In Why the West Rules–for Now, the historian and classical archeologist Ian Morris makes a significant contribution to the great divergence debate by reinterpreting over 16,000 years of history and prehistory. In a book studded with provocative counterfactuals and written in an engaging style, Morris offers readers a detailed comparison of East and West, one based on an index of social development that he has constructed from 14,000 BCE to the present. Although not everyone will agree with his conclusions, his book will have to be taken seriously by anyone interested in the great divergence, in long-run social change, and in the workings of premodern states.

For Morris, the East refers to all the societies that descend from the first neolithic revolution in East Asia, which began in about 7,500 BCE in the area between China's Yellow and Yangzi Rivers. Today the resulting societies stretch from Indochina to Japan, but Morris's focus is upon China itself, and southern Asia is left out. As for the West, it too is a broad label for Morris, because it encompasses all the societies that are the heirs to an even earlier neolithic revolution, some two millenia further back into the past, in the Middle East. Over the centuries, West's core has shifted from the Middle East to the Mediterranean, and then to Europe, and finally on to European settlements in Australia and North America, which are the leading western societies today. But for Morris, the West continues to include the Islamic Middle East, and his ideas are therefore incompatible with arguments (such as Samuel Huntington's) that would define civilization in terms of culture or posit an enduring enmity between Islam and the Occident.

The index Morris devises to compare East and West combines measures of urbanization, military capability, information technology, and per capita energy capture. Each of these four traits is measured relative to the maximum attained in the year 2000. For urbanization, for example, the yardstick is the size of the largest city in the East or the West at any given time. In the year 2000, the biggest eastern city was Tokyo, with 26.7 million inhabitants, while the biggest western one was New York, with a population of 16.7 million. The East, with Tokyo, therefore gets the maximum urbanization score of 250 in the year 2000, while the West for that year has to make do with an urbanization rating of 250*16.7/26.7, or 156.37. One hundred years earlier, the world's biggest city was western—London, with 6.6 million—which gives the West an urbanization score of 61.80 in 1900, while the East that year gets only 16.39, for East Tokyo with its 1.75 million residents. Military capability is based on a subjective index of war making capacity, and information technology on literacy rates and estimates of the speed of communication. Energy capture is simply per capita energy consumption.

Adding the four measures (with equal weights) gives Morris an index of social development for the East and for the West. According to his index, the West took the lead in social development initially, because it had more easily domesticated plants and animals than the East and therefore went through the Neolithic Revolution first. But by the middle of the sixth century CE, the East had caught the West and passed it by. It did not surrender the lead until the late eighteenth century CE, and while the West is still ahead today, it could fall behind again—so Morris suggests—this time to a resurgent China.

Morris's chronology is unlikely to ruffle many feathers, for many of the scholars engaged in the divergence debate share his belief that the East (or at least China) was ahead of the European West during the Middle Ages and the early modern period. But it is worth pointing out that this received wisdom is hardly set in stone. Morris himself admits that the source of much of his information—quantitative archeological data—is often lacking for China, and the sort of indexes that economic historians might prefer, such as real wage rates, have not yet been pushed back in systematic way before the eighteenth century in east Asia. The same holds—despite Angus Maddison's research—for rigorous international comparisons of per capita GDP.

Although Morris's index does not sketch a radically different chronology of the comparative development of the East and the West, it does yield some important insights into the divergence debate. For Morris, the index argues against the West's having any sort of inherent advantage (whether cultural, political, or material) guaranteeing it will always be the leader in social development. The same holds for the East too. Although the West did get a head start thanks to an early agricultural revolution, its lead was not insurmountable, and neither was the lead that the East had in the Middle Ages. Morris is also skeptical of assertions about the virtues of western culture, and his criticisms of such claims are particularly persuasive when the issue is the culture of ancient Greece, no doubt because he himself is a specialist in Greek history and archeology.

Morris's methodological tools do not come from economics—he draws instead upon evolutionary biology, sociology, and environmental geography—but much of his book is nonetheless compatible with economic reasoning. Nonetheless, an economist reader is still likely to balk (or even to object strenuously) at various points in the text. Morris's index will raise the most hackles. Urbanization may seem a reasonable thing to measure, because it is often used as a proxy for per capita income, but why should it be combined (using arbitrary equal weights) with military capability and what amounts to literacy and energy use? In many cases, all these traits are liked to be correlated with per capita income. Why then not just try to estimate per capita GDP?

And what would happen if the correlation is imperfect? Relative prices or institutions might push some societies to specialize, giving them a high score for one trait, but a low one for another. High energy prices, for instance, might diminish the energy score and even Morris's whole index, given his weighting scheme; does that mean the society as a whole has regressed? Similarly, a saber-rattling dictator might spend everything on the military so that he could prey upon more peaceful neighbors. North Korea is an obvious modern example of such a specialist in violence, but there similar examples (eighteenth-century Prussia, Napoleonic France, the Soviet Union) in the past. Does that mean North Korea is “more advanced” than South Korea, or at least not as far behind its southern neighbor as we might think? Finally, can we really combine the highest scores for the traits in various eastern societies to get a composite score for the entire East? And can we do the same for the West? Or are the very terms East and West now almost meaningless in a world where information travels fast and where useful knowledge and human capital can compensate for geographical endowments? Environment and geographical endowments are stressed throughout Morris's book, but their importance (great though it may still be in an age of global warming) is no doubt less that it was three hundred years ago, thanks in large part to our greater stock of human capital and useful knowledge. If so, then Morris's index may well need to be reworked for the final three centuries he studies. But the same could be said for many other innovative works of social science history: they too have driven critics to go to work.