Law of contagion
The law of contagion is a folk belief axiom found in magical thinking which suggests that, once two people or objects have been in contact, a magical link persists between them unless or until a formal exorcism or other act of banishing breaks the non-material bond. The first description of the law of contagion appeared in The Golden Bough by James George Frazer
Conscious belief in the law of contagion
According to this idea, the law of contagion has both dangers and benefits. On the good side, the holiness of a saint, god or other venerated figure confers benefits to relics, as do temples and churches, by virtue of their having religious rituals conducted within them.
On the bad side, this means that, according to the belief system of many cultures, a sorcerer or witch might acquire a lock of hair, nail clipping or scrap of clothing in order to facilitate a curse. Voodoo dolls resemble the victim and often incorporate hair or clothing from them. Cultures that believe in sorcery therefore often exercise care that their hair or nails do not end up in the hands of sorcerers.