Body rocking can be defined as that during the first year of life, most newborns rock their bodies. It generally starts at 6 months of age and has been found to happen most often when newborns are alone in their cribs and listening to music. It generally involves a rhythmic forward and backward swaying of the trunk at the hips, which happens most often in a seated posture but can also happen in quadruped. It might be subtle or forceful enough to cause a crib to tremor or even shift. The highest prevalence of the behavior has been observed to be between 6 and 18 months of age, with a sharp fall in prevalence beyond that age.