A utilitarian criterion embodying both horizontal and vertical equity commands is proposed which determines the equitable income tax. We show that a family size related tax credit cannot be considered equitable, whereas an exemption can if a particular social evaluation function, defined over income and needs, is adopted, one which is, except in the narrowest of circumstances, inconsistent with
... [Show full abstract] the widespread practice among social policy analysts of equivalizing incomes to take account of needs. This finding presents a challenge to find a deeper justification for the use of credits (and indeed, exemptions) by tax designers.