Quantum entanglement is a phenomenon in which entangled systems exhibit correlations that cannot be explained by classical physics. It has recently been suggested that a similar process occurs between people and explains anomalous phenomena such as healing. This paper explores the hypothesis that the therapeutic interaction involves some kind of 'entanglement' between people. There are several different versions of the theory that entanglement is possible between people: the versions differ in the way entanglement between people is derived, and the way it has a therapeutic effect. The two main versions are generalised, specific entanglement, and global, emergent entanglement-specific entanglement. Two research ideas are presented for testing the hypothesis of entanglement between people, both of which are based on predicting variation in outcome in naturally occurring contexts (the naturalistic observational study).