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demography as the Human story

shripad TuLjapurkar

the human story is a grand one, from the long sweep out of africa across the globe over many tens of millennia, to the ebb and flow of peoples and nations over the past millennium. this story, seen from many scales of space and time, is now a hot topic of research, driven in part by genetic studies of human ancestry and by biological and anthropological studies of environments and climate. demography has played a supporting role in the study of human history, but relatively few demographers have chosen to view history broadly through a demographic lens. i find this surprising, since the human story is at its core a story of vital events—birth, death, migration—and economic and sociocultural change, as both malthus and darwin recognized. who could do better than demographers in exploring that story? among the demographers who have looked broadly at history, some have developed powerful concep-tual frameworks that integrate processes of change, for example Ronald lee’s synthesis of malthusian and Boserupian processes. others have explored and illuminated one or another dramatic period of history using the analytical tools of modern demography. this is what massimo livi Bacci (2008, 2010) has done in his two books on the destruction of the american indios.

i find these books fascinating for several reasons. First, for their effec-tive use of original historical materials to add texture and (literally) color to describe the lives of the indios and make them tangible actors in a story that most of us (certainly i) knew only in dim outline. Second, as a splendid display of how formal demography and demographic methods can take us far beyond those appealing but uncertain suspects, “guns, germs, and steel.” His instruc-tive analyses include a discussion of the probability of smallpox transmission between continents and its likely population consequences, and his evalua-tion of whether changes in fertility, family, or mortality could in themselves have led to population decline. the third reason i enjoyed these books is that the subject relates to my own interest in the study of the Polynesian and particularly the Hawaiian islands. Historical studies of these islands grapple with many of the same questions that occupy livi Bacci in these books: what was the trajectory of population and social change in the early history of

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the islands, what was their state before european contact, what drove their populations to follow divergent trajectories after contact? and finally, the discussion and analyses in these books highlight limitations of standard de-mographic models and methods when applied to human history. while these books make clear that demographic analysis has considerable power in the study of history, the study of long-term human change requires new demo-graphic tools. Some of these have been developed but are as yet little known to demographers; others will require development and application.

i begin my discussion by describing briefly the Polynesian, especially the Hawaiian, contact experience and the broader parallels between the story of the indios and other contact stories around the world. i then show that Hawai’i illustrates a range of historical questions that we would like to answer about early human populations and about the challenges the experience of these populations poses for demographic theory. i then outline an approach to the formal demography of prehistory and discuss its application to Hawai’i and its potential for broader application. i close by discussing some of the challenges that remain.

Polynesia is a geographical region that includes many islands spread over the eastern Pacific, including tahiti, Hawai’i, easter island, and new Zealand. the islands are far-flung and their peoples encompass considerable linguistic and cultural diversity. archaeological research suggests that the Hawaiian islands were settled by Polynesian voyagers about 1,200 years ago and then appear to have developed largely in isolation from the other islands. european contact traces back to the late 1700s, nearly three centuries after Columbus’s voyages to the americas. in the case of Hawai’i and many other islands, the first celebrated contacts occurred during the voyages of Captain James Cook. the Spanish themselves attempted an exploration of the Solomon islands with the ill-fated voyage of Álvaro de mendaña de neira from Peru in 1595 but appear to have made no further serious efforts. after contact, many Polynesian islands suffered significant demographic decline (Kirch and Rallu 2007). archaeologists and anthropologists, among others, have long studied and argued about the extent, causes, timing, and broader socioeconomic ef-fects of contact. their discussions revolve around many of the same questions raised by livi Bacci in his discussion of the fate of the indios. these questions about contact are of great interest to many native peoples, including north american native populations in the united States and Canada, and the maoris of new Zealand and australia.

Step back a bit from questions about contact and its consequences, and you realize that what we seek to understand is the demographic history of the pre-contact period. the demographic, social, cultural, and economic milieu that determined the effects of contact was formed in long periods before such interaction. these time periods ranged from several centuries in the case of Hawai’i to millennia in the americas.

