I have developed a schema which consists of a hierarchical list of the phenomena of interest to human scientists, and the links among these. I begin by dividing human science into ten logical categories: two at the individual level, our "genetic predisposition" and "individual differences"; seven at the societal level, "economy", "politics", "social structure" (societal subdivisions such as race and gender), "culture, "art", "technology and science", and "health and population"; and a final "non-human environment" category. These categories are then broken into their constituent parts. For example, culture consists of religion, language, stories, expressions of culture, and four types of values (each of these phenomena is disaggregated further).

I describe the schema in Szostak (2000), providing a complete list of phenomena and a discussion of the types of influence these exert on each other. I note that the schema allows a precise definition of words such as "culture". I justify the schema philosophically, relate my approach to previous discussions of unifying schemas, and argue that the longstanding goal of unity can only be achieved schematically as opposed to through grand theory. I suggest that the schema, and a curriculum it inspires, facilitates interdisciplinary education and skill acquisition, and provides a powerful response to several recent critiques of the Liberal Arts.

Rather than revisit that material, I propose in this paper to apply the schema in a way that shows how it could be used as an organizing device for interdisciplinary courses, while also illustrating one of my major arguments: that all phenomena are causally related to almost all others. I take one of my categories, "health and population", and discuss some of the more important ways in which its constituent phenomena are related to phenomena in all other categories. I cannot pretend to being exhaustive, but hope to illustrate that an understanding of human health and population requires familiarity with a wide range of influences.

Population is completely determined by the experience over time of three phenomena: mortality, fertility, and migration. These in turn determine a fourth phenomenon: the age distribution of a population. The health of a population has a predominant impact on mortality, a significant impact on fertility, and a lesser role in encouraging migration. Nevertheless, health receives much less attention than population in human science. Students of population at times worry about the causes of disease or famine, and health economists worry about the institutional structure for health service delivery. For the most part, health is left to those in health sciences, but health research is rarely grounded in human science theory.

All of our phenomena, except age distribution, can be "unpacked". Mortality can be divided between death by injury and death by disease. For fertility we can distinguish between fecundity, the biological capability of a population to reproduce, and the degree to which the biological maximum is approached. Hornsby and Jones (1993) review the many attempts to establish a typology of migration experiences: these can be distinguished by distance travelled, international versus internal, and temporary versus permanent.

With respect to Health, we can identify thousands of diseases or illnesses. Note, though, that those maladies, including mental disorders, which have their roots in genetics, are treated under "individual differences". That leaves us here to worry about diseases of viral or bacterial origin, as well as any maladies caused by environmental pollution (including cigarette smoke). We must also worry about nutrition. Nutrition can be unpacked in terms of the diverse nutritional requirements of our bodies.

1. Carrying Capacity. While concern with world wide population growth has lessened in the last decade, some worry that we may exhaust the Earth’s capacity to support human life. There must be, after all, some finite limit to the amount of food which can be grown on the earth’s surface. But are we at all close to reaching such a limit? We must recognize that technological change continually expands the earth’s carrying capacity. The development of agriculture, which many scholars suspect was induced by the pressure of population on finite food resources, expanded the earth’s carrying capacity many times, as has a series of improvements in agricultural productivity over the last centuries (Barrett, 1992). Investment in fences, irrigation systems, barns, and the like also enhances productivity. While it is difficult to measure the precise carrying capacity of the earth with existing technology and impossible to know what it might be with future technology, we can still worry that our quality of life decreases as population increases. An increased need for food should have two results: the farming of less productive land and attempts to grow more on existing land. Both generally require a greater work effort, unless we see induced technological change.

Food production per head has increased world wide (except in Africa) both over the last two centuries and last two decades. Nor has the price of food relative to other goods increased; this indicates that we have not had to shift more of our productive resources toward food production (Lindahl-Kiessling, 1994). Indeed, as countries develop, they shift resources out of agriculture into industry and services. Pessimists worry that food per capita will nevertheless fall in the future, due to soil exhaustion from chemical fertilizers, or because of declining rates of technical advance, especially as many countries have slashed funding of agricultural research (Dyson, 1996). Growth in cereal yields in developing countries fell from 3% per year in the 1970s to 2% per year in the 1980s and 1990s. On the other hand, the differences in agricultural productivity across nations are so huge that it would seem that transferring rich world technology to poor countries still has much scope to enhance output. We must remember, though, that even such transfers generally require a fair bit of research in order to apply methods to different soil and climate conditions.

