, especially smallpox, were by far the greatest and most effective allies of the Con­quistadors.13

The thesis that disease has often shaped human history can be applied persuasively to the rise of Western Europe as the dominant world culture and to many other events. But whether or not one accepts all of his arguments, McNeill highlights another element that is even less widely appreciated than the overall historical role of pestilence. That is the role of population size and density in determining the vulnerability of a community to epidemics. Many bacterial and viral diseases that pass directly from person to person depend on cities -- densely packed communities of thousands of people -- for their persistence. Otherwise they run out of susceptible individuals to infect, and they die out. Measles, for example, cannot persist in populations of fewer than about 300,000 people.14 Such diseases were not problems for our hunter-gatherer ancestors.

People have complex relationships with the tiny parasitic organisms that feed on them from the inside. Individuals gain protection by fighting off an invader with their immune systems; populations apparently gain protection by evolving resistance -- that is, individuals of a resistant population on the average are better equipped to fend off a particular disease.15 Populations that have no recent experience with a parasitic disease are susceptible to having a huge proportion of individ­uals infected.

As shown by the Native American experience with smallpox, those who contract the disease in such susceptible populations may suffer very heavy mortality. Moreover, the Native American tragedies are not the most extreme examples of die-offs. In an unidentified epidemic in northwestern China in the early fourth century, mortality was reported to be over 95 percent. The death of half or more of populations suffering novel epidemics was frequently reported before Columbus landed in America. Even the modern world has not proven immune. A new strain of influenza killed some 20 million people after infecting most of Earth's population at the end of World War I. [142]

These episodes should not be dismissed as problems of the past. There is no finite number of diseases that, once conquered, will leave Homo sapiens free of the attack of microparasites. Novel human diseases come primarily from transfers from other animals. Pigs get influenza. Measles may be a form of the rinderpest virus (which causes an often fatal disease of cattle and their relatives) or of canine distemper viruses. It must have been transferred from an animal source within the last six thousand years or so, since human communities large enough to support the virus didn't exist before then.16

Smallpox was likely a gift from cattle or their relatives, as suggested by the close similarity of the smallpox virus to the cowpox virus. Smallpox has close relatives that attack many other organisms. Presumably, then, even if the historic human smallpox strain has been exterminated, another mutation and transfer might someday restore it. Syphilis is believed by some to be an evolutionary development of the spirochete that once caused a serious disease called yaws in the European population,17 having evolved the venereal method of transmission when more clothing and less personal contact made simple skin-to-skin transmission much more difficult. And the influenza virus frequently evolves nasty new strains.

The details of these relationships between human beings and their parasites are not entirely understood and needn't concern us here. The key point is that, for decades now, Homo sapiens has been setting itself up as an increasingly ideal target for a worldwide epidemic -- a pandemic -- as we and others have pointed out repeatedly. The rising numbers of malnourished people living in conditions of poor sanitation and unclean water supply are rapidly increasing the pool of potential disease victims. In addition, ever faster transportation systems have made the pool of susceptible people essentially global.

Increases in the speed of transport can extend chains of infection to previously inaccessible areas. For instance, at the end of the last century, an outbreak of bubonic plague that began in the interior of China spread throughout the world after reaching the seaport of Hong Kong in 1894. In earlier plague outbreaks, the disease would not have reached many distant areas by sea, because on small, slow-moving sailing [143] ships the supply of susceptible people usually would be exhausted before the voyage was over. But the development of steamship fleets after 1870 had changed all that, and within a decade after the Hong Kong outbreak the plague reached all of the world's important seaports. Modern sanitation and knowledge of the role of rats and fleas in disseminating the disease led to its rapid containment in most areas. In India, however, about 6 million people perished before the outbreak was controlled.


Of course, modern air transport means that a person carrying an infectious disease usually can reach anyplace in the world within a day or so. The dangers of rapid transport of smallpox (until its recent eradication) and yellow fever viruses by passengers on jet aircraft have been well recognized by public-health authorities.18 Fast transcontinental transport adds considerably to the threat of diseases that ordinarily produce symptoms in a short time, greatly compounding the problems of quarantine and essentially binding all of Homo sapiens into one gigantic, potentially susceptible pool of hosts for infectious diseases. As Nobel laureate virologist Howard Temin wrote of AIDS in the light of post-World War II social changes, "If anything, the surprise might be that there has been only one major new epidemic."19

In short, the human epidemiological environment has become ever more precarious. We are creating a giant, crowded "monoculture"20 of human beings, which would be at high risk in epidemics even if there weren't millions of people who were especially vulnerable to disease because of their debilitated condition, and even if carriers couldn't zoom around the globe with unprecedented speed. The major barrier between humanity and such disasters is the global public-health system, and it provides the least protection to those who are most vulnerable. The United Nations Children's Fund (UNICEF), however, in recent years has been making heroic efforts to extend immunization for the main childhood diseases to children in poor countries everywhere. [144]

It is true, nonetheless, that humanity has made gigantic strides in its ability to deal with microparasites, and ordinary people in developed nations are certainly much less likely to succumb to a communicable disease than were royalty in the middle of the last century. Yet the likelihood is far from zero even for the richest members of society, as the AIDS epidemic has clearly demonstrated. Malaria, long in decline, is resurgent today among the poor, who have inadequate access to modern medicine and are not able to protect themselves (with screens and repellents) against the mosquitoes that carry the malarial organisms.