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when Cook arrived in 1778–79, Hawai’i was occupied by an isolated population of perhaps 450,000 indigenous Polynesians, with economies based on irrigated and dryland farming, aquaculture, and animal husbandry (for these and other details see Kirch 1985, 1994, and references therein). the size of the colonizing human population was small, probably fewer than 200 persons (periodically, but modestly, supplemented by additional voyag-ers). over the next thousand years, this population achieved a high density (around 150–250 per square km in some areas) supported by a remarkable mosaic of intensified agriculture. this included intensive irrigation on the older volcanic land and intensive dryland farming on the younger and more nutrient-rich land.

over this same time span, Hawaiian society underwent dramatic chang-es. the colonizing ancestral Polynesian society was “house-based,” with a fluid kin structure and heterarchical competition between local groups, each associated with an estate of agricultural land. Between 400 and 600 years later, Hawaiian society was characterized by intensive agriculture, aquacul-ture, and craft specialization; ritualized controls on production; a hierarchical temple system; endogamous classes with divine kings at the apex of society; replacement of a lineage-based system of land control with a territorial system controlled by elites; and a formal system of corvée labor and surplus tribute extraction. at european contact, the Hawaiian islands were divided into four competing, incipient archaic states, centered on the major islands of Hawai’i, maui, o’ahu, and Kaua’i.

Several processes shaped the rise of this complex late-precontact society. these include population change; agricultural intensification, including irri-gated and dryland systems; warfare; and the emergence of competition among chiefly lineages. there are many ways to describe the changes in precontact Hawai’i, including narratives based on the emergence of social hierarchies, the analysis of dated archaeological remains, and ethnohistory. But to under-stand how populations grew and spread and how they gave rise to social and cultural structures, we must take a demographic view. How do we describe demographic change and the evolution of society over this millennium?

the first task is to ask how population dynamics, in particular fertility and mortality, depend on resources. this is an old problem, the same one that occupied malthus. However, demographers have a somewhat schizophrenic view of this question. demographic work on mortality and fertility largely looks at vital rates—mortality and fertility—as period phenomena, and studies patterns, models, and change. the theory of stationary populations is useful for populations that are somehow held at constant numbers, but is of little help in thinking about dynamics. demographers have discussed the impor-tance of covariation between vital rates and population numbers and com-position. that covariation, after all, is the source of the feedbacks that affect population change and that are of great historical interest. But there has been

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little work on the nature and effects of density-dependence. in contemporary populations, demographers have examined the effects of population numbers on economic growth, but the results are ambiguous and provide few useful leads for the student of history.

Happily, useful insights are found in work by historical demographers who have studied the relationship between vital rates and food (or proxies for food) in pre-industrial societies (Bengtsson, Campbell, and lee 2004; lee and Steckel 2006). this work suggests that there are fairly general relationships between food availability—what i will simply call hunger—and both fertil-ity and mortality. Separately, a formal description of how labor produces (or gathers) food and how food is distributed can be based on Ronald lee’s (2003) model of population dynamics with transfers. Combining these, we have a dynamic model in which the working segment of a population produces food subject to constraints (including land, irrigation, nutrients, and so on), con-sumption is shaped by rules about sharing, and the prevailing level of hunger sets mortality and fertility rates. this model (lee, tuljapurkar, and vitousek 2008; lee, Puleston, and tuljapurkar 2009), coupled to biologically-based models of agricultural production, provides a basic tool for understanding population dynamics in human history. i now consider a range of questions that we can study with this approach.

one commonly asked question concerns “carrying capacity,” an ill-defined measure that is usually taken to mean a steady state of population numbers. livi Bacci makes intelligent use of this concept when he discusses estimates of the pre-contact populations of Hispaniola (in Conquest) and of the mojos (in El Dorado). these estimates use geography, estimated food pro-duction, and estimated need to derive total population numbers that were in some sense supportable.

we can do much better with the new models mentioned above. agri-cultural production models can be fed with ecological data on nutrient flows and climate to estimate fairly accurately the productivity of land, given crop choices and agricultural practices (e.g., slash-and-burn, or continuous culti-vation). lee and tuljapurkar (2006) and ladefoged, lee, and Graves (2008) show how this can be done. it is now fairly straightforward to collect the necessary data on soil types, nutrient fluxes, plant production, topography, and so on. it is not inexpensive work, however. there can also be a problem with finding the relevant climate data, data that are only rarely available at the locations of interest. But extensive work has been done on climate his-tory and on spatial correlations of climate, so it is often possible to “borrow” climate history from some nearby location. in addition, good (or at least plausible) climate models can be used to simulate representative climate histories for most of the world. Such work yields fairly robust estimates of production potential. But we still need to link this information to actual production and population.

