Belgian rule reinforced an ethnic divide between the Tutsi and Hutu, and they supported Tutsis political power. Due to the eugenics movement in Europe and the United States, the colonial government became concerned with the differences between Hutu and Tutsi. Scientists arrived to measure skull—and thus, they believed, brain—size. Tutsi's skulls were bigger, they were taller, and their skin was lighter. As a result of this, Europeans came to believe that Tutsis had Caucasian ancestry, and were thus "superior" to Hutus. Each citizen was issued a racial identification card, which defined one as legally Hutu or Tutsi. The Belgians gave the majority of political control to the Tutsis. Tutsis began to believe the myth of their superior racial status, and exploited their power over the Hutu majority. In the 1920s, Belgian ethnologists analysed (measured skulls, etc.) thousands of Rwandans on analogous racial criteria, such as which would be used later by the Nazis. In 1931, an ethnic identity was officially mandated and administrative documents systematically detailed each person's "ethnicity,". Each Rwandan had an ethnic identity card.