Approximately 90% of us are right handed and 10% left handed. But when does this start? By using ultrasound we observed babies sucking their thumb in the womb. Just as with adults, approximately 10% preferred to suck their left thumb and 90% their right thumb.
By observing babies at 10 weeks of gestation, when they first produce independent arm movements, 90% were observed to move their right arm more. It appears as soon as we can show left and right movements our predisposition for being a left-, or right-hander is set.