Blood provides an ideal opportunity for the study of human variation without cultural prejudice. It can be easily classified for many different genetically inherited blood typing systems. Also significant is the fact that we rarely take blood types into consideration in selecting mates. In addition, few people know their own type today and no one did prior to 1900. As a result, differences in blood type frequencies around the world are most likely due to other factors than social discrimination. Contemporary Japan is somewhat of an exception since there are popular Japanese stereotypes about people with different blood types. This could affect choice in marriage partners for some Japanese.
All human populations share the same 29 known blood systems, although they differ in the frequencies of specific types. Given the evolutionary closeness of apes and monkeys to our species, it is not surprising that some of them share a number of blood typing systems with us as well.
These patterns of ABO, Rh, and Diego blood type distributions are not similar to those for skin color or other so-called "racial" traits. The implication is that the specific causes responsible for the distribution of human blood types have been different than those for other traits that have been commonly employed to categorize people into "races." Since it would be possible to divide up humanity into radically different groupings using blood typing instead of other genetically inherited traits such as skin color, we have more conclusive evidence that the commonly used typological model for understanding human variation is scientifically unsound.
The more we study the precise details of human variation, the more we understand how complex are the patterns. They cannot be easily summarized or understood. Yet, this hard-earned scientific knowledge is generally ignored in most countries because of more demanding social and political concerns. As a result, discrimination based on presumed "racial" groups still continues. It is important to keep in mind that this "racial" classification often has more to do with cultural and historical distinctions than it does with biology. In a very real sense, "race" is a distinction that is created by culture not biology.