The four humours were four fluids that were thought to permeate the body and influence its health. The concept was developed by ancient Greek thinkers around 400 BC and developed further by Galen. People were thought to be either Choleric, Melancholic, Phlegmatic, or Sanguine.
During the Middle Ages in Europe, the Latin term complexio served as the translated form of the Greek word crasis, meaning temperament. The term “temperament” referred to the balance of the qualities of hot, wet, cold, and dry; each human body carried a different mixture of the elements. Thus, the Scythians, who lived in a cold climate, were considered “colder and moister” in complexion; the Ethiopians were considered hotter and dryer. Complexion was defined as “that quality which results from the mutual interaction and interpassion of the four contrary primary qualities residing within the elements. These elements are so minutely intermingled as each to lie in very intimate relationship to one another. Their opposite powers alternately conquer and become conquered until a quality is reached which is uniform throughout the whole: this is the complexion.”
As Matthew Simon writes, “since it served as a fundamental concept, not only in physiology but also in pathology and therapy, complexion theory provided important support for the idea that medicine constituted a unified and rational body of knowledge.” By observation and judgment, medieval physicians determined the proper complexion of the individual when healthy. The body was healthy when all was in balance, but diagnosis was difficult, as there was no absolute measure of the right complexion, since this varied for individuals. Balance was thought to be restored by various remedies, which included bloodletting, scarifying, purging, and eating certain foods.
Complexion was thought to be an indicator of one's character. The Spanish work known as Corbacho, written by Alfonso Martínez de Toledo (ca. 1398—ca. 1470), includes a chapter called "De las complexiones." In it he describes the personalities of men of varying complexions: "There are others who are melancholic: these men correspond to the Earth, which is the fourth element, which is cold and dry. These men are very angry, without a sense of tact or moderation... They have no sense of temperance in anything they do, and only bang their head against the wall. They're very iniquitous, petulant, miserable...”
Many surnames arose out of the existence of a complexion whose particularities may have differed from that of the village or town’s population, and thus attracted enough notice to warrant a nickname. The Irish surname Rogan (from Ruadhán) referred to a person with red hair, or a ruddy complexion. The Scottish surname Bain (from bàn) referred to a fair-haired person, while Dunn (from donn) implies brown/dark hair, and Duff (from dubh) implies black hair. The English surname Brown, an extremely common surname in the English-speaking world, was originally applied to anyone with a slightly darker complexion, in the same manner that the surname White was applied to anyone with a particularly light complexion (such as Barry White). The surname Gough is derived from the Welsh goch or coch, meaning "red" or "ruddy." King William II of England was called William Rufus ("the Red") because of his ruddy complexion. Ludovico il Moro ("the Moor") was called as such because of his swarthy complexion.
The variation in complexion has also been used through the centuries to justify racism, the tone of one's skin (and other traits, such as skull shape and size) believed to be proof of one people’s innate inferiority or superiority over another. It is arguable that racism has existed in many forms, since the time when a clear distinction between race could be seen (in skin tone and other physical traits) and/or when two or more different ethnic groups initially came into contact for the first time. In a more modern example, Europeans from the Renaissance onwards, believed the idea that they greatly differed from other groups and constructed a hierarchy of human beings, according themselves a higher status than Africans or Asians. Aryanism, which flourished in the nineteenth and twentieth centuries, declaimed the superiority of the so-called "Aryan complexion" (blond hair, blue eyes, contradicting the Indo-Aryans and Indo-Iranians). In Europe, it is now well understood that the concept of race for human beings does not exist.
A person’s complexion is, however, a biological trait. The protein molecule known as melanin causes variation in tone. Melanocytes insert granules of melanin called melanosomes into the other skin cells of the human epidermis. The melanosomes in each recipient cell accumulate atop the cellular nucleus, where they protect the nuclear DNA from mutations caused by the sun's ionizing radiation. The human body tends to protect itself against harmful surroundings. The epidermis of the body, very sensitive and delicate, reacts almost immediately to most outside affects. People whose ancestors lived for long periods in the regions of the globe near the Equator generally have more active melanocytes, and therefore larger quantities of melanin in their skins. This makes their skins very dark and protects them against high levels of exposure to the sun (it also depends on the country). In areas of the globe closer to the poles, people have far less need for protection from ionizing radiation, so their skin is usually lighter.