Biotechnology is viewed by optimists as the solution to the world’s future demand for food. Pessimists worry that scientists cannot best nature, and that engineered plants expose us to the possibility of environmental catastrophe. Many insects and fungi are developing resistance to pesticides and herbicides. Global warming presents another unknown: scientists disagree on its extent and the effect it would have.

2. Environmental deficiencies. Humans as a species have spread more widely than any other. Still, the environment places limits on where we can live. Humans can live at altitudes of 5000 meters, but only after a lengthy period of adjustment. They can survive without shelter in climates of 10-30 degrees C, though human ingenuity has long extended this range. Soil and climate must be capable of providing food and water, and thus large stretches of desert support no human life (Barrett, 1992).

Even when environmental conditions allow humans to survive, they may affect their health. The tropics, with energy-sapping heat and humidity, and a host of parasitic diseases, may well be a less healthy environment. More broadly, almost all diseases are more common in certain environments than others. Areas with little sunshine may yield a vitamin D deficiency. The absence of seafood can lead to iodine deficiency and goiter. Since our bodies require a host of vitamins and minerals in trace quantities, the potential for deficiency in any self-sufficient locality is large. Modern trade links have reduced this danger.

The World Health Organization estimates that 900 million people per year suffer from diarrhea or other diseases, including typhoid and cholera, spread by contaminated water. An equal number may be afflicted by intestinal worms from the same source. Some 90% of sewage in less developed countries is emptied untreated into bodies of water (along with agricultural and industrial waste). Though some 2 billion people have gained access to treated water supplies in the last two decades, there are still well over a billion who must drink water of poor quality. Various studies have suggested that the costs of clean water provision would be repaid many times in the benefits of a healthier population, especially in heavily populated Asia.

Air pollution also has serious effects on health. As with water, these problems are many times more severe in less developed countries. Indoor air pollution, from primitive cooking and heating appliances in poorly ventilated huts, appears to have the same effects on health as heavy smoking of cigarettes. Outside the home, continued use of lead in gasoline has a range of health effects and is alleged to reduce intelligence. And the small airborne particles spewed out by factories and cars become lodged in lungs; their incidence is highly correlated with mortality rates. The World Bank has estimated that in China alone smog in 1995 was responsible for 178,000 premature deaths and 1.7 million cases of chronic bronchitis.

3. Migration. People move because they think they can make a better life for themselves someplace else. In making this calculation, they must account for the cost and danger of moving. Natural barriers, such as mountains or deserts, can limit migration. Conversely, the great expansion in transport possibilities over the last centuries has greatly facilitated migration. When Ravenstein propounded his laws of migration in the 1880s, one was that migration generally occurred over short distances due to the difficulties both of obtaining information and moving over long distances. These constraints have lessened in severity.

1. Reproduction. This is the most central causal link. If not for our genetic drive to reproduce, we would have disappeared as a species long ago. But is there a danger that in a world of low mortality, our basic drive may force us toward overpopulation? Biologists suggest that certain animal species are observed to limit fertility when faced with overpopulation. It is possible that primitive human societies were programmed with a similar response. We need not rely on this, however. Humans have the ability to consciously overcome genetic programming. And, since we were programmed for sexual activity rather than reproduction, we can use modern birth control technology to limit our numbers while satisfying our drives.

2. Longevity. The age distribution of a population depends to a large degree on life expectancy. Our genes place limits on this (about 120 years) at least within the confines of present medical technology. Genetic selection for reproductive fitness would have found little use for survival beyond an age when one could have raised ones children to adulthood; it may have even been selected against to the extent long-lived individuals attracted scarce resources away from their descendants at or before their reproductive years.

3. Diet. Our physiology would have developed in concert with selection for foraging capabilities. Those who could hunt and gather diverse sources of nutrients would have been most likely to survive; they could in turn bequeath to their offspring complex systems which required varied inputs. Foraging theory, developed by biologists for animals but extended to humans, suggests a tradeoff between foraging time and preparation time. One interesting implication is that as population pressure increases optimal search time and diet diversity is likely to increase (though consumption in calories falls) (Cashdan, 1989). It remains likely that during the millennia between the adoption of settled agriculture and the modern era of low cost trade in diverse foodstuffs, humans tended to get a less diverse diet than that they had been selected for.

