Ben Dantzer had spent several frustrating days trying to capture a single squirrel when the epiphany arrived. Dantzer, a rodent researcher at the University of Michigan, was standing in the Canadian Yukon, scrutinizing the uncooperative squirrel, which was perched high in a spruce tree. Then, all of a sudden, he felt as though he was looking at an optical illusion: When he viewed the squirrel one way, he saw a squirrel; when he viewed it another way, he saw a rat. “I kind of think of squirrels as rats in costumes,” he told me. “Like with a fur coat and a dog’s nose.” It’s true: The two rodents do look remarkably alike. And yet, for all their similarities, they elicit wildly different reactions from humans. Squirrels—aww. Rats—ick. What gives?
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The underlying mechanism at the core of our rat aversion may be disease avoidance, Jakub Polák, a psychologist at Charles University, in Prague, who has studied the relative grossness of different animals, told me. Disgust, Polák said, evolved as a defense mechanism against dangerous pathogens—and rats are notorious transmitters of disease. They’ve long borne the brunt of the blame, deserved or not, for the bubonic plague. And they can also carry a number of other infectious diseases, including hantavirus, leptospirosis, lymphocytic choriomeningitis, tularemia, and salmonella.
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Disease avoidance helps explain how our anti-rat culture came to be in the first place, the University of Michigan psychologist Joshua Ackerman told me. And that culture in turn helps ingrain the genetically inherited predisposition to find rats gross.