Dept. of Population and Family Health Sciences
Johns Hopkins School of Public Health
Presented at the NICHD panel,
"Visions of the Future: A Town Meeting on New Directions in Population Research"
Annual Meeting of the Population Association of America
Before presenting a list of fruitful areas for future work in population studies, I want to spend a few moments setting up a historical and intellectual context for such an exercise. A year numbered "2000" is as good a time as any to take stock and to wax philosophical.
How Can We Crowd and Still Be Kind?
In the middle of the last millenium, Christopher Columbus found something. What he found permitted our ancestors to put off solving one of the great unsolved problems in population studies. Robert Frost puts it best in the final verse of his poem, "America is Hard to See".
Had but Columbus known enough
He might have boldly made the bluff…
But all he did
Was spread the room
Of our enacting out the doom
Of being in each other’s way
And so put off the weary day
When we would have to put our mind
On how to crowd but still be kind.
Crowds are here. They will be here for the rest of our lives. The "Mission to Mars" notwithstanding, another Columbian discovery seems unlikely. We will not have the luxury of another 500 years of stalling on the issue of kindness. Kindness is our problem. Here is the scope of the problem as summarized recently by James Speth in Foreign Affairs:
Among the 4.4 billion people in developing countries around the world, three-fifths live in communities lacking basic sanitation; one third go without safe drinking water; one-quarter lack adequate housing; one-fifth are undernourished; and 1.3 billion live on less than $1 a day. Nearly one-third of the people in the poorest countries, mostly in sub-Saharan Africa, can expect to die by age 40.
Malthus’s Answer: Never Crowd
Columbus delayed the crowding by discovering land, Malthusians sought to solve the problem by stopping crowds. The Malthusian iron law of wages excuses procrastination in solving the problem of crowding with kindness, because the problem is unsolvable. Indeed, for Malthusians there can be no kindness as long as there are crowds because crowding is synonymous with unkindness. Perhaps our successors in 2100 will thank us if we find an effective means of hastening fertility decline, but in the short and medium term the rest of the demographic transition promises to be a bumpy and crowded road.
Technology’s Answer: Faster, Better, Cheaper, More
The 20th century improvements in technology did not solve the problem of kindness, but they certainly made it appear solvable. At the beginning of the last century only a small percentage of world population lived in nations that had advanced very far into the industrial revolution. Dividing the world GDP by the world population in 1900 would have left everybody at a near subsistence level-around $500 (1990) dollars per person per year. World population is now much bigger, but the pie is even bigger. In 1995 dividing the world GDP by the world population would result in a GDP per capita of roughly $5000 per person throughout the world.
Now $5000 per person per year is not a lot of money by the standards of the American middle class, but by world standards it is a solid middle income typically associated with a 70 year life expectancy, infant mortality of roughly 20-30 deaths per 1000 and adult illiteracy of only 20%. An income of $5000 per person per year can support what Sen would call human functioning and it is now in theory achievable for all. There is now plenty to go around. We just do not know how to get it around.
The 20th century offered triumphs for freedom and triumphs in our ability to use freedom to generate an economic surplus. Despite the surplus, few would call the 20th century a century of kindness. Biologists are beginning to suspect that despite their capacity for social living, primate species are not that altruistic. The level of kindness to be expected of a Homo sapiens who is the product of natural selection or who is the subject of a rational decision making model is simply the expedient level—just enough alms to the poor to hold off the revolution.
To this view, there is no market failure stopping us from being kind. The elite classes set up just the requisite institutions that are necessary to conduct the "efficient" level of redistribution. A long-run equilibrium of low level kindness is possible as long as the elites can keep the revolutionaries and demagogues at bay. From a scientific point of view the persistence of poverty in wealthy market economies is not surprising—what has been surprising is how inept historical elites have been in even achieving the efficient amount of poverty alleviation. There have been far too many revolutions driven by starvation and hatred.
Demagogues’ Answer: Redistribute!
Robin Hood’s method for crowding with kindness does not work. Granted, the social systems set up under communism were far more complicated than attempts to simply redistribute wealth by taking from the rich and giving to the poor. But despite their political complexity the demographic record is clear on the link between extreme forms of socialism and famines whose death tolls number in the tens of millions.
We have had enough experience with extreme forms of socialism to realize that a major reason that the market economies’ pie is so big is the retention of incentives to work that are eroded without some protection of private property rights. Coerced redistribution is kind to neither the giver nor the receiver. The famines in the Ukraine in 1930, in China in 1960, and in Korea this last decade need not be repeated for further demonstration.
