Shared psychotic disorder (now termed “other specified schizophrenia spectrum” and “other psychotic disorder” in the DSM-5) is when a delusional belief is shared from one person to another or others.
Most common in nuclear family relationships, folie a deux is when two people share the delusional belief, folie à trois when three people share, it and so on. An entire family can share a delusional belief, in which case the condition is referred to as folie en famille or family madness.
A delusion is a fixed, false belief held with absolute certainty despite strong evidence to the contrary. The belief is also inconsistent with the cultural and religious norms of the affected individual(s), e.g., aliens are stalking your girlfriend or the end of the world will happen next week.
If a large number of people believe the same false information and it generates acute anxiety in most members of the group, it is referred to as mass hysteria rather than shared psychotic disorder.
The chances of developing a shared psychotic disorder are highest if in a cult. Otherwise the disorder is rare.