At a special meeting, virologists have recently gone public with their concerns about the possibility of viral epidemics made difficult or impossible to control because of "population growth and the fact that millions now live in crowded squalor . . . and jet travel allow[ing] infected people to spread a virus even before they are aware they are carrying it . . . [and] various environmental changes [that] could also influence the emergence of viruses."21 They are worried not just about epidemics caused by known viruses, but about viruses that have not previously infected large numbers of people. Dr. Donald A. Henderson, dean of the Johns Hopkins University School of Public Health, "called for development of an international early warning system to detect newly evolving viruses quickly." He suggested establishment of monitoring stations "near rain forests where unknown viruses are thought to lurk, and in densely populated areas to which people have migrated from rural areas, where viruses may have infected only isolated groups."22

There is ample reason to heed their concerns. The vulnerability of human society to viruses that transfer from other animals, especially primates, was demonstrated by the appear­ance in Uganda in 1959 of a "new" virus disease, O'nyong-nyong fever.23 An epidemic of O'nyong-nyong swept through much of East Africa, but remained nonlethal. It suggested, however, the potential that a new lethal virus could have for assaulting humanity -- a potential that has been compared to the impact of the myxomatosis virus on the European rabbit populations in Australia, which it virtually exterminated. [145] Richard Fiennes, in his book on diseases that can transfer from primates to human beings,24 asked rhetorically what would happen if such a disease should appear "in packed human communities," and concluded that mortality could be 90 percent.25

Not all new virus diseases appearing in the human population have been as benign as O'nyong-nyong. The AIDS virus (HIV) is one of several new lethal viruses to threaten us in recent decades,26 and the first to cause a large-scale epidemic, one that will almost certainly kill millions of people. The first new lethal virus to be recognized caused Marburg disease. African vervet monkeys were the original hosts of that virus, a relative of the one that causes rabies. In 1967, a consignment of monkeys carrying the virus passed through London's airport on their way to a laboratory in Marburg, Germany. In the laboratory, twenty-five people in contact with the monkeys or their tissues were infected by the virus, and seven died almost immediately. They passed it on to a few additional people, all of whom survived.27

Humanity had two strokes of luck in this case. First, the incubation period of the virus was short -- only four to seven days. Victims had little chance to contact others and pass on the virus before they became sick and died. This allowed epidemiologists to trace the course of the infection and quickly limit it by isolating carriers. Second, people were not infected by the virus until after the monkeys reached Marburg. If the disease had been passed to people handling the shipment at the London airport, it might have been disseminated around the globe before anything could be done to stop it. Marburg virus is an excellent example of a disease-causing agent which rapid transport could play a key role in spreading, because of its short incubation time.

Marburg virus disease has subsequently been seen outside laboratories and has caused several hundred deaths in Africa.28 There, as the human populations expand, pathogens are more frequently transferring from animal reservoirs into the human population and causing life-threatening infections.

A second serious disease also originated in Africa. That was Lassa fever, caused by another virus that originated in [146] nonhuman mammals, in this case a rat. It first appeared in the Nigerian village of Lassa in 1969 as a highly virulent, contagious hemorrhagic disease. A small but deadly outbreak ensued, and the virus was imported into the United States when sick medical-missionary workers were evacuated. But human luck held again. The originally lethal virus became less virulent as it passed from person to person, and a serum containing survivors' antibodies has helped cure victims. The virus still causes hundreds of cases and many deaths each year in central Africa. In 1989, a forty-three-year-old mechanical engineer died of it in Chicago after flying to the funeral of his parents in Nigeria -- again highlighting the speed with which potential carriers can now move around.


AIDS is caused by a special kind of virus, known as a retrovirus. It invades white blood cells, which play a crucial role in providing immunity to disease.

The original source of AIDS is thought to have been an African monkey, a close relative of the vervets that gave us Marburg virus; but the African origin of this disease has not been conclusively demonstrated, and the suggestion has been highly controversial because of connotations of responsibility. Nonetheless, human populations are exploding at record rates in Africa, ecological situations have been changing dramatically, malnutrition (and thus impairment of immune systems)29 is widespread, and contact with our primate relatives there is more extensive than on any other continent. In addition, close relatives of the AIDS viruses30 have been isolated from various African monkeys but not from wild monkeys living on other continents.31 So the opportunity for transfer from an animal host was probably higher there than anywhere else, and the inference is not unreasonable.32 The question of whether the virus transferred long ago and only "broke out" in response to recent deleterious changes in the human ecology of Africa, or whether the virus only invaded human beings in the last few decades, is much more in doubt.

