Greater wealth strongly correlates with property rights, the rule of law, more education, the liberation of women, a free press, and more social tolerance. The enduring puzzle for political scientists is how do the social processes that produce freedom and wealth get started in the first place?

Many political theorists have associated democracy with the rise of wealth and the establishment of a large middle class. As Ronald Inglehart, a political scientist at the University of Michigan, and Christian Welzel, a political scientist at Jacobs University in Bremen, Germany explain in the 2009 edited volume Democratization, "Growing resources are conducive to the rise of emancipative values that emphasize self-expression; and these values are conducive to the collective actions that lead to democratization."

A group of researchers led by Harvard University economist Jeffrey Sachs recently noted that a billion people live on less than a dollar per day [PDF] and "are roughly as poor today as their ancestors were thousands of years ago." Why? The researchers suggest that high disease burdens create persistent poverty traps from which poor people cannot extricate themselves. High disease incidence lowers their economic productivity so that they can't afford to create the resources needed to improve sanitation and medical care, which in turn leaves them vulnerable to more disease. And so it goes.

University of New Mexico anthropologists Randy Thornhill and Corey Fincher pushed the disease thesis further with their "parasite hypothesis of democratization," arguing that disease not only keeps people poor, but it also makes them illiberal. The two researchers test this hypothesis "using publicly available data measuring democratization, collectivism–individualism, gender egalitarianism, property rights, sexual restrictiveness, and parasite prevalence across many countries of the world." The idea is that the lower the disease burden, the more likely a society is to be liberal.

Thornhill and Fincher argue that the risk of infectious disease affects the willingness of elites to share power and resources, the general social acceptance of hierarchal authority, and the openness of innovation. Their central idea is that ethnocentrism and out-group avoidance function as a kind of behavioral immune system. Just as individuals have immune systems that fight against pathogens, groups of people similarly evolve with local parasites and develop some resistance to them. People who are not members of one's group may carry new diseases to which the group has not developed defenses. "Thus, xenophobia, as a defensive adaptation against parasites to which there is an absence of local adaptation, is expected to be most pronounced in regions of high parasite stress," assert Thornhill and Fincher. In another study, they find that where disease prevalence has been historically high, cultures tend toward collectivist values such as ethnocentrism and conformity. Why? Because these inward-looking cultural values inhibit the transmission of diseases.

Using prevalence data for 22 diseases, the researchers find a correlation with a number of cultural values, including democratization, property rights, gender equality, and sexual liberalization. Where disease prevalence remains high, autocracy reigns, property rights are weak, women have fewer rights, and sexual behavior is restricted. Disease prevalence lessens the further one gets away from the equator. Thus, Thornhill and Fincher argue that it is not surprising that the development of democratic institutions began in high latitude Western Europe and North America. In 1820, Britain's average life expectancy of 40 years was the highest in Europe; France's was 37 years and Germany's 32 years. (Britons and American colonists had more available calories per capita which also boosted their ability to fight off disease.)

Furthermore, Thornhill and Fincher assert that more recent advances in medicine and public health are similarly implicated in the post-1950s wave of liberalization that swept over the United States and Western Europe. The advent of penicillin, polio vaccines, the elimination of malaria, chlorination of drinking water, and the reduction in foodborne illnesses all combined to dramatically reduce disease prevalence. The authors suggest that if people actually experience few infections as they grow up, they perceive strangers and novel ways of life as safe. Tolerance and the embrace of social, economic, and technological innovation follow. They note that areas of the world in which disease rates remain high have not experienced this trend toward liberalization.

A new study led by University of Maryland psychologist Michele Gelfand published last week in Science looks at the "differences between cultures that are tight (have many strong norms and a low tolerance for deviant behavior) versus loose (have weak social norms and a high tolerance for deviant behavior)." In this case, Gelfand and her colleagues consider a wider number of possible threats including disease prevalence, but also population density, resource scarcity, and territorial conflicts. Again, adversity correlates with higher levels of social conformity, autocratic rule, religiousness, and media control. Of the 33 countries in Gelfand's survey, Pakistan scored highest on tightness (12.3 points) while the least tight country was Ukraine (1.6 point). The United States scored a pretty loose 5.1 points.

Disease causes autocracy, in turn causing poverty, resulting in more disease, producing continued autocracy, and so on. However, if Sachs, Thornhill, Fincher, and even to some extent Gelfand is right, then reducing disease burdens in a country would promote the rise of liberal institutions. In fact, Fincher and Thornhill explicitly conclude, "If the parasite hypothesis of democratization is supported by additional research, humanitarian efforts to reduce human rights violations and to increase human liberties and democracy in general will be most effective if focused on the most fundamental causal level of infectious disease reduction." Unfortunately, ignorant ethnocentrism has gotten in the way of eradicating diseases like polio.

Nevertheless, as life expectancy across the globe has increased, liberal institutions have spread. The nonprofit Freedom House reports that since 1972 the percentage of free countries has risen from 29 percent to its current rate of 45 percent. During that same time average global life expectancy has risen from 58 years to 70 years. If these studies are right, they bode well for the future of humanity. Biomedical and sanitation innovations developed by already-free and rich countries will likely continue to spill over to poor autocratic countries sparking in them the virtuous circle of health producing wealth, which eventually promotes liberty.

Ronald Bailey is Reason's science correspondent. His book Liberation Biology: The Scientific and Moral Case for the Biotech Revolution is now available from Prometheus Books.