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

Many political theorists have linked liberal democracy to the rise of wealth and the establishment of a large middle class. "Growing resources are conducive to the rise of emancipative values that emphasize self-expression," write political scientists Ronald Inglehart of the University of Michigan and Christian Welzel of Jacobs University in their contribution to the 2009 book Democratization, "and these values are conducive to the collective actions that lead to democratization."

That same year, a group of researchers led by the Harvard economist Jeffrey Sachs noted in the Proceedings of the Royal Society B that a billion people live on less than a dollar per day and "are roughly as poor today as their ancestors were thousands of years ago." Sachs and his colleagues suggest that heavy disease burdens create persistent poverty traps from which poor people cannot extricate themselves. High disease rates lower their economic productivity so they can't afford to improve sanitation and medical care, which in turn leaves them vulnerable to more disease.

In a 2008 article for Biological Reviews, two University of New Mexico biologists buttressed the disease thesis with their "parasite hypothesis of democratization." The researchers, Randy Thornhill and Corey Fincher, argue that disease not only keeps people poor but makes them illiberal. Thornhill and Fincher tested 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 lower the disease burden, they found, the more likely a society is to be liberal.

Thornhill and Fincher argue that the risk of infectious disease affects elites' willingness to share power and resources, the general social acceptance of hierarchical authority, and the population's openness to 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 pathogens, groups of people 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," Thornhill and Fincher write, "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."

In another study, published in the Proceedings of the Royal Society B in June 2008, Thornhill and Fincher found that where disease prevalence has been historically high, cultures tend toward collectivist values such as ethnocentrism and conformity—because, they argue, these inward-looking cultural values inhibit the transmission of diseases. The pair examined prevalence data for 22 diseases, looking for correlations with various cultural values, including democratization, property rights, gender equality, and sexual liberalization. Where disease prevalence remains high, they found, autocracy reigns, property rights are weak, women have fewer rights, and sexual behavior is restricted.

It is well-known that disease prevalence falls the further one gets away from the equator. Hence it is not surprising, Thornhill and Fincher say, that the development of modern 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 was 32. (Britons and American colonists had more available calories per capita, which boosted their ability to fight off disease.)

Thornhill and Fincher believe that more recent advances in medicine and public health are implicated in the post-1950s wave of liberalization that swept over the United States and Western Europe. The advent of penicillin, the arrival of polio vaccines, the elimination of malaria, the chlorination of drinking water, and the reduction in food-borne illnesses all combined to dramatically reduce disease. The authors suggest that if people 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 such liberalization.

A new study, published in the May 27 issue of Science, lends a bit of additional support to Thornhill and Fincher's theory. Researchers led by University of Maryland psychologist Michele Gelfand looked 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)." Gelfand and her colleagues considered a wider range of possible threats, including not just disease prevalence but population density, resource scarcity, and territorial conflicts. They found that adversity correlates with higher levels of social conformity, autocratic rule, religiousness, and controls on the media. Of the 33 countries in Gelfand's survey, Pakistan scored highest on tightness (12.3 points); the loosest was Ukraine (1.6 points). The United States scored a pretty loose 5.1 points.

If Sachs, Thornhill, Fincher, and Gelfand are right, reducing a country's disease burdens should promote the rise of liberal institutions. "If the parasite hypothesis of democratization is supported by additional research," Thornhill and Fincher write, "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, the ethnocentrism that may have emerged as a protection against diseases sometimes gets in the way of eradicating health threats. In 2003, for example, the Global Polio Initiative's vaccination campaign was derailed by a boycott in northern Nigeria after some Muslim religious and political leaders endorsed rumors that oral polio vaccine was an American conspiracy to spread HIV and cause infertility. During the boycott, polio became resurgent and spread to 15 other countries. Polio still has not been eradicated globally.

In any event, as life expectancy across the globe has increased, liberal institutions have spread. The human rights group Freedom House reports that since 1972 the percentage of free countries has risen from 29 percent to 45 percent. During that same time, average global life expectancy has risen from 58 to 70 years. If these studies are right, they bode well for the future of humanity. Biomedical and sanitation innovations developed by countries that are already relatively rich and free likely will continue to spill over to poor autocratic countries, setting off a virtuous circle in which health produces wealth, which eventually promotes liberty.

Science Correspondent Ronald Bailey is the author of Liberation Biology (Prometheus).