Of central importance for understanding the development of handedness is the answer to the question of when in development it is actually determined whether a child will be left-handed or right-handed. It was long thought that handedness could only be reliably determined in elementary school, when a child learns to write. However, this assumption is incorrect. In fact, scientific studies show that left‐handedness is established in many children long before elementary school ─ interestingly, even before birth in most people. In such studies, the hand and arm movements of unborn children in the womb are recorded using ultrasound images. Using this technique, it was shown that a clear preference for the movement of the right arm exists as early as 10 weeks after fertilization. In this study, ultrasound images of 72 unborn children 10 weeks after fertilization were evaluated and 85% showed more movements of the right arm than the left. This number is already very close to the approximately 89.4% right-handers among adults.