Do picture books affect counting directionality in preliterate children? Developmental course and potential mechanisms

• Explored experiential causes, development, and mechanisms of lateral biases in mental number line in preliterate children. • A mirror-printed picture book independently changed lateral biases in counting and mental model construction. • Older children showed initial lateral biases that were not present in younger children. • Picture books can cause early, script-linked lateral biases before reading and writing practice. Why the number line in preliterate children is already consistent with the direction of the script? Here we aimed to 1) show that being read a mirror-printed picture book is able to change lateral biases in counting; 2) trace the development of preliterate biases; and 3) test the role of mental model construction processes. Spanish-speaking 3 and 4 year-olds (N = 155, white, 87 female) carried out a task of counting a row of objects and a task in which they built a toy scene before and after being exposed to a mirror-reversed storybook. The left-to-right or right-to-left directionality of their responses was recorded. Only the older group showed pre-test lateral biases. The mirror book changed the lateral biases in counting, and induced a congruent, but smaller change in the model construction task. The two tasks did not correlate, against the implication of shared mechanisms in them.