4. Genes and Disease. Our genes have evolved over time in concert with various sorts of parasites. While the ideal from our point of view would be a defence which destroyed any parasite attack, selection among both humans and parasites often favored an equilibrium where the parasite would not kill the host immediately. It is clear that some of us are programmed better than others with respect to particular diseases. Most humans can carry diseases such as cholera and meningitis without showing ill effects, while a minority will suffer severely and may die. Those with type B blood are most resistant to streptococci and those with type A to the plague. The distribution of blood types across human groups reflects imperfectly the relative likelihood of encountering particular parasites in particular locales. Over the next decades, geneticists will likely identify at least some of the genes which allow some individuals or groups to resist certain afflictions. This raises the possibility of gene therapy for those less fortunately endowed. In the meantime, some tricky legal and ethical questions about ownership of genetic material need to be resolved.

I am working on a book which surveys links to and from culture, and will thus be especially brief here. I would note that various cultural attitudes (to work, marriage, leisure) affect our decisions to have children. There is also a correlation between religion and birth rate; it has changed markedly over time, suggesting that the effect may be cultural rather than scriptural. Societies differ in how seriously they view particular maladies (based largely on their incidence), whether they view illness as a problem to be borne stoically by the individual or a cause for collective support, whether they stress cures or the alleviation of symptoms, and the degree to which they rely on traditional versus modern medicines. Various aspects of culture influence health: cuisine, proxemics (how close we get while talking), religious belief (perhaps indirectly through opposition to smoking or drinking), and perhaps willingness to seek advice. The degree of cultural cohesion has an important negative impact on outmigration, while ensuring that migrants move in large groups. Differing cultural attitudes toward death may have an increasing importance as medical technology extends our life spans. Various values are in conflict when we consider euthanasia: we value life, dignity, and the ideal of personal choice, but worry that impatient family members will hasten death.

1. Fecundity. Among women, various factors, such as too much or especially too little body fat, disease, stress, poor nutrition, and mental illness, have been found to lower fecundity. There is also evidence that certain genes increase fecundity (Daugherty and Kammeyer, 1995). Men also differ in fecundity, and for similar reasons. Certain couples may have difficulty procreating, either due to genetic mismatch or the multiplicative effect of small deficiencies on both parts.

2. Intelligence and Birth Rate. Among the explosive arguments made by Herrnstein and Murray (1994) was that those with low IQ scores had higher birth rates, and thus an underclass of increased size was being generated. On the latter part of this argument, we should note both that the link between parental and child intelligence is complex and incompletely understood but apparently fairly weak, and that average IQ scores have risen steadily for decades. The link between intelligence and birth rate is also small, and likely not a direct causal link, but mediated by such factors as low education and income.

3. Teenage Pregnancy. Many teenage pregnancies result solely from a misuse or failure of birth control technology. In most cases, though, there was either a desire to become pregnant or at least a lack of concern about the possibility. Teenage pregnancy is often associated with low levels of self-esteem. For a girl who holds out little hope of success in school, work, or relationships, bearing a child at one stroke provides her with a social role (motherhood) and an infant that has no choice but to shower her with affection for a few years. By the time she recognizes that she has underestimated the cost of motherhood, it is too late.

4. Migration. Empirical studies rarely grapple with the fact that in almost all cases of migration some people stay and others leave. Still, some of the more obvious determinants of individual migration choice have been identified. Those in their late teens or twenties are most likely to migrate (though in developed countries people are also likely to migrate at the time of retirement). Men are generally more likely to migrate than women, though exceptions occur. One can explain the age distribution of migrants fairly simply: the young are less likely to have sunk costs at home C seniority on the job, real estate C and will have a longer time horizon over which they can recoup the costs of migration. Men may be more likely to migrate simply because they are more likely to obtain a financial reward at the other end. However, it is also possible that the young and the male are more likely to be ambitious, willing to take risks, and adventurous. The effect of such personality traits on migration has rarely been examined. There is evidence, though, of differences in gene frequency between immigrant groups and the source population. This, as well as casual observation, suggests that personality traits play a significant role in determining migration.

5. Mental State and Health. There is now a substantial body of research in support of the widespread belief that there is a positive relationship between psychological wellbeing and physical health (see Ellis, 1993). In clinical trials of new treatments, patients receiving placebos are often observed to fare better than the population average. The mere belief that they are being cured encourages exactly that result. Support groups for patients with similar diseases often slow the spread of the disease (those with advanced breast cancer doubled their life span from 18 to 36 months in one study), whether because people feel better from knowing they are not alone, or take heart in positive stories from group members.

