If I ask you where you’d put the number 4 in relation to the number 12, chances are you’d put 4 to the left and 12 to the right. Most adults have some form of a mental number line that starts with smaller numbers on the left and progresses to bigger numbers on the right.
There’s no reason to think that this number line is innate, of course. It might be influenced by the fact that we read from left to right, or that we’re taught to represent smaller numbers as being on the left.
But a startling new study raises the possibility that newborns actually do have a left-to-right mental number line. In the study, infants who were an average of two days old were shown pictures of different numbers of squares that appeared on either the right or the left side of their visual fields.
After being shown pictures of 12 black squares inside a white square, the newborns were then shown groups of either 4 squares or 36 squares, on either the left or the right. As it turned out, the newborns looked longer when the group of 36 squares was on the right side and the group of 4 squares was on the left.
One possible explanation is that the newborns were paying attention to the size of the square groups, not the number specifically, so the researchers did a followup experiment. They resized the squares so that the group of 4 squares was larger than the initial group of 12 squares and the group of 12 squares was smaller. Even with this modification, the newborns continued to pay more attention when lower numbers of squares were on the left and higher numbers on the right.
Although this experiment doesn’t show that newborns have a mental number line that exactly resembles that of adults, it does suggest some general tendency toward putting larger numbers on the right and lower numbers on the left. The idea that our mental number line comes from the fact that we learn to read left-to-right is intuitively appealing, but these findings raise the possibility that the reality is more complicated.