One crucial aspect of naturalisation for Zainichi Korean is that both Mindan and Chongryon link Korean ethnic identity to Korean nationality (Japanese and South Korean nationality laws do not allow multiple citizenship for adults). By their definition, opting for a Japanese passport means becoming a Japanese, rather than a Korean Japanese. In order to be naturalized as Japanese citizens, Zainichi Koreans had to go through multiple, complex steps. In order for Zainichi Koreans to complete these steps, they were required to collect information about their family and ancestors that stretched ten generations. They could collect this information if they paid money to an organization such as Mindan, however with prohibitively expensive cost, so many were unable to afford such information. However, these processes became much easier as time passed and today, it is easier for Zainichi Koreans to naturalize into Japanese citizens. Though there are a few cases of celebrities who naturalize with their Korean name, the majority of Zainichi Koreans formally choose a name that is both read and appears ethnically Japanese. This means that the rate of naturalisation can be taken as a rough measure of assimilation. During post-World War II period many Zainichi Koreans married with other Zainichi Koreans and it was a rare case for them to marry with Japanese citizens. This was because Japanese citizens had a xenophobic prejudice against Zainichi Koreans due to the stigma stemming from decades of discrimination. Therefore, Japanese citizens, especially the parents were refusing marriage with Zainichi Koreans. However, there were also some problems about marriage between Zainichi Koreans too. As stated in the previous section, Zainichi Koreans were hiding their identity and living as a Japanese citizens at that time. Because of this, it was very hard for Zainichi Koreans to know of other people who had the same nationality as them. They were married mostly through arranged marriages supported by Mindan. Tong-il Ilbo () or Tōitsu Nippō (), a Korean Japanese newspaper, reported that according to statistics from the Japanese Health and Labour ministry, there were 8,376 marriages between Japanese and Koreans. Compared to 1,971 marriages in 1965, when the statistics began, the number has roughly quadrupled and it now constitutes about 1% of the 730,971 total marriages in Japan. The highest number of marriages between Japanese men and Korean women was 8,940, in 1990. Since 1991, it has fluctuated around 6,000. On the other hand, there were