Baby Chicks' Mental Number Line Looks Like Ours

Like humans, animals seem to have an innate preference for small numbers on the left and larger numbers on the right

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Think of a number. Now think of a bigger number. Now imagine them in front of you. If you are seeing the smaller number on the left, you have just confirmed an oft-repeated finding: people tend to map numbers onto space from left to right. Mounting evidence, including research on preverbal infants, suggests this tendency is innate, although it can be easily overwritten by culture. Now, in a study published this past January in , a team of researchers at the University of Trento in Italy, led by cognitive psychologist Rosa Rugani, has shown that infants of a different species altogether also prefer to see bigger numbers on the right.

The team trained three-day-old chicks to walk around a panel for food. First, a group of chicks learned to find food behind a panel displaying five dots. Then the researchers replaced that panel with a pair of new panels. When these new panels displayed two dots each, the chicks walked to the left panel first 70 percent of the time. When the panels showed eight dots each, the chicks tended to choose the right panel—as if preferring to see smaller numbers on the left and larger numbers on the right.

The researchers then repeated the experiment with different chicks that were trained using 20 dots and tested using eight or 32 dots. In both experiments, the chicks veered left for relatively smaller numbers and right for larger numbers. The scientists chose the number eight to be smaller in one context and bigger in the other, showing that the effect depends on relative amounts rather than any absolute preference.

The findings strongly confirm the idea that the left-to-right tendency is innate. Research has shown that this tendency can be modified quite easily by experience, however, so overriding it most likely presents little problem to young brains in a culture that writes from right to left. Arabic speakers, for example, show the reverse spatial tendency. Others who write language from right to left and digits from left to right, as in Hebrew, display no particular spatial preference.

The authors suggest their results are related to the fact that brains are not symmetrical. The right hemisphere dominates visuospatial processing, leading to a preference for the left side of space to dominate attention—perhaps explaining why we naturally think of the “first” numbers there as we count. The spatial mapping might also arise from a physical map of numbers in the brain, which has been found in humans in the right posterior parietal cortex but has not yet been seen in animals.

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