Humanists’ Answer: Locate Kindness
The solution to the problem of kindness relies on there actually being kindness in hearts of free human beings who use their freedom to voluntarily redistribute their private property. The source of any current dis "ease" requires a rudimentary bit of ethical theory to locate. To keep things simple I will ignore over 2000 years of philosophical advance and return to the ethics of Socrates and Plato.
Classical Political Philosophy
In Plato’s dialogue, the Protagoras, we meet a Socrates who many classicists feel speaks for the historical Socrates. This Socrates argues that virtue is the right estimation of the painful and pleasurable consequences of our actions. Were Socrates here he would tell us of our 20th century virtues. Socrates is impressed with knowledge. Because 20th century academics know so much about what leads to pain and pleasure we would be considered paragons of virtue. For Socrates, our knowing that measles vaccine reduces child mortality would be virtuous. Our knowing that collectivized farming can lead to famine would be virtuous. Our knowing how to maintain political stability despite economic inequality would be virtuous. Socrates equates all mental activity with cognition and sets out the objective of cognition as hedonistic maximization of utility. (For obvious reasons, neoclassical economists are quite at home in a Socratic world.)
But neither Socratic ethics, nor a neoclassical economist can account for any of the uneasiness some among us feel about tolerating poverty in a world of plenty. Even back in ancient Greece, the Socratic vision of mankind did not sit right with Socrates’ own pupil who found it paradoxical that knowledgeable (and therefore virtuous) people might do unjust things . Plato found that the root of the Socratic paradoxes came from a model where all mental activity was simply knowledge. The resolution to the paradox as set out in The Republic was a view of man’s soul as a triad. Plato taught that mental activity was not just knowledge, but that there were three parts of the soul: The rational, The appetitive, and the spirited. The virtue of a single human being was derived from achieving balance and harmony among the three parts of the soul. The virtue of a state was derived from achieving balance and harmony among the rational (philosopher-kings), appetitive (merchant class), and spirited (guardian or warrior class).
Social Decay as Disharmony of a Triad
Plato spawned a genre of social commentary in which the political maladies of any age could be rooted in an imbalance or disharmony of the soul. Other mental triads have been spawned including Freud’s Ego, Id, and Superego, and modern neuroanatomists’ accounts of the three brains: the human neocortex, the mammalian cortex, and the reptilian limbic system. Stebbins has offered an account of human evolution that relies on a triad of three uniquely human capacities of artisanship [the evolution of tool making into complex manufacturing and construction activities], conscious time binding [the ability to plan ahead while benefiting from present and past experiences] and imaginal thinking [the ability to abstract from reality, necessary for planned achievement] . No intellectual historian would seriously entertain a thesis that attempted to find complete homology between each and every triad that has been advanced. Freud’s ego is not Plato’s rationality. Nor is there any obvious reason why the number three keeps recurring in accounts of human psychology. The number three just happens to be the smallest integer that is not two—and suggests complexity without the full encumbrances of complexity.
In the recent outpouring of Y2K literature, commentators on the history of the last millenium and the last century have chosen to link the low moments in human history to the failures to achieve harmony in whatever trichotomous account of humanity appears most apt. The dark ages can be cast as a period in which the human capacity for imaginal thinking dominated thought. Dogmatic adherence to calcified spiritual teachings slowed the progress of artisanship and planfulness . The 19th and 20th century can be cast as a period in which the tables were turned: artisanship and planfulness became ascendant. Spirit ceded ground.
We have all just survived humankind’s most overwhelmingly scientific century. For those of us living in market economies, it has also been the most appetitive century as well, more cravings than ever before for material goods have been fulfilled. Consumption rates have never been higher in the US—this trend is likely to continue at least until the retirement of the baby boomers begins in 2010 leads to a sharp rise in the total dependency ratio. Some economists are busily trying to put off our reckoning with a day on which our aging workforce forces all of us to live with less consumer goods. Other economists have been pointing out that for all our consumer goods we are not getting more of what we really want out of life after basic needs are met. The imbalance of the soul pointed out by Fogel in his presidential address to the American Economic Association, is the atrophy of spirit and spiritual capital in our cultural institutions and in our academic discourse. An attempt to find balance for the 21st century would need to address the imbalance between spirit and science.