Negative feelings can make one unhealthy. While a stress-free existence is not to be sought, as it would prevent self-growth and might also have adverse physical effects by under-stimulating nervous and cardiovascular systems, many people have stress levels that are too high in the sense of causing physical deterioration. The hormone adrenaline which is generated in stressful situations interferes with the creation of antibodies to fight disease. Hormones that act with adrenaline may produce proteins that foster the multiplication of cancer or virus cells. Various empirical studies have shown that, though individuals differ in their resistance, those under the most stress are most likely both to catch diseases, and suffer from these.

Can we move beyond these discoveries to establish whether certain personality types are more prone to certain diseases? Research here has been limited to a few chronic diseases, and severely hampered by limited communication between human scientists and medical researchers. Cardiologists have suggested that "Type A" individuals are much more likely to suffer heart problems. Type A’s are defined as too achievement-oriented, competitive, impatient, hostile, and with overly vigorous speech and manner. Debate has raged over which of these characteristics are the most important, with hostility followed by vigor the most favored (Contrada, Leventhal, and O’Leary, 1990). Psychologists have in turn debated how Type A’s might be viewed from the perspective of standard personality typologies: paranoia, compulsiveness, impulsiveness, and insecurity have been suggested as key variables (Miller, 1990).

6. Drug Abuse. We will discuss below the effects of drugs on health. We focus here on the fact that both genetic and environmental factors affect the likelihood of drug abuse. Children of alcoholics are thus more likely to become alcoholics, both because they may have inherited a genetic predisposition, and because they will have been raised in a conducive environment (some are instead turned off by parental behavior). There appear to be several genes involved in alcoholism, and thus the precise causal pathways are unclear. Miller (1990) discusses the possible links between personality dimensions and alcoholism. While noting that there are some friendly drunks, he suggests that most alcoholics are antisocial, impulsive, and aggressive. He finds that depression is often associated with alcoholism. Those with attention deficit disorder are also more likely to become alcoholics. In both cases, it remains to be seen whether there is some direct genetic link or whether depression and/or ADD decrease one’s chances of success in life, and thus one’s desire for escape. Miller concludes that those with poor internal language skills, and thus poor ego development, are subject to surges of impulse and emotion which they use drugs to counteract.

1. Reproduction. Human infants require years and years of nurture by those older than themselves. Parents and wider communities face limits in their ability to devote time and resources to children. Families or communities which are unable to provide enough food to children will experience higher rates of child mortality (and may kill some children, especially the weak or disabled). Finally, we could expect families unable to support more children to practice birth control of some sort, and thus reduce fertility on average.

Throughout most of human history, we can observe the positive relationship between income and population growth rates suggested by the above analysis. Within societies, the wealthy have usually had more children than the poor, especially where polygamy is countenanced. More prosperous societies display more rapid population growth. Variance in income also has an impact: families in environments characterized by frequent interruptions in food supply tend to have extra children as "insurance".

In modern developed countries, the link between income and fertility has been broken. Birth rates have fallen as incomes have grown. And birth rates vary little across income groups. Economists explain this "demographic transition", which occurred in all developed countries over the last century or so, and in most developing countries more recently, in terms of the rising cost and declining benefit of children. Industrialization decreases the benefit of children, as they can no longer be productively employed in farm work. It also increases their cost: it is much easier for childbearing to be combined with farm work than with paid employment outside the home. Another characteristic of modern society is the increased importance of skill acquisition. Parents who want their children to succeed will want to support them while they gain an education, as well as devoting personal time and energy to teaching them. Parents then face a tradeoff between child quality and quantity. Modern societies also institute social security and pensions: parents no longer need many children to support them in old age. Moreover decreases in infant/child mortality generally precede the demographic transition; parents can thus be more confident that their children will survive. Note that these parental decisions do not explicitly account for the effects on the wider society. Parents internalize neither the social costs of public education nor the social benefit of a future taxpayer.

2. Nutrition. It is now widely believed by economic historians that improved nutrition was the most important single determinant of declining mortality in nineteenth century Europe, and still responsible for half the twentieth century mortality decline (in large part because well-fed people are less prone to disease). This in turn reflected an increase both in the quantity and variety of food (Fogel, 1994). Fogel has calculated that the bulk of the eighteenth century European peasantry was too undernourished to work hard all day; 10% of French peasants would have been too weak to work at all. Increased agricultural productivity gradually increased the volume of food per capita over the next couple of centuries in Europe. In many parts of the Third World, however, the modern situation is similar to that of eighteenth century England.

