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.