In cultures that do not coerce spiritual consumption, the disposal of both public and private resources today fuels scientific research into the production of more goods. The manifest preferences of the free market indicate that the harvest reaped from the scientific paradigm is more suited to the tastes of free consumers than competing possibilities. There is really cool stuff for sale and it never existed before. No wonder people are buying it.
But the fact that the power of science and industry is built on individual free choice does not render it immune from abuse. The ascendant power of the scientific research establishment can be subject to just as much abuse of power as was witnessed 700 years ago when the church wielded absolute power over European life. Consumerism can engender the same self-perpetuating cultural forces that were seen with the rise of state religions.
The Academic Triad
So we haven’t learned to crowd and still be kind. The academy that might have been able to discover and to teach this lesson did not. The lesson of kindness that has eluded the academy is not simply a matter of finding a new technological discovery, it is not a matter of a new political system, it is not even a matter of moral leadership: it is all of these things. It is a balance and a harmony in the soul. If we are disturbed by a world of disharmony and unkindness, our best response is to do as Gandhi suggested and "be the change we wish to see."
The traditional trichotomy of academia has also been no stranger to calls for unity and balance. Academics from science, social science, and the humanities have heard from an army of deans, provosts, and NIH program officers, of the need to form interdisciplinary linkages. If the academic triad ever was in balance it certainly is not in the year 2000. One plea for unification and "consilience" that has recently arisen was little more than a bid for a hostile takeover of social science and the humanities by biology . A Platonic perspective on the unity of academia would caution against any one of the three areas wielding hegemony power. A century spent devoting resources to the fruits of social scientific inquiry would be just as bad as any other excess or imbalance in the triad. (On the other hand power is nice and the other branches of inquiry have already had a turn at the helm….!)
So with that lengthy digression as background, I come to the business at hand: making a junior researcher’s list of promising areas for research. I am here to deliver the old saw that there should be greater cooperation between the social sciences, the physical sciences and the humanities. Interdisciplinary cooperation means both give and take. Population studies and social sciences more generally have much to offer to inquiry in both the natural sciences and the humanities. The opportunities I would like to discuss are ones that I believe will create more balance. They are also opportunities that help create a solution to the problem of crowding with kindness that faces ourselves and our children.
What Social Scientists Can Offer To The Natural Sciences
In the study of socioeconomic differentials in mortality, the final common pathway must play out in human tissue. At this moment many natural scientists are engaged in working out the pathophysiological pathways of the stress response thought to be responsible for the differentials. Chronic cortisol poisoning is one of the proximal determinants, but being low in the social hierarchy is thought to be a possible distal determinant. There is agreement that living conditions, and social, and economic institutions are in some way responsible for the stress response. Population scientists need to be dealt in here. If the problem of socieconomic differentials in mortality remains in the hands of physiologists alone, the solution for it is going to resemble a new pharmaceutical product or neurological procedure. Cooperation might disclose unexpected interplay between biology and social structures.
Equitable Distribution of Scientific Technology
Even if solutions to important problems in public welfare are judged to have "technical" solutions, the mere invention of a medication or a procedure is demonstrably inadequate to solve the problem. Every day inexpensive and nearly affordable health interventions remain out of the hands of the world’s poor. Effective vaccines, medicines, and technologies that were invented decades ago still remain on shelves in wealthy countries because the economic problem of their fair distribution has not yet been solved. As of October1998, Hepatitis B vaccine (at a cost of $1.00) had still not been adopted by 104 of 179 countries for which there was data . As of July, 1998 there were at least 10 countries where measles vaccine (costing <$.50) has not reached more than half the eligible children and over 25 countries where coverage is less than 80%. Even the great success stories in vaccine-led control of epidemics occurred after long delays in which the rich countries benefited long before the poor.
There can be no argument that the development of new and better chemical technologies against the world’s diseases would be welcome. The Global Forum for Health Research has persuasively made the point that the North’s vast army of human resources trained in the development of biomedical technology is being deployed unjustly. That 90% of health research targets only 10% of the world’s burden of disease is unconscionable. From the perspective of the human species, the dual treasure of scientific knowledge and scientific manpower is being squandered. Yet the long delays in deployment and the mal-distribution of existing technology suggest that even if biomedical research priorities were rectified new products would remain out of reach for the poor.