If AIDS, like humanity, did originate in Africa, that is [147] certainly no reason to blame Africans. It is the virus that causes the disease, not people, and people had nothing to do consciously with the appearance of the virus. Assigning blame simply makes dealing with this massive public-health problem much more difficult -- and it is also wrong and unfair.33

Fringe groups have demanded that AIDS victims be quarantined, and blood tests are now required of immigrants into some nations. If the epidemic is not controlled, disruption of trade and international recriminations are possible outcomes because of irrational fears and the desire to blame others, especially foreigners or other scapegoats. Remember, transport systems are a key part of agricultural systems; anything that disrupts them will seriously compromise humanity's ability to feed itself. Should some American cities be seen as hotbeds of disease, one could imagine truckers, confused about actual risks, refusing to enter them. Under such circumstances, local food shortages could become acute, even if over­all food production were maintained. Remember that much less complex and interdependent societies than the United States have virtually collapsed when confronted by epidemics like bubonic plague.34 Nothing in the rules says that AIDS or some other epidemic could not have a similar effect, since the more effective public-health organizations of modern societies could be counterbalanced by the increased vulnerability inherent in modern distribution systems.

Whether or not AIDS can be contained will depend primarily on how rapidly the spread of HIV can be slowed through public education and other measures, on when and if the medical community can find satisfactory preventatives or treatments, and to a large extent on luck. The virus has already shown itself to be highly mutable,35 and laboratory strains resistant to the one drug, AZT, that seems to slow its lethal course have already been reported.36

A virus that infects many millions of novel hosts, in this case people, might evolve new transmission characteristics. To do so, however, would almost certainly involve changes in its lethality. If, for instance, the virus became more common in the blood (permitting insects to transmit it readily), the very process would almost certainly make it more lethal.37 Unlike [148] the current version of AIDS, which can take ten years or more to kill its victims, the new strain might cause death in days or weeks. Infected individuals then would have less time to spread the virus to others, and there would be strong selection in favor of less lethal strains (as happened in the case of myxomatosis).38 What this would mean epidemiologically is not clear, but it could temporarily increase the transmission rate and reduce the life expectancy of infected persons until the system once again equilibrated.

If the ability of the AIDS virus to grow in the cells of the skin or the membranes of the mouth, the lungs, or the intestines were increased, the virus might be spread by casual contact, by inhalation, or through eating contaminated food. But it is likely, as Temin points out, that acquiring those abilities would so change the virus that it no longer efficiently infected the kinds of cells it now does and so would no longer cause AIDS. In effect it would produce an entirely different disease. We hope Temin is correct, but another Nobel laureate, Joshua Lederberg, is worried that a relatively minor mutation could lead to the virus infecting a type of white blood cell commonly present in the lungs. If so, it might be transmissable through coughs.

In the United States and Europe, most AIDS patients are members of "high-risk" groups -- promiscuous male homosexuals, intravenous drug users, and hemophiliacs. In central Africa, where the virus may have been in the human population the longest, it appears to infect men and women about equally.39 Reliable statistics from that continent are scarce, but in some localized areas as much as one quarter of the population is claimed to be carrying the virus. The Centers for Disease Control in Atlanta more conservatively estimates for central Africa a still horrifying rate of 7 percent. In any case, it seems certain that the death rate from AIDS in nations like Rwanda and Uganda will move rapidly upward.40

The demographic impact of AIDS in Africa and elsewhere is uncertain -- too little is known about patterns of transmission of current strains of HIV and about the potential for the virus to evolve a more efficient mode of infection and higher survival times. Computer projections suggest that, even in Africa, [149] mortality from the disease alone (as opposed to social breakdown or economic effects) is unlikely to bring an end to population growth.41 While AIDS could turn out to be the global epidemic that brutally controls the population explosion by raising death rates, the strains of the virus that have so far been observed seem not to have that capacity. In truth, it is impossible at the moment to predict what will happen.

Whatever the outcome of the coevolutionary battle between Homo sapiens and the AIDS virus, we can be confident that our species will face other deadly microparasites in the future. Until the size of the human population is diminished and the proportion of people without adequate diets, clean water supplies, and medical care is reduced, the danger will remain that one of those parasites could cause a devastating global epidemic that could threaten everyone's future.


We want to mention briefly the conviction of ecologists that a growing human population is making subtle changes in the environment that are bringing more people into contact with old disease-causing organisms, and permitting some organisms that never before had a chance to cause problems for people the opportunity to do so. ,

An example of the latter is a previously nonparasitic soil amoeba (a protozoan related to the ones often studied in high-school biology classes) which has been allowed by nitrate pollution to invade lakes in the southeastern United States. There it has invaded the bodies of a few swimmers and caused a fatal inflammation of their brains.42 Of course, no one knows precisely what environmental changes may have opened the door for the much more serious transfers of viruses from animals to previously uninfected human populations, but the suspicion is that they involve expanding human populations coming into closer contact with formerly isolated virus carriers.

