65% of newborn infants tend to turn their heads to the right when lying in a crib, with 15% turning their heads to the left, and about 20% showing no particular preference. Most infants of a few months of age, whether they ultimately turn out to be right-handed or left-handed, tend to hold a rattle in their right hands longer than in their left. However, it is not seriously claimed that this kind of phenomenon in very early life can be genuine evidence of handedness.

Handedness only truly emerges when some degree of skill is required, and hand specialization of any kind only really begins to show after about one year of age. Even then, though, there is usually a period of experimentation and constant change in hand preferences, sometimes referred to as the “chaotic phase”. This is further confused by the fact that the right hemisphere tends to mature before the left in the brains of human infants between the ages of about one and three, and then this asymmetry shifts back towards the left hemisphere after about three years old, during the development of language abilities. It has been noted that, in general terms, the route to handedness tends to be more stable among right-handed children than among left-handers, who tend to exhibit more fluctuations en route to establishing their hand preference.

Hand choice, therefore, does not usually stabilize until around three or four years of age, when more complex activities like writing, cutting, etc, are learned, including the habit of “crossing the midline” (using one hand to carry out activities which are physically on the other side of the body). Other children, however, may not establish a hand preference until the age of six, seven or even eight.

The Geschwind-Galaburda Hypothesis proposed by Norman Geschwind and Albert Galaburda tries to explain sex differences in the cognitive abilities of young people by relating them to the lateralization of brain functions. According to the Hypothesis, male brains mature later than female brains, and the left hemisphere matures later than the right.

Some child educators believe that it is important for a child to develop a distinct handedness, one way or another (with the other hand relegated to the status of assistant or helper hand) by the time they are three or four, so that the motor programs in the brain can be laid down consistently, and the child can develop speed and skill in fine motor tasks. Failure to do so could lead to motor and/or cognitive problems later in childhood. This “choosing”, it should be stressed, is not the deliberate encouragement of a particular handedness, merely a demonstration or confirmation of the child’s naturally occurring handedness, which could be either right or left. Interestingly, this need for a definite handedness seems mainly to apply to writing, and mixed-handedness in other tasks such as eating, throwing, etc, is not associated with motor or cognitive problems.

On the other hand, very strong handedness tendencies at an early age (before 18 months) is usually considered a warning sign of possible hemiplegia (paralysis of one side of the body) and potential future developmental problems.