Clearly the world is not making effective use of past successes in medical technology. Given its obvious shortcomings, the present system of global distribution of medical advances should not be retained if we can do better. It is a disservice to the preceding generation of biomedical developers that the fruits of their work are not reaching the intended beneficiaries. The past approach of public health has been to hope and pray that a deep pocket such as a government, an NGO, or a multilateral institution will decide to subsidize services for the poor. Relative to world military spending the amount of subsidies required for such subsidies would be a pittance. But after decades of attempting to cajole donors to put greater priority on subsidizing health care for the poor, global inequities remain. Perhaps one day ethicists will develop just the right moral argument that, once uttered, will compel all haves to help have-nots. Until the day that there are sufficient subsidies to achieve equity, other approaches to the distributional problem need to be explored with the same zeal as we commit to solving the biomedical problems.
One long known and little implemented approach to generating subsidies, is known to economists as Ramsey pricing and involves charging higher prices to those who are willing to pay them and lower prices to those who are not. Unfortunately those customers who discover that they are paying the higher prices don’t call it Ramsey pricing—they call it gouging. Firms with monopoly power automatically try to engage in Ramsey pricing in attempt to capture higher profits. It is usually difficult to sustain the tiered prices without the complicity of a government regulator to run interference—and the government regulators usually demand a cut of the take. For example ATT is a classic example of a regulated monopolist who was forced by a regulator to subsidize construction of rural phone networks on the proceeds of the more profitable urban networks. City dwellers and businesses paid far more for phone service than the cost of laying wire in a city. Country dwellers paid far less.
Many economists would like to use an international system of price tiering to generate subsidies that can fund distribution of new vaccines and medications to poor nations at prices at or below the cost of producing them.
Modeling Human Behavior in Scientific Studies of Health and Fertility
In my own realm of population economics we offer mathematical accounts of why human beings expose themselves to risk, why they decide to try to reproduce, why they decide to seek medical care. We have been able to conduct statistical tests of these models and to show that they are superior to black box models that assume that risk behavior, fertility and medical utilization are random and exogenous events. As more and more large administrative data sets come on line from managed care companies and disease management units there will be more opportunities to evaluate pharmaceuticals, medical interventions, and health systems through secondary data.
What Social Scientists can contribute to the Humanities
Measures of Outcome Equity
Social scientists and particularly, demographers can offer ethicists and historians important tools with which to assess the equality achieved by different societies. Old tools that focussed on the distribution of resources such as the Gini ratio are now being joined by tools that focus on the distribution of health outcomes. These new tools can advance discussions of what it means to call a society just.
The simplest example of such a tool is the concentration index, which is simply a Gini ratio applied not to "income" by income percentile, but mortality rate by income . Measures of outcome equity that attempt to focus on the pure variance of death rates without requiring data on income were reviewed by Wilmoth and Horiuchi who find a high degree of equivalence among the various measures.
Valuations of Equity
Philosophers since Hume have been aware of the naturalistic fallacy which says that one cannot go from an "is" to an "ought". For example, one could not have simply taken a survey of 18th century southern plantation owners in order to judge whether slavery was wrong. Nevertheless, ethicists seem unable to contain their curiosity when economists disregard Hume’s warning and survey samples of our fellow citizens on how they would value various outcomes in population health.
For example in one empirical implementation of Rawlsian political philosophy a sample of Swedes was asked to hypothetically place themselves behind a veil of ignorance and to make judgements on the merits of societies that varied in the distribution of life expectancy probabilities . In a now classic study, Peter Ubel and colleagues asked a sample of Philadelphians if they would prefer to spend a fixed amount of public funds on a targeted intervention that saved 1100 people or a universally applied intervention that saved 1000 people. Roughly half of the respondents (56% of the prospective jurors) preferred the universally applied intervention.
Ultimately the question of spending public money on equalizing access to care vs. maximizing the number of saved life years cannot be decided by a field survey. Empirical results have had a stimulatory effect on the amount of debate on these issues. In the past, public health planners could blithely engage in maximizing the number of saved statistical lives. Nowadays the equity-efficiency tradeoff is starting to sink in as a binding constraint for policy makers who used to just save lives.
What Social Scientists Can Learn from the Humanities
Interdisciplinary work is a two way street. As much as we might like to pump up what demographers can offer fellow academicians, we have plenty to learn.
Assistance with Narrative
Unlike members of the PAA, most of the world’s inhabitants do not communicate with each other using numbers. If we are to understand the life stories told by our research subjects we are going to have to understand narrative data better. A combined assault on a new problem must start by employing qualitative analysis. Unlike our computers we are able to encounter our subjects through eyes, ears, and noses (other senses require a rather lax Human Subjects Committee.) Economic theorists acknowledge the role of their own life experience and intuition in setting up models of the constraints facing economic beings.