In the United States, two "ecological" diseases have become widespread in recent years. One is caused by Giardia lamblia, which causes serious diarrhea, weight loss, and abdominal pain. It has made drinking the water from lakes and [150] streams in the mountains of the western states a risky game, even at very high altitudes. Giardia's emergence as a major problem appears related to the greatly increased density of hikers in those mountains.

The other is Lyme disease, a tick-borne infection related to syphilis which has as reservoirs deer and most other warm­blooded wildlife, from birds to squirrels and raccoons. The disease was only identified in the United States in 1975, but is spreading rapidly and is now considered to be, after AIDS, the most important "new" infectious disease in the nation.43 An initial rash and flulike symptoms can be followed (as can the initial chancre and secondary rash of syphilis) by serious later complications, including recurrent arthritis, abnormal heart rhythms, and nervous-system problems. What triggered the epidemic is unclear. The spirochete that causes the disease may have been accidentally introduced from Europe. Its spread may have been facilitated by a combination of expanding deer populations in the Northeast (as woodlands invaded abandoned farm fields) mixed with human population growth and suburbanization (which brought people, ticks, and woodland animals into close proximity). Connecticut, where the disease was discovered, increased in population size from 2 million to 3 million people between 1950 and 1970.

Finally, in the subtropics and the tropics, irrigation projects that help to keep food production ahead of population growth have contributed to the spread of the parasites that cause schistosomiasis (called bilharzia in Africa).44 It is second only to malaria as a serious disease afflicting large numbers of human beings. Like most forms of malaria, unless fairly drastic medical treatment is taken, the disease becomes one of chronic debilitation, although it also can kill.

In sum, as human populations grow they inevitably alter their environments, and that, not unexpectedly, will change relationships with parasites. Some may be able to attack us more readily, others may find the job more difficult. [151]


There is, of course, another epidemic already upon us that threatens the health of billions of people. That is the epidemic of poverty. Looking at the crude division of the world into rich and poor nations produces striking statistics on the root causes of this epidemic. Over 80 percent of the world's wealth is held in the industrialized nations (which have about 23 percent of the people), and they have some 94 percent of the scientists and technologists. With all those handicaps the poor are deeply in debt to the rich, and suffering greatly in their attempts to service that debt. In 1987, $38 billion was paid, in 1988 $45 billion -- a flow that former Chancellor of West Germany Willy Brandt called "a blood transfusion from the sick to the healthy."45

The poverty epidemic results in reduced health, and thus reduced life expectancy, for most people in poor nations (as well as many poor people in rich nations). This epidemic shows up clearly even in aggregated statistics. The fifth of the world's people in rich nations live an average of seventy-three years; the four fifths in the poor nations live an average of sixty. These differences do not represent differences in the remaining life expectancy of twenty-year-olds; they reflect very high infant and child mortalities among the poor. Of each 1,000 babies born to the rich, only about 15 die before they reach the age of one; in the poor countries an average of 84 die.

The numbers are even more dramatic if the very poor are examined separately. In Pakistan, life expectancy is fifty-four years and infant mortality 120 per 1,000. In Mali, life expectancy is forty-three years, and infant mortality 175 per 1,000 (or almost one in every five babies). Contrast these statistics with those of Japan, where a child at birth today can expect to survive seventy-eight years, and infant mortality is less than 5 per 1,000.

This poverty epidemic has many population-related causes, especially malnutrition, contaminated water supplies, lack of adequate medical care, and lack of education. These conditions often lead poor women to use "formula" instead of breast-feeding, a choice that, under crowded conditions with [152] poor sanitation, seriously threatens the health of their infants. Not only are the babies exposed to a multitude of germs in the water used to mix the formula, they are denied the immunizational and other benefits of mother's milk. And some poor mothers dilute the formula to save money, starving their infants as a result.

Because of the scale of the problem -- it involves at least a billion people -- it would not be unfair to call poverty the greatest public-health problem today. And there is no question that rapid population growth among the poorest people is a major factor in keeping them impoverished. Instead of being able to put aside surplus physical and financial resources for use in raising the average individual's standard of living, less-developed nations must keep plowing much of their capital back into providing subsistence for ever-growing numbers of people. It is a sad treadmill: the more people there are in those nations, the more difficult it is for them to escape from poverty.

It is sometimes said, of course, that rather than population growth being a contributing cause of poverty, poverty is the cause of population growth.46 There is some truth in this thesis. Poor people often are malnourished and suffer higher infant and child mortality rates than the well-off. Since they also lack social security and therefore need surviving sons to care for them in their old age, they have larger families. But the problem is partly circular, since high fertility also contributes to those high death rates.47

But the argument is almost moot. First of all, prosperity has not brought birthrates down to the necessary level even in rich nations such as the United States, and family sizes remain much too high even in Costa Rica, that most exemplary of developing nations (which has little severe poverty and an infant mortality rate comparable to eastern-European countries). Second, bringing the poor up to the levels of affluence of today's rich nations will produce unsupportable stresses on Earth's ecosystems unless the rich decide to engage in a massive redistribution of wealth. Third, there is no sign that the rich would seriously consider such a step. Fourth, whatever "charitable" steps are taken to help the poor will be more beneficial if the numbers of poor are smaller. And, fifth, population growth itself is a major barrier in preventing the poor [153] from helping themselves. In short, the "does population growth cause poverty or vice versa" argument is counterproductive if the goal is to provide everyone with a decent life. If that is the goal, then all of us should be working very hard to end both poverty and population growth, not wasting our efforts trying to determine which causes which.


