Disability eligibility expanded to include mental retardation, requiring psychiatrist certification and rule-based employability assessment for tax relief. The provision is amended to include mental retardation as a qualifying condition for disability-based tax relief where it substantially reduces capacity for gainful employment, subject to rules made by the Board; claims based on mental retardation require a psychiatrist's certificate from a Government hospital and the Board must consider the nature of the condition and its likely effect on employability when specifying qualifying disabilities.