Fogel found that the decreased incidence of famine over the last centuries in Europe was responsible for only a fraction of observed mortality decline. Individual famines can nevertheless have a devastating effect on regional populations. Modern scholars of famine prefer to define these as "life threatening decreases in access to food for at least some members of society", rather than in terms of an absolute food shortage. This reflects the fact that famines can indeed occur without a food shortage. People can lose access to food because of a loss in income, a loss of legal access to supplies, or because the price has risen (or official availability fallen). Famines are most likely after bad harvests or during times of war or civil unrest but can happen without these. A small shock can have a devastating effect if it encourages hoarding of food. While increasing agricultural production can reduce the incidence of famine, then, it will not necessarily eliminate famine. Various government policies, such as stockpiling food and dispersing it to the needy, or improving transport infrastructure, have been observed historically to reduce/eradicate famine.

Even when there is enough food for all people to lead lives free of hunger and weakness, there may be problems of an unbalanced diet. We have, for example, identified 13 vitamins essential to good health. Not all can be found in any one type of food. As agricultural productivity increased, farmers were able to grow a variety of crops, rather than focussing on the crop with which they could wrest the most calories from the soil. Moreover, as transport costs have fallen, it has become increasingly easy to eat a variety of foods. In the Third World, however, a recent World Bank study estimated that one in 2500 South Asians die from vitamin deficiency, and thousands more lose their sight or are rendered unable to work.

Nutrition also influences fertility. Frequency of ovulation and sperm counts will both decrease in the face of undernourishment. Moreover, it is believed that improved nutrition is largely responsible for the fact that boys and girls become sexually mature years earlier than was the case in the eighteenth century.

Once a woman becomes pregnant, her nutritional level has important effects on fetal development. Various birth defects are traced to a shortage of key nutrients at particular stages of development. Insufficient folic acid intake can hinder fetal nervous system development. Insufficient maternal calorie intake will be reflected in an underweight and underdeveloped baby prone to disease. The fetus devotes energy first to the brain and only later to liver and kidney, so these organs can be especially underdeveloped.

3. Pollution. Pollutants have been fingered in recent decades for contributing to a variety of health problems, especially cancer and asthma, as well as a troubling (if true) fall in sperm counts in most western countries. It is hard, though, to determine which of the thousands of chemicals produced by humans is at work. Laboratory experiments with animals are often derided for using much higher exposures than are found outside the laboratory (see Bailey, 1995). And both falling sperm counts and disease incidence might reflect changes in lifestyle, such as urbanization or office work, more than chemicals.

Economic historians have uncovered evidence of a decline in health and increase in mortality associated with the earliest stages of industrialization in many European countries. They blame this primarily on water pollution. Countries such as France in which urbanization proceeded more slowly appear to have suffered less if at all. Countries which industrialized after the recognition of the role of germs in disease were able to reduce or eliminate this negative effect by early investment in clean water and sewerage.

4. Migration. Daugherty and Kammeyer (1995) review various models of migration. In all of these economic and family considerations loom largest, though political considerations can sometimes dominate, and it is generally recognized that an individual’s perception of circumstances at home and abroad is the final determinant of migration. Barrett (1992) asserts the primacy of economic motives; Daugherty and Kammeyer worry that these may simply be easier to measure at an aggregate level than family concerns. One longstanding debate considers whether migrants are more attracted by evidence of higher average wages in another locale or evidence of a greater probability of getting a job (Plane and Rogerson, 1994).

5. Psychotropic drugs. Humanity has been familiar with the mind-altering effects of various natural substances such as alcohol, cannabis, and opium for at least tens of thousands of years. Economic circumstances determined which of these natural substances people come in contact with, while cultural and social considerations determined attitudes toward use. Alcohol production arose with settled agriculture, for example, though it spread through trade to nomads. Desert tribes resisted alcohol, and imposed these views on others when they conquered agricultural areas (Goodman, Lovejoy, and Sherratt, 1995).

The range of substances available to humanity as a whole changed little over the millennia. Within the last century, however, a number of synthetic drugs have been created. These have greatly extended the list of psychotropic drugs available, though drugs based on natural substances still dominate world markets.

