A study of group therapy in over 200 normal college students conducted by Yalom and Lieberman found that 9.1% of the students who completed over half of a series of "encounter groups" using attack therapy had psychological damage lasting at least six months.
An investigation of group therapy involving more than 200 normal college students, led by Yalom and Lieberman, discovered that 9.1% of those who participated in over half of a series of "encounter groups" utilizing attack therapy suffered from psychological harm lasting at least six months.