Method: A systematic review of the literature revealed two groups of studies. The first group comprised 15 clinical trials and compared the effects of EMDR therapy with eye movements to those of EMDR without the eye movements. The second group comprised 11 laboratory trials that investigated the effects of eye movements while thinking of a distressing memory versus the same procedure without the eye movements in a non-therapy context. The total number of participants was 849.

Results: The effect size for the additive effect of eye movements in EMDR treatment studies was moderate and significant (Cohen’s d = 0.41). For the second group of laboratory studies the effect size was large and significant (d = 0.74). The strongest effect size difference was for vividness measures in the non-therapy studies (d = 0.91). The data indicated that treatment fidelity acted as a moderator variable on the effect of eye movements in the therapy studies.

Conclusions: Results were discussed in terms of current theories that suggest the processes involved in EMDR are different from other exposure based therapies.