This report examines the effects of the Mental Health Parity Act of 1996 (MHPA). 

From the SAMHSA report:

Results indicate that the effects of MHPA have been largely positive. Only about half of the health plans subject to the Act had to make changes in their mental health benefits. Of these, the large majority did not judge that the mandate required compensating changes in other benefit provisions. Further, MHPA had an unintended beneficial effect of also improving coverage for substance abuse benefits in many plans. Nevertheless, some employers did make changes that would tend to nullify any beneficial effects of the legislation, and a very small number actually dropped mental health coverage.

Full report: http://mentalhealth.samhsa.gov/cmhs/ManagedCare/Parity/ParityActEffcts.asp 

U.S. Dept. of Health and Human Services, Substance Abuse and Mental Health Services Administration (SAMHSA). (1998). Effects of the Mental Health Parity Act of 1996.