Mosquitoes, malaria and the spread of slavery in the US

In 1641 Massachusetts legalized slavery, the first North American colony to do so. Slavery was to persist in the colonies and territories, and then the United States for another two centuries, until formally abolished by the Thirteenth Amendment in 1865. Numerous theories have been advanced about why the US, a nation that values individual liberty so highly, introduced and institutionalized the practice of slavery. And why the slavery of African slaves in the southern states, in particular, became the prevalent form of slavery. Now, Elena Esposito publishes new research showing how the spread of malaria in North America may explain the pattern of growth of slavery in the US.

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Elena Esposito is an applied economist with research interests in the fields of development economics, economic growth, political economy, and economic history.

The establishment, spread, and persistence of slavery in the USA, is a period of American history that has been the cause of much debate and controversy. Slavery is such an emotive topic that it is difficult to discuss the US slave trade dispassionately. Yet such important periods of world history cannot and should not be ignored. It falls upon academics to undertake the challenging task of analyzing these moments in time, viewing the issues through the lens of their particular expertise, and using robust evidence to support their arguments.

It is in this empirical academic tradition that Elena Esposito, assistant professor at HEC Lausanne, after noting the ruinous legacy of slavery – the deep seated political and economic inequalities, fractionalization and distrust – examines a relatively overlooked factor in the establishment, geographical location, and nature of slavery in the US: the role of malaria.

Esposito examines the rationale behind three key aspects of slavery in the US: why slavery grew and persisted predominately in the South; why its codification and institutionalization grew substantially towards the end of the seventeenth century; and, crucially, why it involved Africans, and certain African populations in particular, rather than other sources of slave labor.

There has long been a debate about the causes of slavery. Whether settlers to North America brought the institution of slavery with them, via persistent, ingrained attitudes, for example. Or alternatively, if the distribution of slavery in the colonies and then the US might be the product of geographical factors that the settlers adapted to, such as climate and the suitability of soil types for labor intensive crops. However, while not discounting the influence of such factors, Esposito asserts that one single geographical disease factor – malaria – provides a stronger explanation for all three aspects of slavery.

“Esposito asserts that one single geographical disease factor – malaria – provides a stronger explanation for all three aspects of slavery.”

Esposito argues that where conditions were most favorable for the rise of malaria, this led to labor scarcity, and therefore increased costs of labor. As such this created a set of economic conditions that encouraged slavery. At the same time it led to a high demand for malaria-resistant labor. Populations that are exposed to malaria over time can build up innate resistance to the effects of the disease. These people, although still affected by malaria, may not suffer from the most debilitating and lethal aspects of the illness. Compared with Native Americans and Europeans, Africans had a significantly higher resistance to malaria, and that was especially true of those Africans from Sub-Saharan malaria-infested areas of Africa.

Esposito shows that, at a time when slavery was considered politically acceptable and economically logical, the incidence of malaria in the southern states led to an increase in demand for malaria-resistant African slaves. In just a few decades the African workforce in these areas went from a small minority to making up one third of the population, and slavery became socially institutionalized.

Other commentators have speculated on the role of malaria and other diseases in the establishment of slavery in the more southern states. However, Esposito goes further towards providing substantial and convincing evidence for a link. Using census data for 1790, 1860, and when only considering slave states, she shows a strong positive correlation between malaria incidence and the share of African slaves across US counties within states. Although precise and complete data on malaria incidence is lacking, Esposito uses indexes of predicted malaria transmission, constructed based on mosquito biology and long-term climate, to reliably predict the risk of being infected with malaria for a particular county.

In addition, Esposito specifically looks at difference in intensification of slavery, before and after the introduction of the most deadly and virulent malaria species – falciparum malaria (untreated mortality rates of falciparum malaria range between 20% and 40%). The data shows that, following the arrival of falciparum malaria in the 1680s, the colonies where conditions were more favorable to endemic malaria also showed the greatest rise in slavery – regardless of the types of crops grown. In all, the introduction of falciparum malaria explains about 75% of the dramatic rise in the proportion of African slaves in the US south after 1690.

“The records show that higher prices were paid for Africans from more malaria-ridden regions where malarial resistance was greatest…”

Esposito also demonstrates the link between malaria and slavery using historical data from the official Louisiana slave records for 1719 to 1820. The records contain details for over 3000 slaves, including prices paid and birth place. The records show that higher prices were paid for Africans from more malaria-ridden regions where malarial resistance was greatest, results that remain unaffected even when controlling for a variety of other factors such as the size of the slave or their agricultural skills.

There was other strong evidence for the positive correlation between the introduction of malaria and the institutionalization of slavery. In the post 1690 period, following the introduction of falciparum malaria, for example, Esposito uses records from 12 colonies from 1640 to 1780 to demonstrate that regions that were more malaria-suitable were more likely to codify the practice of slavery. Equally, higher malaria transmission intensity across US counties ties in with an increase in pro-slavery attitudes and ideology, as evidenced by the support for the Democratic party during the 1860 and 1864 presidential elections (at the time the Democrats were the pro-slavery party).

There has been a long running debate about the factors involved in the establishment, spread, and nature of slavery in the US. Understandably, it is a discussion that inspires considerable passion. Esposito makes an important contribution to our understanding of the way slavery evolved in the US. In doing so she provides a strong, evidence backed, case for an economic rationale linked to the introduction of a mosquito borne disease that has caused centuries of misery and suffering across the globe.

Related research paper: Side Effects of Immunity: The Rise of African Slavery in the US South, Elena Esposito, Department of Economics Working Papers Series, HEC Lausanne, University of Lausanne