I’m going to ask Daniel, as the professional, for a more complete answer, but my recollection from Tax Law class is that the “marriage penalty” is really a function of accounting that cannot be made equitable.
It all comes down to whether you consider a married couple’s income as one income or two. If you count it as one, then the couple, while getting a bit more in deductions, loses out because the combined income means more is taxed at a higher marginal rate. If you count it as two, then singles lose out because marrieds usually have less expenses, as they share living space, etc. An additional problem is that married couples can cheat by transferring pre-tax income from the higher-earning spouse to the lower earning spouse, thereby having more money taxed at a lower rate.
The irony is that the marriage penalty wouldn’t exist if marrieds acted as the social conservatives pushing for the rebate wanted them to – the wife stays at home with the kids. Under that scenario, no income from the wife means less income taxed at the higher marginal rates. Combined with higher deductions for married couples, and the traditional family pays less taxes.
Please correct any errors.
Sua