White Southerners have long lagged behind Northerners in aptitude tests and real world academic achievement. Part of the explanation of the historical gap can be attributed to the high Southern prevalence of hookworm infection, which both lowers IQ and increases absenteeism.
Hookworm is usually acquired when people walk around barefoot in wet soil. It is still common in places like Brazil where modern IQ testing has been used to give a sense of how mentally sluggish it makes schoolchildren: 
When the connection was made between hookworm and various ailments in 1910 the John Rockefeller Society launched a thorough survey which discovered about 40 percent of Southerners to be infected. ncbi.nlm.nih.gov/pmc/articles/P…
Many teachers were very impressed of the effects of the great eradication campaign: "In short, we have here in our school-rooms today about 120 bright, rosy-faced children, whereas had you not been sent here to treat them we would have had that many pale-faced, stupid children."
"Children who were listless and dull are now active and alert.....Some of the 45 children who had never attended school, having been treated, have come in during the year. Others have declared their intention to enter in the fall".
Everywhere it is currently studied children with hookworm are not just duller, but so drained of energy that they skip classes altogether. One of the ways widespread infection probably affected all Southerners was that it increased adult tolerance for playing hooky.
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