Quality of life
In 1988, economist Amartya Sen and philosopher Martha Nussbaum convened a conference on the Quality of Life at the World Institute for Development Economics Research in Helsinki. The conference permitted a chance for economists and philosophers to cooperate in the critical development of more helpful welfare measures than the GNP/capita. Two years later, with Sen’s guidance the UNDP began its remarkable series of Human Development Reports offering world comparisons of a "Human Development Index" that incorporated indicators of life expectancy, education, and GDP. Ten years after his conference with the philosophers, Sen was awarded a Nobel prize for lifelong contributions to economics.
What Social Scientists can learn from the Natural Scientists
Accounting for tastes
Members of the Population Association of America study human populations. While the behavior of a whole (population) may not all be directly inferred from the sum of the parts (persons), reductionism has been so fruitful that we would be irresponsible not to consider the health and fertility behavior of individual persons. Once we admit persons as research subjects, we admit their reproductive biology and neurobiology and sociobiology to our discourse. Bongaarts model of the proximate determinants of fertility is overtly biological and fundamental to any discussion of fertility.
By the same token models of the proximate determinants of child mortality must ultimately appeal to human physiology . As social scientists, we may find the distal determinants of both fertility and mortality of more professional interest. But behavior comes from brains, and as my neuroanatomy professor was fond of pointing out, "The brain is the largest sex organ in the body". As much as Becker and other economists would like to account for human tastes and preferences as adapted to current regimes of price and income, that can never be the complete story . Because of natural selection, each of the 6 billion brains now responsible for demographic behavior was inherited from a lineage of primate ancestors who succeeded in the encounter of human tissue with terrestrial and social environment. For example, sugar does not create the mental sensation of "sweetness" because of socialization, or price, or income. Ancestors who sought the sweeter redder apples on the tree automatically derived higher calorie intakes and fitness than any mutant competitors who preferred the flavor of tree bark.
One field of inquiry in the natural sciences that is particularly ripe for harvest by demographers is the study of primate sexual behavior. Dixson’s systematic review of knowledge in this area makes it particularly accessible at this point in time. Readers will be astonished by the sheer variety of successful reproductive strategies that are adopted by creatures with similar biological requirements to our own. Of particular interest to demographers concerned with gender biases in health is evidence for primate regimes that favor preferential treatment of daughters in groups where female offspring inherit their mother’s ranks. In baboon and macaque groups, male offspring emigrate after attaining sexual maturity and derive little benefit from maternal rank. Daughters of high-ranking females retain a fitness advantage and the birth sex ratios of high-ranking rhesus macaques are observed to favor daughters. Conversely in groups such as spider monkeys where maternal rank is non-heritable and the returns to high maternal rank are derived chiefly through access to a high ranking sire it is beneficial for high ranking females to invest more in sons and rear them to peak physical condition. Successfully rearing an alpha male son who will sire an entire troop pays the highest dividends to the mother and the birth sex ratios for spider monkeys are observed to yield more sons.
Sociobiological Accounts of Social Structures
The relevance of primate dominance hierarchies may turn out to be relevant to the study of persistent SES differentials in human survival. The production of glucocorticoids and catecholamines has been linked to stressful encounters within a dominance hierarchy. Abundance of these two hormones can be linked to atherosclerosis, diabetes, and cancer. Baby rats who are licked more by their mothers mature to have a dampened adrenal response to stress . Mothers lick their infants more when they themselves are fondled and stroked by investigators. The full picture of the relationship between human social behavior and health is incomplete if it is left in the hands of neurophysiologists. By the same token it is incomplete if left only to social scientists.
Can Sociobiology Explain The Disharmony of the Academy?
Given the context of this essay one may speculate that the dominance hierarchies within academic groups and between academic groups and the territoriality of academic groups may be isomorphic to the social behavior in other primates. Emerging cooperation between ethologists and social scientists may shed light on disharmony and non-cooperation within the academy.
We live in an era in which the natural sciences enjoy tremendous success in the political, economic, and academic marketplace. A Platonic perspective on harmonic cooperation in a social group suggests that yielding too much ground to the play of one of the specialized parts of a group can damage the success of the whole group. The traditional call for greater cooperation between the sciences, the social sciences and the humanities is one that deserves to be repeated. Social scientists have much to contribute to both the natural sciences and humanities and much to gain from close cooperation. The biological, social, and human obstacles to greater cooperation between the disciplines may themselves be elucidated through collaborative work.
