Sai, who is from Japan, can easily tell whether another Asian person appears to be from China or Thailand. However, he has trouble distinguishing among White people and thinks they all look basically the same. This is due to what psychological tendency?

This is the so-called cross-race effect, or cross-race bias. It's easier for us to recognize the faces of people of racial groups we are familiar with than to recognize the faces of people belonging to other cultures. We see the example here, Asians will more easily recognize other Asian people's faces that faces of European people. And viceversa, many white people go to Asian countries and they have difficulties in telling faces one from another in the beginning. Only after you have spent some time, maybe one month or two months, you start telling differences which, of course, exist everywhere.