The culture of the Shuar is deeply rooted in spiritualism, magic and war. An account of the tribe published in National Geographic in 1921 mentions that there's no established priestly class; rather, through the use of hallucinogenic drugs, like the brewed concoction called natema, a Shuar tribesman can directly contact and consult with spirits. This is particularly relevant to the topic of head shrinking, because the Shuar believe that the death of a tribesman is the direct result of a spiritual or magical attack of another, living tribesperson.
This belief forms the basis of the culture of vengeance that largely characterizes the Shuar. Not simply violent death, but death by natural causes is attributed to an unseen, remote enemy attack. The concept of revenge is deeply rooted in Shuar culture. From a very young age, boys are drilled in the standing feuds his family has with other families. He is also taught that not exacting revenge in the form of violence is to welcome retribution from his fallen ancestors [source: Jandial, et al].
When a family member dies -- regardless of the cause -- a tribesman consults a member of the spirit world under the influence of natema to learn the culprit. The answer comes in a vision, and the family sets about recruiting other nearby households to join them in a loose confederation with the purpose of exacting revenge. Over the course of several weeks or months, the group prepares for war, at times even alerting the intended target of its plans.
Ethnographies differ on who is killed during attacks on rival households. An earlier sketch of the Shuar describes them as polygamous and mentions that men from the raiding party may capture women from the targeted household as wives. A more recent history of the Shuar suggests that while a single tribesman is identified as the source of the family member's death is thus the central target for revenge, anyone found with that person may be killed. Indeed, an entire household, including women and children, may be subject to slaughter, although the heads of children aren't taken.
The heads that are taken are removed by cutting the skin at the extreme base of the neck, just above the clavicles and in a "V" shape meeting at a point between the nipples. Once the head is removed, a hair band or vine is passed through the open neck and out of the mouth, creating a loop that makes for easy transport as the warrior swiftly makes his way back home.