The visual cliff was created to test what age infants develop depth perception. The visual cliff is an apparent, but not actual drop from one surface to another. The visual cliff is a sheet of heavy glass which is supported a foot or more above the floor. There is a patterned board laid underneath it. Half the board is directly underneath the glass; the other half is laid on the floor, which creates the "cliff." Psychologists assumed around eight months old was when babies developed depth perception.
In a test done, researchers placed babies on the side with the pattern directly under it. They had parents stand on the other side. They wanted to see if the babies would cross the cliff to get to their parents. Six months old wiggled to the other side, while ten month olds refused to cross the cliff.
However, later studies showed babies as young as three months old refusing to cross the cliff, thus having already developed depth perception. This study has been done several times. Sometimes, parents were waiting on the other end. Other times, the parents were told to make a happy face, implying that it was okay to cross or a scared face, implying danger. A few studies placed a toy on the other side, as an incentive to cross the cliff.