I started by setting out what I believe is the great unsolved problem for our age. I have presented several concrete examples of areas for research cooperation relevant to demographic and behavioral sciences that can address this problem. It may perhaps be more useful to consider some general principle that will be of more lasting value. I can do no better than Mahatma Gandhi who left us with a touchstone of enduring relevance.
"Apply the following test: Recall the face of the poorest and weakest man whom you may have seen and ask yourself if the step you contemplate is going to be of any use to him."
Andersson, F. and C. H. Lyttkens (1999). "Preferences for equity in health behind a veil of ignorance." Health Econ 8(5): 369-78.
Becker, G. (1996). Accounting for Tastes. Cambridge, MA, Harvard University Press.
Bongaarts, J. (1978). "A Framework for Analyzing the Proximate Determinants of Fertility." Population and Development Review 4: 105-127.
Currat, L. J., A. A. Hyder, et al. (1999). The 10/90 Report on Health Research 1999. Geneva, Global Forum for Health Research.
Dixson, A. (1998). Primate Sexuality. New York, Oxford University Press.
Easterlin, R. A. (2000). "The Worldwide Standard of Living Since 1800." Journal of Economic Perspectives 14(1): 7-26.
Fogel, R. W. (1999). "Catching Up with the Economy." American Economic Review 89(1): 1-21.
Frances, D., F. Champagne, et al. (1999). Maternal Care, Gene Expression, and the Development of Individual Differences in Stress Reactivity. Socioeconomic Status and Health in Industrial Nations. N. E. Adler, M. Marmot, B. S. McEwen and J. Stewart. New York, New York Academy of Sciences. 896: 66-84.
Hopkins, D. R. (1985). The Spread of Jenner's Vaccine. Universal Childhood Immunization by 1990. Unicef. New York, Unicef: 225-230.
MacLean, P. (1982). "A triangular brief on the evolution of brain and law." J. Social Biol. Struct. 5: 369-379.
McEwen, B. S. and T. Seeman (1999). Protective and Damaging Effects of Mediators of Stress: Elaborating and Testing the Concepts of Allostasis and Allostatic Load. Socioeconomic Status and Health in Industrial Nations. N. E. Adler, M. Marmot, B. S. McEwen and J. Stewart. New York, New York Academy of Sciences. 896: 30-47.
Miller, M. A. and L. McCann (2000). "Policy Analysis of the Use of Hepatitis B, Haemophilus influenzae Type b-, Streptococus pneumoniae-conjugate and Rotavirus Vaccines in National Immunization Schedules." Health Economics 9(1): 19-36.
Moore, H. D., M. Martin, et al. (1999). "No evidence for killer sperm or other selective interactions between human spermatozoa in ejaculates of different males in vitro." Proc R Soc Lond B Biol Sci 266(1436): 2343-50.
Mosley, W. H. and L. C. Chen (1984). "An Analytic Framework for the Study of Child Survival in Developing Countries." Population and Development Review 10(Supp.): 25-48.
Satcher, D. (1999). "Polio Eradication by the Year 2000." JAMA 281(3).
Sen, A. (1993). Capability and Well-being. The Quality of Life. M. Nussbaum and A. Sen. New York, Oxford Clarendon Press.
Sirageldin, I. (1999). Sustainable Human Development in the 21st Century: An Evolutionary Perspective. Cairo, Economic Research Forum for the Arab Countries, Iran, and Turkey.
Speth, J. G. (1999). "The Plight of the Poor." Foreign Affairs 78(3): 13-22.
Stebbins, G. L. (1982). Darwin to DNA, Molecules to Humanity. San Francisco, W.H. Freeman & Company.
Wagstaff, A. (1999). Socioeconomic Inequalities in Child Mortality: Comparisons Across Nine Developing Countries. Brighton, England, University of Sussex.
Weil, D. N. (1997). The Economics of Population Aging. Handbook of Population and Family Economics. M. Rosenzweig and O. Stark. New York, Elsevier: 967-1014.
WHO (1998). Coverage Rate With Measles Vaccine in Infants, WHO. 2000.
Wilmoth, J. R. and S. Horiuchi (1999). "Rectangularization Revisited: Variability of Age at Death Within Human Populations." Demography 36(4): 475-496.
Wilson, E. O. (1998). Consilience. New York, Knopf.