How have researchers come to know that babies understand that the image looking back at them in a mirror is their own?

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Researchers have used the "mirror test" to determine whether babies (and other animals) recognize themselves in a mirror. This test involves placing a mark on the baby's face and then observing whether they touch their own face or the mirror when looking at their reflection. Babies who understand that the image in the mirror is their own will touch the mark on their face rather than the mirror.

How Have Researchers Come to Know That Babies Understand That