Correct answer: Because fingerprints are influenced by environmental factors during intrauterine life. Identical twins, unlike fraternal twins, come from one fertilized egg that divides into two embryos which thus have the same DNA profile. Their fingerprints are similar enough in structure that, before genetic testing came along, they were used to determine whether twins were identical or fraternal (fraternal twins' fingerprints are no more alike than the fingerprints of siblings born separately). Although very similar, the fingerprints of identical twins are nevertheless nearly unique because they are not completely determined by genetics: they are an example of phenotype, which refers to the physical characteristics of an individual that are determined by the interaction of genes and the environment. Various aspect of the embryo’s development influence the formation of fingerprints, which takes place between the 10th and 16th weeks of pregnancy when the cell layers at the end of the fingers fold. This process is affected by the speed of the finger's growth, which can be different between the twins. In addition, Merkel cells in the skin will rearrange themselves when the skin is folding, with a significant variability caused by an attraction and repulsion phenomenon between the cells. Every baby therefore has a nearly unique imprint. Globally, the chance of finding two people with the same fingerprint is infinitesimally small. The fingerprints on our left and right hands differ for the same reasons. What purpose do fingerprints serve? Apart from improving our grip on objects, they allow us to tell textures apart thanks to little vibrations transmitted by the valleys and ridges to the nerve endings under the skin. And because the area of contact is larger and more complex, our sense of touch is very precise. The uniqueness of fingerprints also means they can be used in forensic science and criminal investigations, with techniques that can identify up to 200 measurement points on each finger. 95% of fingerprints fall into one of three categories: loops, whorls (coils or spirals) and arches. With thanks to Christophe Champod, professor of Forensic Science at the University of Lausanne.