"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. These measurements were however not used during the implementation of the identity card. In 1931, the ethnic identity was officially mandated and administrative documents systematically detailed each person's "ethnicity,". Rwandans had to apply for their identity card. During their application they were asked basic information such as their name, age and ethnicity. They could choose between Hutu, Tutsi or Twa. Most Rwandans were Hutu and answered as such, although when people started to suspect being Tutsi came with special privileges some lied and stated their ethnicity as Tutsi. When they applications were over however, the ethnic identity was fixed, as it was inherited patrilinealy. 84% of Rwandans were assigned Hutu, 14% Tutsi and around 1% Twa. Each Rwandan had an ethnic identity card.[19][20][21]

A history of Rwanda that justified the existence of these racial distinctions was written. No historical, archaeological, or above all linguistic traces have been found to date that confirm this official history. The observed differences between the Tutsis and the Hutus are about the same as those evident between the different British social classes in the 1950s. The way people nourished themselves explains a large part of the differences: the Tutsis, since they raised cattle, traditionally drank more milk than the Hutu, who were farmers.

The fragmenting of Hutu lands angered Mwami Yuhi IV, who had hoped to further centralize his power enough to get rid of the Belgians. In 1931 Tutsi plots against the Belgian administration resulted in the Belgians' deposing the Tutsi Mwami Yuhi. The Tutsis took up arms against the Belgians, but feared the Belgians' military superiority and did not openly revolt.[22] Yuhi was replaced by Mutara III, his son. In 1943, he became the first Mwami to convert to Catholicism.[23]

From 1935 on, "Tutsi", "Hutu" and "Twa" were indicated on identity cards. However, because of the existence of many wealthy Hutu who shared the financial (if not physical) stature of the Tutsi, the Belgians used an expedient method of classification based on the number of cattle a person owned. Anyone with ten or more cattle was considered a member of the Tutsi class. The Roman Catholic Church, the primary educators in the country, subscribed to and reinforced the differences between Hutu and Tutsi. They developed separate educational systems for each,[citation needed] although throughout the 1940s and 1950s the vast majority of students were Tutsi''.[citation needed]

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