A wide variety of species 
tested—–including chicks, 
infant rats, kittens, puppies, 
lambs, and snow leopard cubs—–
showed a strong preference for 
the shallow side and typically 
were very reluctant to go to the 
deep side of the "cliff." Infant 
monkeys as early as three days 
after birth showed signs of 
distress when placed on the 
glass over the deep side. 
Ironically, research by Bennett 
Bertenthal and Joseph Campos 
indicates that young human 
infants do not display much 
fear of height (and thus do not 
avoid the cliff’s deep side) and 
do not show cardiac 
acceleration when held over 
that side until several weeks 
after they have begun to crawl. 
But, in other behavior, human 
infants do indicate that they 
perceive the depth of the 
"deep" side much earlier. 
Studies testing depth 
perception via stereoscopic 
vision indicate that it is 
present in infants by the age of 
three and one-half months. 
Thus, experiments resolve the 
long-standing controversy: 
Distance perception is present 
very early in an organism’s life.

    Which cues on the "visual 
cliff" enabled the animals 
tested to perceive distance? 
Experiments suggest, by a 
process of elimination, that 
motion parallax is a critical 
factor. Yet the study of motion 
parallax described earlier 
indicates that this factor is not 
always effectual among adult 
subjects. Thus this question 
remains an open one.