It has long been realized that high densities in human populations made them -- all else being equal -- subject to high rates of disease.48 Indeed, it was only about a century ago that cities stopped being, in McNeill's term, "population sumps."49 Until then, most cities had such high death rates from diseases that they could not maintain themselves without a constant flow of immigrants from relatively healthier rural areas. But today cities tend to be centers of the best medical care, even in poor nations, and disease is now a relatively minor factor in urban death rates. This does not mean, however, that cities are free of population-related problems -- problems that should properly be considered part of "public health."

In rapidly growing, less-developed nations, migration to cities is going on at unprecedented rates. In 1920, about 100 million people lived in Third World cities; by 1980, ten times as many, about a billion, did so. Furthermore, if current trends continue, by the turn of the century there will be over 1.9 billion urban dwellers in poor nations, or about as many people as occupied the entire planet during World War I. In 1950, only three cities, New York, London, and Shanghai, were inhabited by over 10 million people. In 2000, there are projected to be twenty cities of more than 10 million, seventeen of them in developing nations. Overall, about half of humanity will be living in cities before 2010 -- more than 3 billion people.50 While developed countries have been overwhelmingly urbanized for many decades, the change is dramatic for developing countries, which until the last decade or two have been mainly agrarian. Now, clearly, the promise of the city, no matter how unlikely to be fulfilled, is more attractive than the poverty of the countryside.

But the cities in poor countries are growing at staggering [154] rates. The largest, Mexico City, held some 17 million souls in 1985 and, barring disaster, will have over 25 million (about equal to the present populations of Ireland, Denmark, Norway, Sweden, and Finland combined) by 2000. Sao Paulo will be around 21 million;51 and Calcutta and Greater Bombay will each have over 15 million and Delhi over 13 million.

The prospects for these gigantic agglomerations are not bright. They have grown so fast that they have far outstripped their ability to care for their inhabitants. Lack of sewage systems, inadequate water supplies laced with pathogens, air pollution, and gigantic garbage dumps (often occupied and "mined" by the poorest of the poor) plague these overgrown metropolises. Although their water supplies are generally superior to those of rural areas, a very optimistic estimate is that at least a quarter of the people living in Third World cities lack safe drinking water -- over 250 million people, comparable to the entire population of the United States.52

Half of the population of Delhi are now slum dwellers, and according to the Delhi Planning Authority that fraction will be more like 85 percent at the end of the century.53 In the summer of 1988, millions in Delhi went without water during the drought; when the rains finally arrived, wells were polluted by the human feces that are everywhere (because of the inadequate sewage system), and a cholera epidemic broke out among the poor. In Bombay, shantytowns make up half the housing, and social workers estimate that 200,000 to 500,000 people sleep in the streets.54

Mexico City has so many people without sanitary facilities that a "fecal snow" often falls on the city as winds pick up dried excrement. Its air pollution is rated the worst on the planet. Sao Paulo, Brazil, in the state of the same name, has been gathering problems in spite of its relative wealth. Sao Paulo State once was described as a "rich country plunked down in the middle of an India," but now "India" is creeping in. In the narrow verges of huge freeways, naked children now play among the cardboard shacks of favelas (shantytowns) within inches of eight lanes of roaring traffic. About a million people now live in the favelas (which first appeared in the mid-1970s), and about 4 million more in hideous slums.55 [155]

Many attempts are being made to resolve urban problems in poor nations, including the encouragement of industry to locate in secondary cities to divert some of the people fleeing from the countryside and reduce pressure on the megacities. Most migrants are peasants displaced by industrialized agriculture. Ironically, that industrialization is one of the strategies that have helped keep food production up with population growth globally. But, as long as population growth continues at anything like current rates, trying to solve those urban problems is like trying to bail out the ocean with a thimble.

Even rich nations are having trouble maintaining the livability of their giant cities. In New York City, the waiting time for a vacancy in public housing is eighteen years, and thousands of people are homeless. In most American cities, the need for shelters has doubled or tripled since Ronald Reagan moved into the White House.56 In Britain, which is sliding downhill compared to the rest of the European Economic Community, over $40 billion is needed to rebuild crumbling sewer systems, $8 billion to repair gas mains, and many billions more to restore disintegrating government housing.

