Transitivism -- a phenomenon first discovered by Charlotte Bühler -- refers to a special kind of identification often observed in the behaviour of small children.
For example a child can hit another child of the same age on the left side of his face, and then touch the right side of his own face and cry in imagined pain.
For Lacan, transitivism illustrates the confusion of ego and other which is inherent in imaginary identification.
The inversion (right to left) is further evidence of the function of the mirror.
Transitivism is also evident in paranoia, in which attack and counter-attack are bound together "in an absolute equivalence."