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the power of the demographic models mentioned above is derived from combining production potential with demography, environment, and technology—the size of the labor force, the number of hours worked, the choice of technology (e.g., did people use mulching to increase nutrient in-take in their fields?), the extent of available area, climate variation in time and space—to generate both population trajectories and the likely range of population size and composition. the models also tell us about population trajectories, both during expansions into new lands and when populations were constrained by space. as with all demographic models, they describe the welfare of the population in terms of fertility rates, mortality of infants and children, life expectancy, and so on. Finally, they tell us about the marginal product of labor, usually low when a population is near steady state, and this in turn tells us about the extent of extractable labor and the effects of taxation. all of these are questions that livi Bacci considers in one form or other, but he has to analyze them separately and then piece things together.

the range and synthetic power of these models is novel and valuable, and i hope to see the methods come into wider use over time. But many challenges remain. Perhaps the central one is to model and analyze the evo-lution of social complexity and its effects on social behavior. in Hawai’i, we would like to understand the evolution of hierarchical societies, their effects on inheritance and kinship, and whether and how they influenced fertility and mortality. these problems are closely related to the problems of describ-ing interactions (including warfare) between individuals and groups, and the relationship between these interactions and the distribution and spread of populations across space. there are as yet few useful theories for the evolu-tion of social structure, even fewer that can be reasonably incorporated into quantitative models. evolutionary, especially game theoretic, models suggest tempting directions to investigate, but such models ignore demography com-pletely and hence also ignore social structure and constraints. demographers can find rich problems to explore in such topics, and i hope we will find some hardy souls who will help us to follow massimo livi Bacci along more journeys into our past.

Note

i would like to thank Pat Kirch, Charlotte lee, Cedric Puleston, and Peter vitousek for helping to educate me on the subjects discussed here.

References

Bengtsson, t., C. Campbell, and J. Z. lee (eds.). 2004. Life under Pressure: Mortality and Living Standards in Europe and Asia, 1700–1900. Cambridge, ma: mit Press.

Kirch, P. 1985. Feathered Gods and Fishhooks: An Introduction to Hawaiian Archaeology and Prehis-tory. Honolulu: university of Hawaii Press.

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———. 1994. The Wet and the Dry: Irrigation and Agricultural Intensification in Polynesia. university of Chicago Press.

Kirch, P. v. and J-l. Rallu. 2007. The Growth and Collapse of Pacific Island Societies: Archaeological and Demographic Perspectives. university of Hawaii Press.

ladefoged, t. n., C. t. lee, and m. w. Graves. 2008. “modeling life expectancy and surplus production of dynamic pre-contact territories in leeward Kohala, Hawai’i,” Journal of Anthropological Archaeology 27: 93–110.

lee, C. and S. tuljapurkar. 2006. “Risky business: temporal and spatial variation in preindus-trial Pacific dryland agriculture,” Human Ecology 34(6): 739–763.

lee, C., S. tuljapurkar, and P. vitousek. 2008. “Population and prehistory i: Food-dependent population growth in constant environments,” Theoretical Population Biology 73:473–482.

lee, C. t., C. Puleston, and S. tuljapurkar. 2009. “Population and prehistory iii: Food-depen-dent demography in variable environments,” Theoretical Population Biology 76: 179–188.

lee, R. 2003. “Rethinking the evolutionary theory of aging: transfers, not births, shape senes-cence in social species,” Proceedings of the National Academy of Sciences of the United States of America 100: 9637.

lee, R. and R. H. Steckel. 2006. “life under pressure: an appreciation and appraisal” (review of t. Bengtsson et al. (eds.), Life under Pressure: Mortality and Living Standards in Europe and Asia, 1700–1900), Historical Methods 39(4): 171–176.

livi Bacci, massimo. 2008. Conquest: The Destruction of the American Indios. translated from italian by Carl ipsen. Cambridge, uK and malden, ma: Polity Press.

———. 2010. El Dorado in the Marshes: Gold, Slaves and Souls Between the Andes and the Amazon. translated from italian by Carl ipsen. Cambridge, uK and malden, ma: Polity Press.

Puleston, C. and S. tuljapurkar. 2008. “Population and prehistory ii: Space-limited human populations in constant environments,” Theoretical Population Biology 74: 147–160.

tuljapurkar, S., C. lee, and m. Figgs. 2007. “demography and food in early Polynesia,” in P. v. Kirch and J-l. Rallu (eds.), The Growth and Collapse of Pacific Island Societies. university of Hawaii Press, pp. 35–51.