Dehumanization is the process whereby opponents view each other as less than human and thus not deserving of humane treatment or what are generally accepted as fundamental human rights. It is necessary, psychologically, to so categorize the enemy if it is to be possible to engage in warfare or otherwise violate the generally accepted norms of behavior regarding one's fellow man.
"Negro Boy and Apes. On the left side of the figure there is a young
Chimpanzee, and on the right a young Orangutan." Taken from Shufeldt, R.W.'s America's Greatest Problem: The Negro (1915).
Dehumanization is actually an extension of a less intense process of developing an "enemy image" of the opponent. An enemy image is a stereotype-a negative oversimplification--which usually views the opposing group as evil, in contrast to one's own side, which is seen as entirely good. Enemy images are usually black and white. Shades of gray (meaning one's own faults or one's enemis' values) are usually discounted, denied, or ignored.
This is accentuated, according to psychologists, by the process of "projection"-in which people "project" their own faults onto their opponents. This means that people or groups who tend to be aggressive or selfish are likely to attribute those traits to their opponents, but not to themselves. This improves one's own self-image and group cohesion, but it also escalates the conflict and makes it easier to dehumanize the other side.
While the formation of enemy images is very common, it is a dangerous process that becomes especially so when it reaches the level of dehumanization. Once the enemy is considered to be less than human, it becomes psychologically acceptable to engage in genocide or other atrocities such as those that occurred in Rwanda, Cambodia, and the former Yugoslavia. (source: Conflict Research Consortium, University of Colorado)