Are drugs harmful? Cavalli-Sforza (1981) concludes that they are fitness-reducing, and wonders why we do not have stronger resistance to drug abuse. Indeed, we must recognize that drug use is a universal: all human groups have explored the drugs available in their locales (Goodman, Lovejoy, and Sherratt, 1995). Moreover, we are physiologically designed to become addicted to a variety of substances, though there is no reason to believe we were selected for drug addictions (T. Smith, 1992). And many drugs, including alcohol, are observed to increase sexual desire while lowering fertility.

What exactly are the effects of drugs on health? While most societies have firm ideas about which drugs are dangerous, the medical basis for such decisions is often thin. Many researchers believe that some illegal drugs are in fact less harmful and no more addictive than alcohol and tobacco. While particular negative effects are well documented C nicotine and cancer, alcohol abuse and heart attacks C the full range of effects of all drugs is still very poorly understood. Some of these effects are positive. A glass or two of wine a day may do more good than harm. More controversially, studies have suggested that marijuana can control glaucoma, suppress the nausea associated with chemotherapy, stimulate appetites among AIDS sufferers, and relieve pain associated with multiple sclerosis. Criminalization of some drugs complicates the evaluation procedure. Many researchers may be hesitant to proclaim results which suggest laws are misguided. Those interested in testing the beneficial effects of illegal drugs often have difficulty obtaining government approval. Addicts may suffer much more from irregular supply than they would if the drugs were readily available (heroin addicts are commonly given access to methadone for this reason). Moreover, dirty needles, impurities in street drugs, bad diets, prostitution, and violence are further health-reducing characteristics of illegal drug use which may stem more from the illegality of the drugs than their chemical composition.

Debate about the psychological effects of drugs is perhaps even less conclusive. Few would argue that those with severe mental illnesses should not be given drugs which clearly reduce their symptoms. But should the average person regularly take drugs of any sort to numb pain or anxiety or shyness or depression? For many the answer is a resounding yes: if drugs make us happy we should use them. Konner (1982) sounds a defiant "no": he argues that real pleasure in life comes from overcoming our fears and weaknesses, and thus the easy escape offered by drugs will lower our longterm sense of wellbeing.

If social attitudes toward drug use can not be explained by a straightforward appeal to medical evidence, we are guided to look at cultural and social influences. Drugs are an important part of religious and cultural rituals in most human societies, and have often been central to socializing. Cultural attitudes toward drugs have changed markedly in western societies over the last century: alcohol was banned but is now accepted, tobacco is viewed increasingly harshly (largely because of medical evidence), addicts were once viewed as victims but are often now blamed. Attitudes toward drugs depend to a great deal on which social groups are using particular drugs. Opium was disdained in late nineteenth century America in large part due to its association with Chinese immigrants. Prohibition was powered by a public perception that working class drunkenness reduced productivity and created social problems (Davis and Stasz, 1990). Cocaine excited little controversy in the 1970s when it was used by the middle and upper classes, but became a public scourge when in the form of crack cocaine it spread to the poor and racial minorities. Marijuana came to be viewed as a symbol of youthful rebellion in the 1960s; it is not yet clear how the passage of generations will affect its public standing. Society has tended to ignore drug abuse by women (laudanum in the nineteenth century or valium in the twentieth), perhaps because it rendered women more docile.

1. Fertility. By providing a range of tax breaks and subsidies for parents, states decrease the cost of children. Since most families in developed countries have far fewer children than they can afford, it is unlikely that these financial incentives have a huge impact on the fertility decisions of all but the poorest. If parents had to pay the cost of schooling and health care, fertility might fall slightly (Birdsall, 1994).

Laws governing sexual activity, such as bans on underage sex or incest or rape, will to the extent they actually decrease sexual activity serve to decrease fertility. More importantly, both the legality of birth control (and abortion) and the extent of government provision of birth control advice will have a huge impact on people’s ability to have sex without unwanted pregnancy. Many Third World countries have had considerable success in lowering birth rates in recent decades via provision of birth control services.

Governments can affect the costs and benefits of having children in many other ways. By providing villages with running water, for example, they may severely reduce the need for the child labor involved in fetching water. And social security removes the need to rely on one’s children in old age.

Arguably the greatest single political factor in fertility, though, is education. The fact that children in developed countries go to school for many years has increased the cost of children markedly. On the other hand, within a modern urban setting, schooling takes on the role of free day care for many families. In developing countries, education has a huge negative impact on the future fertility of young girls. They thus learn more about prevention of unwanted pregnancy. Most of the effect, though, is widely attributed to the fact that educated women are more powerful (in large part because more employable) within families. Since women bear more of the costs of childbearing, this increased relative power results in lower fertility.

