historians can formulate only statistical trends. With a very high probability of being correct, I can predict that, of the next 1,000 babies born at the University of California Medical Center, where I work, not fewer than 480 or more than 520 will be boys. But I had no means of knowing in advance that my own two children would be boys. Similarly, historians note that tribal societies may have been more likely to develop into chief-doms if the local population was sufficiently large and dense and if there was potential for surplus food production than if that was not the case. But each such local population has its own unique features, with the result that chiefdoms did emerge in the highlands of Mexico, Guatemala, Peru, and Madagascar, but not in those of New Guinea or Guadalcanal. Still another way of describing the complexity and unpredictability of historical systems, despite their ultimate determinacy, is to note that long chains of causation may separate final effects from ultimate causes lying outside the domain of that field of science. For example, the dinosaurs may have been exterminated by the impact of an asteroid whose orbit was completely determined by the laws of classical mechanics. But if there had been any paleontologists living 67 million years ago, they could not have predicted the dinosaurs' imminent demise, because asteroids belong to a field of science otherwise remote from dinosaur biology. Similarly, the Lit- 4 Z 4 ' EPILOGUE tle Ice Age of a.d. 1300-1500 contributed to the extinction of the Greenland Norse, but no historian, and probably not even a modern climatoiogist, could have predicted the Little Ice Age. Thus, the difficulties historians face in establishing cause-and- effect relations in the history of human societies are broadly similar to the difficulties facing astronomers, climatologists, ecologists, evolutionary biologists, geologists, and paleontologists. To varying degrees, each of these fields is plagued by the impossibility of performing replicated, controlled experimental interventions, the complexity arising from enormous numbers of variables, the resulting uniqueness of each system, the consequent impossibility of formulating universal laws, and the difficulties of predicting emergent properties and future behavior. Prediction in history, as in other historical sciences, is most feasible on large spatial scales and over long times, when the unique features of millions of small-scale brief events become averaged out. Just as I could predict the sex ratio of the next 1,000 newborns but not the sexes of my own two children, the historian can recognize factors that made inevitable the broad outcome of the collision between American and Eurasian societies after 13,000 years of separate developments, but not the outcome of the 1960 U.S. presidential election. The details of which candidate said what during a single televised debate in October 1960 could have given the electoral victory to Nixon instead of to Kennedy, but no details of who said what could have blocked the European conquest of Native Americans. How can students of human history profit from the experience of scientists in other historical sciences? A methodology that has proved useful involves the comparative method and so-called natural experiments. While neither astronomers studying galaxy formation nor human historians can manipulate their systems in controlled laboratory experiments, they both can take advantage of natural experiments, by comparing systems differing in the presence or absence (or in the strong or weak effect) of some putative causative factor. For example, epidemiologists, forbidden to feed large amounts of salt to people experimentally, have still been able to identify effects of high salt intake by comparing groups of humans who already differ greatly in their salt intake; and cultural anthropologists, unable to provide human groups experimentally with varying resource abundances for many centuries, still study long-term effects of resource abundance of THEFUTURE OF HUMAN HISTORY AS A SCIENCE • 415 human societies by comparing recent Polynesian populations living on islands differing naturally in resource abundance. The student of human history can draw on many more natural experiments than just comparisons among the five inhabited continents. Comparisons can also utilize large islands that have developed complex societies in a considerable degree of isolation (such as Japan, Madagascar, Native American Hispan-iola New Guinea, Hawaii, and many others), as well as societies on hundreds of smaller islands and regional societies within each of the continents. Natural experiments in any field, whether in ecology or human history, are inherently open to potential methodological criticisms. Those include confounding effects of natural variation in additional variables besides the one of interest, as well as problems in inferring chains of causation from observed correlations between variables. Such methodological problems have been discussed in great detail for some of the historical sciences. In particular, epidemiology, the science of drawing inferences about human diseases by comparing groups of people (often by retrospective historical studies), has for a long time successfully employed formalized procedures for dealing with problems similar to those facing historians of human societies. Ecologists have also devoted much attention to the problems of natural experiments, a methodology to which they must resort in many cases where direct experimental interventions to manipulate relevant ecological variables would be immoral, illegal, or impossible. Evolutionary biologists have recently been developing ever more sophisticated methods for drawing conclusions from comparisons of different plants and animals of known evolutionary histories. In short, I acknowledge that it is much more difficult to understand human history than to understand problems in fields of science where history is unimportant and where fewer individual variables operate. Nevertheless, successful methodologies for analyzing historical problems have been worked out in several fields. As a result, the histories of dinosaurs, nebulas, and glaciers are generally acknowledged to belong to fields of science rather than to the humanities. But introspection gives us far more insight into the ways of other humans than into those of dinosaurs. I am thus optimistic that historical studies of human societies can be pursued as scientifically as studies of dinosaurs—and with profit to our own society ay> by teaching us what shaped the modern world, and what might shape our future. acknowledgments It IS A PLEASURE FOR ME TO ACKNOWLEDGE THE CONTRIBUtions of many people to this book. My teachers at Roxbury Latin School introduced me to the fascination of history. My great debt to my many New Guinea friends will be obvious from the frequency with which I cite their experiences. I owe an equally great debt (and absolution from responsibility for my errors) to my many scientist friends and professional colleagues, who patiently explained the subtleties of their subjects and read my drafts. In particular, Peter Bellwood, Kent Flannery, Patrick Kirch, and my wife, Marie Cohen, read the whole manuscript, and Charles Heiser, Jr., David Keightley, Bruce Smith, Richard Yarnell, and Daniel Zohary each read several chapters. Earlier versions of several of the chapters appeared as articles in Discover magazine and in Natural History magazine. The National Geographic Society, World Wildlife Fund, and University of California at Los Angeles supported my fieldwork on Pacific islands. I have been fortunate to have John Brockman and Katinka Matson as my agents, Lori Iversen and Lori Rosen as my research assistants and secretaries, Ellen Modecki as my illustrator, and as my editors Donald Lamm at W. W. Norton, Neil Belton and Will Sulkin at Jonathan Cape, Willi Kohler at Fischer, Marc Zabludoff and Mark Wheeler and Polly Shulman at Discover, and Ellen Goldensohn and Alan Ternes at NaturalHistory. FURTHER READINGS THESE SUGGESTIONS ARE FOR THOSE INTERESTED IN READing further. Hence, in addition to key books and papers, I have favored references that provide comprehensive listings of the earlier literature. A journal title (in italics) is followed by the volume number, followed after a colon by the first and last page numbers, and then the year of publication in parentheses. Prologue Among references relevant to most chapters of this book is an enormous compendium of human gene frequencies entitled The History and