There are many other problems that appear at least partially traceable to the cramming of more and more people into cities. Recently, an "urban stress test" was applied to all 192 U.S. cities with populations of over 100,000 people.57 The test scored each city on a scale of one (a model city) to five (highly stressed) on eleven criteria: population change, crowding, education, violent crime, community economics, individual economics, births to women under age twenty, air quality, hazardous wastes, water quality, and sewage treatment.58

The results of the survey showed a strong relationship with population. Cities with fewer than 100,000 people had an average score of 2.5; those over one million, 3.8; with those in between generally having intermediate scores. Cedar Rapids, Iowa (population 109,000), Madison, Wisconsin (176,000), and Ann Arbor, Michigan (108,000) had the best scores with 1.6, 1.7, and 1.8 respectively; Gary, Indiana (137,000), Baltimore, Maryland (753,000), and Chicago, Illinois (3 million) had the worst with 4.2, 4.1, and 4.1. The twenty-two cities with the best scores (1.6 to 2.3) averaged 116,000 people, with about [156] 3700 per square mile. The twenty worst (scoring 3.8 to 4.2) averaged 1,154,000, with 8200 per square mile. Only two cities with populations over half a million scored below 3.0: Columbus, Ohio (2.6), and San Francisco (2.8).

The results, when the four environmental measures alone were considered, were similar. The nineteen cities with the best scores had an average population size of about 120,000; the thirteen worst averaged about 786,000. The message seems clear: measured either by social and environmental indicators together or by environmental indicators alone, more people mean more problems in American cities. Of course, one could argue that "correlation is not causation." The other possible explanation of the association of large cities with signs of stress -- that crime, poverty, lack of education, and environmental deterioration cause cities to get larger -- is more complex and we think less likely.59

These results, naturally, came as no particular surprise. Large American cities fill the evening news with their problems: Washington, D.C., as the "murder capital of the U.S.A.," New York City and Seattle having to ship their garbage out of state (or even overseas), Miami's drug wars, San Francisco's high rate of infection with AIDS, Los Angeles's choking smog. Almost two decades ago, the report of the United States Commission on Population Growth and the American Future stated: "Population growth in the United States has multiplied and intensified many of our domestic problems and made their solution more difficult." The commission explicitly included the problems of the cities, including those of law enforcement.60

Of course, some of these problems are more clearly tied to population size and density than others. The connection with smog and demand for water is obvious. That with sewage treatment is less direct: proper treatment costs money, and larger cities tend to be poorer per capita (the rich have moved to the suburbs).

While the incidence of violent crime is higher in large cities than in small ones, experimental studies of people under crowded and uncrowded conditions suggest that high density per se is not the cause.61 Crowding merely seems to intensify [157] each individual's reaction to a given situation, whether that reaction is helpful or harmful. Crime rates have long been thought to be tied to rates of city growth, which in turn often are connected with size and density.62 High crime rates in rapidly growing cities may result from the associated lack of community feeling and coherence. And there doubtless is an "anonymity factor": in larger cities, the chance of being recognized while committing a crime is smaller. But doubtless also crime rates are tied in complex ways to employment rates, education, racial prejudice, teenage pregnancy, and other factors that themselves have connections to population size, density, and age composition. Much more research is needed to sort out what causes what in American cities, and final answers may elude us permanently. That American cities generally have much higher crime rates than those of similar size in Europe indicates that numbers per se must be a minor factor when compared with social and cultural factors.

We must emphasize again that in the area of public health the population connection is often relatively weak. People can live at very high densities, as in Tokyo, and still largely avoid many of the problems that are often associated with crowding. It would be a mistake to expect that simply reducing population densities would eradicate the problems considered in this chapter. It would surely ameliorate many of them, though, and in so doing add to the quality of our lives. But ending population growth and starting a slow decline is not a panacea; it would primarily provide humanity with the opportunity of solving its other problems.


1. For more detail, see P. Ehrlich, A. Ehrlich, and J. Holdren, Ecoscience: Population, Resources, Environment (Freeman, San Francisco, 1977).

2. Garbage from New York City is now hauled to the Middle West; Seattle is negotiating to send its refuse to eastern Oregon.

3. We are using the term loosely, since in this case no single factor of production is "fixed" in the classical sense.

4. The point where diminishing returns are encountered will vary with the specific situation, but in most cases humanity is now well beyond that point.

5. M. Renner, "Car Sick," World Watch, November/December 1988.

6. S. Postel, "Global View of a Tropical Disaster," American Forests, November/December 1988.

7. Renner, "Car Sick." Much of the following material is also from this source.

8. I = 0.67, P = 1.25, AT = .54; if P had remained at 1, then I = .54. Note that with the per-capita number of cars increased as well as the mileage driven (A), the T factor has been impressively reduced by more efficient engines with superior pollution-control equipment. A has probably almost doubled, meaning that, on average, emissions per mile driven (T) have been reduced some 75 percent (so that AT is roughly .5).

The reductions, of course, vary with the substance. Lead has been reduced 96 percent, for example; carbon monoxide, only 39 percent. This detail is lost when lumped estimates are given (M. Weisskopf, "A Qualified Failure: The Clean Air Act Hasn't Done the Job," Washington Post Weekly, June 19-25, 1989).