2. Mortality. Sadly, the greatest cause of death among young inner-city black males is criminal violence. Possible changes in a variety of government laws and programs which might reduce the incidence of crime, and thus indirectly improve mortality figures. Such policy changes would also affect the more direct impact the state has on mortality through its police, prisons, and the use of capital punishment. Debate still rages as to whether state-sanctioned death serves to legitimize murder in the eyes of a significant segment of the population, or whether fear of severe penalties acts as a deterrent. States also declare wars, and may encourage internal rioting and rebellion by unfair or unpopular policies.

3. Health. Governments are in most countries the main providers of medical services. They certainly regulate the licensing of professionals and the dispensing of drugs. They are also generally responsible for providing clean water, decent sewerage, and environmental regulation. And almost every country affects the price of food, and thus its citizens’ diet, through a range of price supports and/or subsidies and trade restrictions. Rare too is the state which does not affect the price and quantity of clothing and shelter. Finally, governments mandate a range of health and safety guidelines.

4. Migration. In the short term at least, by far the most important state policy affecting population size is that governing migration. Virtually every country in the world limits the number of legal immigrants, and attempts to restrict illegal immigration. Some countries also attempt to limit emigration. While rare in the developed west, such policies were common during the communist era in eastern Europe, and are pursued in some Third World countries which fear an exodus of their most talented. Differences in tax rates encourage some to migrate, though the size of this effect is much debated. People are also likely to migrate to areas in which various government services, such as schools, health, and police, are judged superior. Even more indirectly, it should be noted that empirical studies regularly find that the likelihood of migration increases with educational attainment, likely because it increases the chance of getting a job (and a visa).

Empirical studies almost always find differences along ethnic, marital status, and class lines in terms of all of our categories, while genders almost always differ in terms of health, mortality, and migration. The causal links involved are generally indirect.

1. Class and fertility. While in some societies, notably those which allow polygamy, the rich have more children, in developed countries the reverse if anything is true. The rich apparently concentrate on the quality aspects and forego the possibility of many progeny. Some of the poor may choose a different tradeoff: if pessimistic about any one child’s chances of success they may decide to have several.

2. Race and Fertility. Black Americans have long had higher birth rates than white Americans. Yet the trends in birth rates have been remarkably similar, indicating that members of both groups respond to similar forces. In recent years the major source of fertility differences has been the high rate of teen pregnancy in the black community (Daugherty and Kammeyer, 1995). This in turn largely reflects a pessimism about the future among black teenagers. It is noteworthy that immigrants from high fertility countries display high fertility only for the first generation or two. This indicates that cultural attitudes toward fertility adapt quickly.

3. Gender Relations and Fertility. Since women bear most of the costs of childbearing, increased power of women within families tends to result in a decline in fertility.

4. Health. Within developed countries, ethnic differences in health generally reflect class divisions: poorer ethnic groups--aboriginals, blacks--are also the least healthy. This reflects in large part inferior access to food, clothing, shelter, clean water, education and health care, as well as increased likelihood of criminal activity and hazardous occupation (the poor are also observed to snack more and eat more junk food). African countries are often healthier than South Asian countries, despite being poorer. Some of this anomaly might be explained by greater access to food among less dense populations. Disease incidence is also likely greater in densely populated south Asia. A further difference is the lower status of women in south Asia society, which likely leads to undernourished fetuses and babies. The status of women naturally also affects their relative health. Amartya Sen has compared the ratio of men to women across societies and calculated that in a handful of developing countries there appear to be about 100 million women "missing". Whether through infanticide or merely substandard care, women live much shorter lives than men there.

In the developed world women live longer. At least part of this difference is genetic. For example, testosterone weakens the body’s immune system, and thus men are more prone to disease than women.

5. Mortality. Can the increases in life expectancy which we have seen over the last centuries be continued, or are we rapidly approaching biological limits to longevity? Pessimists suggest that developed countries have now reached a point where even the poor can achieve good diets and access to sanitation, and thus the "easy" gains in mortality which result from nutrition and sanitation are behind us. As well, we have beaten most epidemic diseases, and thus future medical advances may be sluggish. One puzzle that faces this approach is that poor Americans still experience much higher mortality than rich Americans. In particular infant mortality differs considerably. Explanations include poor diet, drug abuse, and poor health care. Among adults more subtle influences are likely at work. Daugherty and Kammeyer (1995) note that those with jobs with little stress or decision making may be more prone to heart attacks, for example.

