THE ATLANTIC: Why Do Humans Still Have a Gene That Increases the Risk of Alzheimer's?

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“It’s consistent with other studies indicating that E4 isn’t always the bad guy,” says Ruth Itzhaki, from the University of Manchester, whose own work showed that E4 might protect against liver damage caused by hepatitis C. Other researchers have found that E4-carrying children in Brazilian shanty towns did better in mental tests when carrying intestinal parasites, while those in Mexico City were better able to cope with lead exposure. This doesn’t explain why E4 remains common in many Western populations, but it does at least hint that the variant might be beneficial in other contexts that we haven’t discovered.