The way we arrange numbers in our mental space has a biological origin, not a cultural one, as one might think. A group of researchers has clarified this scientific controversy by demonstrating that the mental ordering of the number line is a biologically determined phenomenon, although it can be modified over time by literacy habits. The researchers observed the Himba, an indigenous population from Namibia who have only an oral culture, limited mathematical knowledge, and no formal schooling. They then compared their behavior with that of Italian adults and preschool children. The findings showed that, in experiments typically used with non-verbal living beings, such as animals and infants, the result was the same: all three population samples place small numbers on the left and large numbers on the right. The first author of the study is Elena Eccher from the Interdepartmental Center for Mind and Brain at the University of Trento, with the collaboration of colleagues from Cimec and several French university research laboratories. The study was coordinated by Professor Manuela Piazza and Professor Giorgio Vallortigara, both faculty members at Cimec. The subjects involved in the study underwent two types of experiments. In a first "explicit" task, where individuals had to arrange tokens with different numbers of discs on them in order, only the adult Italians showed a mental number line ordering, while the Himba and preschool children placed the objects randomly. However, when asked to perform a second "implicit" task, computerized and similar to those used with animals and infants, to measure the reaction time to a variation in quantity of a stimulus presented first centrally on the screen and then, either larger or smaller in quantity, to the right or left, all three groups behaved the same way, responding more quickly when small numbers were placed on the left.