6. Drug Abuse. The incidence of drug abuse varies considerably across ethnic groups. As with health in general, much but not all of this can be attributed to income/status differentials. The poor are much more likely to turn to drugs (or, some would argue, just more likely to be arrested for doing so). Cultural differences in acceptance of particular substances and/or behavior are also important.

7. Families and Migration. Members of nuclear families are more likely to migrate than members of extended families. Single men are the most likely to migrate, though young couples are also common migrants. Families in which one child inherits everything encourage younger children to move. In all of these cases, family structure influences both the economic situation and emotional ties which individuals have to their family, and through them to their locality. Once one member of a family has moved, they will usually communicate news of the new environment. If the news is good, other family members often follow.

The space devoted to a causal link in this paper is an imperfect predictor of the importance of that link. In our present case, the impact of technology on health and population over the course of history is so overwhelming that we need but list several of the most important effects.

1. Birth Control. Birth control serves to de-link our sex drive from decisions about reproduction. It thus severely reduces the likelihood that parents will have more children than they desire. The reliability of birth control methods has increased markedly in recent centuries and decades. There is, though, relatively little research at present. Moreover, various religious and ethnic groups remain suspicious of birth control.

2. Medical Technology. It is doubtful that doctors had much impact on health or mortality until the nineteenth century, though there was a gradual accretion of understanding. Doctors’ more recent success is associated with a range of drugs and machines. Ironically, doctors and patients both viewed machinery with suspicion a century ago, for it violated the tradition of manual healing. There are still huge gaps in our knowledge. Not only have cancer and heart disease proven difficult opponents, but the optimism of the 1950s concerning the imminent conquest of infectious diseases has receded (see Garrett, 1994). Some viruses and bacteria are becoming resistant to penicillin and antibiotics. And a handful of recent studies suggest that people who do not catch childhood infectious diseases are more prone to chronic adult ailments such as asthma, hay fever and diabetes, perhaps because the immune system is less active. It is entirely possible that genetic engineering, if ethical concerns can be dealt with, will yield the greatest future advances.

Finally, we should note that scientists do not fully understand all of the effects which various drugs have on the human body (and the average doctor even less so). Many drugs have known side effects, though our ability to predict who will experience these is far from perfect. There are also likely a vast of unknown side effects. One recent estimate suggests 100,000 Americans die annually from the side effects of correctly administered drugs. This is a small figure compared to the million likely saved by the same drugs, but nevertheless signals the value of more fully understanding the effect of drugs on the human body.

3. Production Technology. Long before medical technology made a dent on mortality, technology in agriculture and transport had revolutionized diets, and better pumps and pipes provided clean water and sewerage to urban masses. Effects have not always been positive: industrial pollution turned cities into death traps, and international shipping allowed diseases to attack populations with no immunity. Looking to the future, optimists feel that agricultural and industrial innovation will allow future billions to enjoy high living standards, while pessimists fear environmental degradation and shortages of both food and water.

We can briefly list some of the more obvious ways in which Health and Population in turn affect other phenomena. Population growth encourages pollution and resource depletion. Health affects cultural attitudes toward disease, family, children, and death, among others. Migration is a major source of cross-cultural diffusion of attitudes and practices. Health has a major impact on an individual’s outlook on life. Our position in the age distribution affects both our attitudes and chances of success. Healthy people are more productive. Population growth has both positive and negative effects on economic growth, though the latter may dominate in densely populated countries. Aging populations place great demands on health and social security systems, while high birth rates also require spending on schools and hospitals. Undernourishment, gender imbalance, and a youthful population, are all conducive to political instability. Governments may divert attention with wars. "Excess" immigration may also be destabilizing (though immigrants generally develop an attachment to their new country fairly quickly). Differential population growth rates affect the relative power of societal (especially ethnic) groups. Population pressure on resources has at times provided a major spur to technological innovation.

I would emphasize that the links discussed, while organized in terms of major category, in fact operate for the most part at lower levels of aggregation. We could have discussed a number of other links. Hopefully, we have nevertheless succeeded in illustrating the value of the schema as an organizing device, and the ubiquity of causal links among phenomena.