9. "Group Gives Gloomy Prognosis," Denver Post, July 18, 1989.

10. The biggest problems with industrialized agriculture are that it is generally unsustainable and does serious damage to ecosystem health. Cancer statistics do not indicate the upsurge many of us feared two decades ago when pesticide contamination of food was first widely recognized; however, on first principles we still would recommend stiff laws to control residues and a general policy of minimizing the use of many synthetic chemicals of questionable safety.

11. W. McNeill, Plagues and People (Doubleday, New York, 1976).

12. A. Crosby, The Columbian Exchange: Biological and Cultural Consequences of 1492 (Greenwood Publishing, Westport, Conn., 1972), p. 52.

13. A. Crosby, op. cit. and Ecological Imperialism: The Biological Expansion of Europe, 900-1900 (Cambridge Univ. Press, Cambridge, 1986).

14. F. Black, "Measles Endemicity in Insular Populations: Critical Community Size and Its Evolutionary Implications," Journal of Theoretical Biology, vol. 11, pp. 207-11 (1966).

15. The existence of nonspecific inherited resistance is somewhat controversial and its mechanism uncertain. Nonetheless, on evolutionary grounds it seems very likely to be a major explanation for the high susceptibility of populations to new infections and their high morbidity-mortality rates in comparison to populations long associated with pathogens that are capable of inflicting high mortality. See also T. A. Cockburn, "Infectious Diseases in Ancient Populations," Current Anthropology, vol. 12, pp. 45-54 (1971), and the discussion following.

16. Black, "Measles Endemicity"; T. A. Cockburn, "Infectious Diseases in Ancient Populations."

17. Syphilis is caused by the spirochete Treponema pallidum; yaws, by its very close relative, T. pertenue. Yaws is now largely restricted to hot, humid areas. In earlier times when people huddled together for warmth naked or semiclothed in the winter, it was common in Europe.

18. M. Burnet, and D. 0. White, The Natural History of Infectious Disease, 4th ed. (Cambridge Univ. Press, Cambridge, 1972), p. 122.

19. H. Temin, "Is HIV Unique or Merely Different?" Journal of Acquired Immune Deficiency Syndromes, vol. 2, pp. 1-9 (1989).

20. A monoculture in agriculture is a substantial area in which a single crop is grown. As we have seen, humanity is getting close.

21. L. Altman, "Fearful of Outbreaks, Doctors Pay New Heed to Emerging Viruses," New York Times, May 9, 1989. Similar concerns were expressed in 1968 in The Population Bomb, pp. 70-71.

22. Altman, "Fearful of Outbreaks . . ."

23. The disease was transmitted by mosquitoes and perhaps bedbugs. It produced severe joint pains, headache, fever, swollen glands, and an itching rash; but after about a week they subsided, and recovery was ordinarily complete. The relationship of the virus to that of another disease, Chikungunya (Chik), that occurred in Tanganyika and the Congo around the same time is not clear. For details, see R. Fiennes, Zoonoses of Primates: The Epidemiology and Ecology of Simian Diseases in Relation to Man (Weidenfeld and Nicolson, London, 1967), from which we've drawn much of our information on O'nyong-nyong fever.

24. Ibid., pp. 7-8.

25. Such a mortality rate creates a very strong selective pressure on the pathogen to become less lethal, which is exactly what happened to the myxomatosis virus (see F. Fenner, B. McAuslan, C. Mims, J. Sambrook, and D. White, The Biology of Viruses [Academic Press, New York, 1974]). Of course, that isn't much comfort for the victims who create the selective pressure.

26. In addition to those mentioned below, there is also monkey-B virus and several new hemorrhagic-fever viruses.

27. Ehrlich, Ehrlich, and Holdren, Ecoscience, chap. 10.

28. "New Outbreak of Marburg Disease," New Scientist, Oct. 28, 1976, p. 199.

29. R. Chandra, "Nutrition, Immunity, and Infection: Present Knowledge and Future Directions," Lancet, March 26, 1983, pp. 688-91.

30. Called simian immunodeficiency viruses.

31. AIDS Monitor, "French Probe New Virus from Wild Chimp as Other Monkey Viruses Shed Light on Origin of HIV," New Scientist, June 9, 1988, p. 40.

32. A discussion of the possible African origin of AIDS with extensive documentation can be found in P. Epstein and R. Packard, "The Social Context of AIDS in Africa," Science for the People, January/February 1987, pp. 10-17, 32. It is not at all clear that the speculations in this article are correct, but it provides a sense of the complexity of the issue. See also M. A. Gonda, "The Natural History of AIDS," Natural History, 1986, no. 5, pp. 78-81.

33. For an interesting discussion of many of the issues surrounding "blame" for the AIDS epidemic, see Renee Sabatier, Blaming Others: Prejudice, Race and Worldwide AIDS (Panos Institute, London, 1988).

