Peggy Mason, a renowned professor of neurobiology at the University of Chicago, set out to discover the biological basis of empathy. To do this, Mason and her team created a unique test for her lab rats, which started by placing two rats in a large box. The first rat was free to move about, and the other was placed in the middle of the box in a trap that could only be opened from the outside. Rats, in general, do not like to be out in open spaces, where they can be spotted by predators—but, astonishingly, the free rat risked its own exposure to rescue the rat in the trap. Mason had discovered empathy in rats.

Every discovery leads to more questions, and in this case, Mason wondered, how deep did the empathy go? She reasoned that her lab rats were familiar with each other—bunking and often learning new skills together. But would they rescue rats they had never met? Mason put newcomers into the traps. Lo and behold, her rats rescued the strangers.

Was the empathy linked to genetics? Scientific labs use white rats for their experiments, and most of these rats have been crossbred for generations. Mason brought in some genetically distant black rats, and when one of these was put into the trap, the result was dramatically different. The free white rat did not go to the black rat’s rescue.

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