Annually, 14-41 per 100 000 infants get mildly to lethally injured or severely disabled through violent shaking. The incidence and mortality of inflicted head injury by shaking trauma (IHI-ST) are highest in the early months and decrease with age. This may partly be due to the age-related physical characteristics of infants. Younger, smaller infants are more vulnerable owing to their size and material properties. In addition, from basic biomechanics, it is expected that larger or heavier infants may be more difficult to fiercely shake and will exhibit different motion patterns when being shaken violently. Therefore, the aim of this study was to compare the kinematics of shaking a smaller versus a larger infant dummy. We recorded the kinematics of two dummies, representing a 6-week-old and a 1-year-old, while they were violently shaken by volunteers. We found that participants induced higher head and torso accelerations when shaking the 6-week-old, than with the 1-year-old dummy. Moreover, higher peak sagittal angular accelerations coincide with smaller radii of rotation in the 6-week-old than in the 1-year-old. Because it has been suggested in the literature that sagittal angular acceleration of the head is an important mechanism in inducing the injuries associated with IHI-ST; the results of this study show that shaking a smaller/younger infant is more likely to cause the kinematics possibly responsible for IHI-ST.