Mirror writing of digits: Is there a difference between boys and girls?

The neurological process of mirror generalization in memory, also known as mirror symmetrization, presents a real dilemma for typically developing 5- to 6-year-olds when learning to write characters (digits and letters). Should they write the digit 3 oriented to the left, that is correctly, or to the right, which leads to its mirror image ε? It has been anecdotally suggested that boys are more prone to mirror-writing than girls, but there is no scientific evidence for this idea. The present article gathers data from 691 children in the upper section of the French école maternelle (age between 5 and 6 ½), who each wrote the digits 0 to 9 four times under dictation and not necessarily in their natural order. Both simple and complex (mixed-effects linear regression) statistical comparisons on the percentages of digit reversal, show a substantial difference: girls produce more mirror reversals than boys. And yet the reversal curves as a function of the digits are quite similar between the two sexes (r = .97). It has been proposed that mirror reversal of characters results from the left orientation of some of them (e.g., 3, 7, J, Z), that is, in an orientation contrary to the direction of writing in our Western cultures. The present investigation shows that (1) this character orientation hypothesis (choosing to write characters in the same orientation as sentence writing) better explains reversals than the counterclockwise hypothesis (children are trained to draw circles counter-clockwise to prepare for attached cursive writing); (2) the study of the stability of reversals additionally supports the explanation of mirror writing by the left orientation of the digits (1, 2, 3, 7 and, less obviously, 9); but (3) neither of the preceding findings (left-right orientation and stability) provided a convincing explanation for the aforementioned gender difference.