According to Roberto Belvís, coordinator of the USP Dexeus Neurology Service, the right-left dominance (right-left-handed) in some functions such as hearing (the ear we put on the watch), vision (the eye we put in the telescope ), manual manipulation (the hand with which we write) or foot (the foot with which we kick a ball) is determined by genetic and environmental factors and is established around five years. However, ultrasound studies in pregnant women show that 10% of fetuses now preferentially use the left hand in movements (and this despite the fact that the fetuses have not yet been subjected to environmental factors).
Being left-handed does not entail any problem as long as there is homogeneity in dominance (foot-hand-eye-ear). Homogeneous lefties have no more learning problems than homogeneous right-handers. The problem appears when the dominance is not homogeneous but crossed, as learning problems may appear regarding calculation, writing and reading .
