This 1998 report examined the state of mental health and substance abuse parity under the Mental Health Parity Act of 1996 and state parity laws.  The study summarized characteristics of state parity laws, conducted detailed case studies of five state parity laws, analyzed actuarial estimates of the costs of parity, and provided estimates of premium increases under full and partial parity. 

From the report:

Health plans offered by employers typically provide less coverage for mental health and substance abuse (MH/SA) treatment than for general medical and surgical services. States and the federal government have begun to require that mental health and/or substance abuse treatment be covered in the same way as other medical care. This concept is known as "parity."In 1996, Congress passed and President signed the Mental Health Parity Act. Effective January 1, 1998, this law requires that health plans provide the same annual and lifetime limits for mental health benefits as they do for other health care benefits. The act does not affect service limits, such as limits on outpatient visits, or cost sharing, such as deductibles. Nor does it apply to substance abuse benefits.

States have mandated parity, as well. By September 1997, 12 states had passed laws that, to various degrees, require parity in mental health and/or substance abuse benefits. Others have enacted legislation conforming to the federal mandate.

Opinion differs as to the costs and effects of parity mandates. This study was designed to address these issues by:

• summarizing the characteristics of state parity laws,
• conducting detailed case studies of five states with such laws,
• analyzing previous actuarial estimates of the costs of parity, and
• providing updated estimates of premium increases due to full and partial parity.

Full report: http://mentalhealth.samhsa.gov/publications/allpubs/Mc99-80/Prtyfnix.asp 

U.S. Dept. of Health and Human Services, Substance Abuse and Mental Health Services Administration. (1998). The costs and effects of parity for mental health and substance abuse insurance benefits. Sing, M., Hill, S., Smolkin, S. and Heiser, N.