In wayne dennis's study of infants in iranian orphanages, only 15 percent of the orphans were walking alone by 3 to 4 years of age because

The answer to the question above is this: they spent their days lying on their backs in cribs. Wayne Dennis has studied infants in Iranian orphanages. The infants in these orphanages were more on left lying on their backs on their cribs and this results in the delay of their physical development such as walking and other physical activities. This made the children in the orphanages walk at the age of 3 to 4 instead of 1 year old or earlier.