34. W. Langer, "The Black Death," Scientific American, February 1964.

35. Like all other RNA viruses.

36. B. Larder, G. Darby, and D. Richman, "HIV with Reduced Sensitivity to Zidovudine (AZT) Isolated During Prolonged Therapy," Science, vol. 243, pp. 1731-34 (1989).

37. Temin, "Is HIV Unique or Merely Different?" Our discussion of changed transmissibility leans heavily on this source, but see the more pessimistic view of Lederberg, cited below (quoted in R. Weiss, "Waiting for the Real-life Andromeda Strain," Washington Post Weekly, Oct. 16-22, 1989).

38. F. Fenner, B. McAuslan, C. Mims, J. Sambrook, and D. White, The Biology of Viruses, 2nd ed. (Academic Press, New York, 1974).

39. This may be explained by factors other than differences in viral strains, such as a high incidence of sexually transmitted diseases whose lesions aid the passage of HIV, less male circumcision, and more female circumcision.

40. See N. Kreiger, "The Epidemiology of AIDS in Africa," Science for the People, January/February 1987, pp. 18-20: S. Kingman, "Ten Times More AIDS Cases in Africa," New Scientist, Sept. 22, 1988, p. 20. For a discussion of the social downside of examining the origins of AIDS, see R. Sabatier et al., Blaming Others.

41. M. John, "A Model of HIV-1 Transmission in Developing Countries" (1988). Manuscript.

42. J. H. Callicott, "Amoebic Meningioencephalitis Due to Free-living Amoebas of the Hartmanella (Acanthomoeba) naegleria Group," American Journal of Clinical Pathology, vol. 49 (1968), p. 84.

43. T. Daniels and R. Falco, "The Lyme Disease Invasion," Natural History, July 1989.

44. K. Warren, "Precarious Odyssey of an Unconquered Parasite," Natural History, May 1974.

45. Quoted by Peter Raven in Calypso Log, June 1989. This is also the source of the figure on scientists and technologists.

46. B. Commoner, "How Poverty Breeds Overpopulation and Not the Other Way Around," Ramparts, August/September 1975.

47. See discussion in Ehrlich, Ehrlich, and Holdren, Ecoscience, especially pp. 778-79.

48. What is often not equal is the quality of sanitation and the availability of medical care. Japan is quite crowded and the population density in Tokyo is extraordinary, yet public-health measures keep mortality from disease very low.

49. McNeill, Plagues and People, p. 275.

50. UN Centre for Human Settlements, Global Report on Human Settlements 1986 (Oxford Univ. Press, New York, 1986); WRI and IIED, World Resources 1988-89 (Basic Books, New York, 1988).

51. Recently, city planners have scaled back previous forecasts of 25 million for Sao Paulo -- see E. Robinson, "Straddling the First and Third Worlds," Washington Post Weekly, July 10-16, 1989.

52. WRI and IIED, World Resources 1987 (Basic Books, New York, 1987).

53. E. Goldsmith and N. Hildyard, The Earth Report: The Essential Guide to Global Ecological Issues (Price Stern Sloan, New York, 1988).

54. R. Weintraub, "Bombay -- Hell in a Very Small Place," Washington Post Weekly, Dec. 19-25, 1988.

55. WRI and IIED, World Resources 1988-89; PRE and AHE personal observations in 1987; Robinson, "Straddling the First and Third Worlds."

56. E. Hollings, "Decaying America: The Underside of the Reagan Legacy," Washington Post Weekly, May 8-14, 1989.

57. Administered by the organization Zero Population Growth in 1988.

58. The criteria (in italics) were assessed as follows: For population change, cities growing rapidly or shrinking were reckoned to be less desirable places to live in than those with relatively stable populations. Crowding was measured by the percentage of housing units classed as crowded by the Census Bureau, and education by per-pupil expenditures and proportion of the adult population having finished high school. Violent crime was evaluated both by the rate of such crimes per 100,000 population and by changes in that rate. Community economics was measured by unemployment rates and Moody's rating of the city's municipal bonds; and individual economics by the percentage of families and individuals below the poverty line and changes in per-capita income between 1979 and 1985. Births to women under the age of 20 was another criterion, as were four general measures of environmental quality: air quality (compliance with EPA standards), hazardous wastes (number of hazardous or potentially hazardous sites), ground and surface water quality (U.S. Geological Survey ratings), and sewage treatment (EPA reports on quality and capacity).

59. No doubt there are complex interactions among these factors that may never be sorted out fully. A careful study of the degree to which population change, poverty, and lack of education may be causative variables would be desirable. Teenage pregnancy, of course, is a factor in population growth and thus in making cities larger.

60. United States Commission on Population Growth and the American Future, Population and the American Future, 6 vols. (U.S. Government Printing Office, Washington, D.C., 1972).290 Notes, pages 156-160

61. P. Ehrlich and J. Freedman, "Population, Crowding, and Human Behavior," New Scientist, April 1, 1971.

62. J. Forrester, "Counterintuitive Behavior of Social Systems," Technology Review, vol. 73, pp. 52-68 (1971).