Stockholm syndrome is a proposed condition to explain why hostages occasionally develop a psychological bond with their captors. It is named after an attempted bank robbery in 1973, in Stockholm, Sweden, in which four people were held hostage but later refused to testify against the robbers. Stockholm syndrome has never been included in the Diagnostic and Statistical Manual of Mental Disorders (DSM), the authoritative guide for diagnosis of psychiatric conditions used in the United States, due to the lack of any academic study of the condition, and an increasing body of evidence leading to doubt about the legitimacy of the condition.