History is not often thought of as a science, but it can be if it uses the comparative method. Jared Diamond, professor of geography at the University of California, Los Angeles, and James A. Robinson, professor of government at Harvard University, employ the method effectively in the new book they have co-edited, Natural Experiments of History (Harvard University Press, 2010). In a timely study comparing Haiti with the Dominican Republic, for example, Diamond demonstrates that although both countries inhabit the same island, Hispaniola, because of geopolitical differences one ended up dirt poor while the other flourished.
Christopher Columbuss brother Bartolomeo colonized Hispaniola in 1496 for Spain, establishing the capital at Santo Domingo on the eastern side of the island. Two centuries later, during tensions between France and Spain, the Treaty of Ryswick in 1697 granted France dominion over the western half of the island. Because France was richer than Spain at this time and slavery was an integral part of its economy, it turned western Hispaniola into a center of slave trade with staggering differences in population: about 500,000 slaves in the western side of the island as compared with only 15,000 to 30,000 slaves in the eastern side.
That difference in population pressures, along with Frances hunger to import more timber from Haiti, magnified the influence of geographic factors. Weather fronts for Hispaniola come from the east and dump rain on the Dominican side of the island, leaving the Haitian side naturally drier and with less fertile soils for agricultural productivity. Haitis need for farmland and timber rapidly deforested the already sparse trees on its side of the island, with disastrous consequences: soil erosion, loss of timber for building and of wood for charcoal fuel, heavier sediment loads in rivers and decreased watershed protection that reduced the potential for hydroelectric power. This negative feedback cycle of environmental degradation for Haiti set it up for squalor.
When both the Haitians and Dominicans gained their independence in the 19th century, we see other comparative differences. Haitian slave revolts were violent, and Napoleons draconian intervention for restoring order resulted in the Haitians distrusting Europeans and eschewing future trade and investments, imports and exports, immigration and emigration. Haitian slaves had also developed their own Creole language spoken by no one else in the world, which further isolated Haiti from cultural and economic exchanges. Collectively, those barriers meant that Haiti did not benefit from factors that typically build capital, wealth and affluence and that might have led to prosperity under independence. In contrast, Dominican independence was relatively nonviolent; the country shuttled back and forth for decades between independence and control by Spain, which in 1865 decided that it no longer wanted the territory. Throughout this period the Dominicans spoke Spanish, developed exports, traded with European countries, and attracted European investors, as well as a diverse migr population of Germans, Italians, Lebanese and Austrians, who helped to build a vibrant economy.
Finally, even when both countries succumbed to the power of evil dictators in the mid-20th century, Rafael Trujillos control of the Dominican Republic involved considerable economic growth because of his desire to enrich himself personally, but his policies led to a strong export industry and imported scientists and foresters to help preserve the forests for his profiteering timber holdings. Meanwhile Haitis dictator Franois Papa Doc Duvalier did none of this and instead further isolated the Haitians from the rest of the world.
Diamond acknowledges that many other factors are involved in the long history of this island but that the comparative method, he writes, consists of comparing--preferably quantitatively and aided by statistical analyses--different systems that are similar in many respects but that differ with respect to the factors whose influence one wishes to study.
At the heart of all science is the isolation of a handful of powerful factors that account for the majority of the variance in what is being measured. Employing the comparative method with such natural experiments of history is no different from what sociologists and economists do in comparing natural experiments of society today. So it is time for scientists to respect history as a science and for historians to test their historical hypotheses by the comparative method and other techniques.