In a classic study on depth perception, gibson and walk placed infants of various ages on a fabric-covered runway that ran across the center of a clever device called a visual cliff. in the study,
girls refused to cross the visual cliff but boys crossed it
boys crossed the visual cliff faster than girls
infants crossed the cliff when their mothers encouraged them to do so
eight out of ten crawling infants refused to cross the seemingly unsupported surface of the visual cliff

The study found that infants crossed the cliff when their mothers encouraged them to do so.

In this 1960 study by psychologists Eleanor J. Gibson and Richard D. Walk, infants aged 6 to 14 months are placed at one end of the runway and their parent stands at the other end of the plexiglas surface. The idea is to evaluate the child's perception of height and depth: if they perceived the visual (simulated) cliff, and if they became scared of it, they would be reluctant to crawl over to their parent.

An overwhelming majority (27 out of 36 children tested) crossed the surface, motivated by their parent's stimulus.