question: Why do single taxpayers in the US have to pay more federal taxes than married taxpayers?
comment: The total income of the couple is treated approximately as if it was split between the two members of the married couple rather than by one person, which results in a lower tax burden. It would still be slightly better if it was actually split between the two people, so if both members of a married couple make the same amount, it may actually be disadvantageous to file jointly, but in cases where one spouse is primarily supporting the other, the government treats it as two people each earning a smaller income, rather than the larger income that is supporting a single person.
passages: The taxpayer using the Head of Household filing status is entitled to use the Head of Household tax rates, which feature wider tax brackets. Also, the taxpayer is entitled to a larger standard deduction ($18,000 for tax year 2018) than taxpayers using Single or Married Filing Separately ($12,000 for 2018).
  
  Section::::Considered unmarried.
   Although the joint return often produces lower taxes, the opposite is sometimes the case. To accommodate for such circumstances, married couples may decide to file separately for a taxable year.
  
  Married couples filing separately does not create an identical situation to the two parties filing as single. There are different brackets for unmarried taxpayers from the ones for married taxpayers who file separately. Unmarried taxpayers enjoy wider tax brackets and so pay less tax on the same amount of income.
   When the rates applied by the tax code are not based income averaging, but rather on the "sum" of individuals' incomes, higher rates will usually apply to each individual in a two-earner households in a progressive tax systems. This is most often the case with high-income taxpayers and is another situation called a marriage penalty.
   If the incomes of the two persons are similar, then at the lower end of the tax schedule there is no difference between filing as singles and filing as a married couple (ignoring the question of deductions, see below). But at the higher end of the tax schedule, there is a penalty for a married couple whose incomes are similar, compared to what they would pay as singles.
  
  For example, the following chart shows the US federal tax rates for 2013:
   On the other hand, being married can result in less tax. If one person earns twice the sum of the standard deduction and the personal exemption (so he earns around $20,000 a year) and the other earns nothing, the wage earner would pay around 5% of his total income as tax as a single, but as a couple their taxable income would be zero so they would pay no tax.
  
  Section::::Social security/Medicare burden and subsidy to sole breadwinners and nonearning parents.
   Under these tax rates, two single people who each earned $87,850 would each file as "Single" and each would pay a marginal tax rate of 25%. However, if those same two people were married, their combined income would be exactly the same as before (2 * $87,850 = $175,700), but the "Married filing Jointly" tax brackets would push them into a higher marginal rate of 28%, costing them an additional $879 in taxes.
    For households, determining the amount of aggregate income generated over the course of a calendar year can be advantageous when calculating the total taxes due for that period. In a number of nations, federal tax agencies provide some incentives for spouses to file joint tax returns, rather than each spouse filing separately. By choosing to aggregate their generated income for the tax period, the household is likely to owe fewer taxes and thus receive a higher joint return than they would have received with individual returns.
   In the United States, married couples that submit a combined tax return may face a marriage penalty, where tax credits for low-income single earners are not applied to the combined income.
   The US tax code fixes different income levels for passing from one marginal tax rate to another, depending on whether the filing is done as a single person or as a married couple. For lower incomes, the transition points for married couples are twice those for single persons, which benefits a couple that gets married if their incomes are sufficiently different. This is equivalent to "income splitting", meaning that the tax due is the same as if the two persons use the schedule for single persons, but with each declaring half the total income. At higher incomes, this equivalence is lost but there is still an advantage if the two incomes are sufficiently different. To make an extreme example, if one person had $466,950 taxable income in 2016 (with itemized deductions amounting to more than twice the standard deduction) and the other earned nothing, the tax due by the earner as a single person would be $466,950*0.396−$43,830.05 or $141,082.15, whereas filing jointly they would pay $466,950*0.396−$54,333.70 or $130,578.50, a saving of $10,503.65.
   In some countries a married person or couple benefits from various taxation advantages not available to a single person. For example, spouses may be allowed to average their combined incomes. This is advantageous to a married couple with disparate incomes. To compensate for this, countries may provide a higher tax bracket for the averaged income of a married couple. While income averaging might still benefit a married couple with a stay-at-home spouse, such averaging would cause a married couple with roughly equal personal incomes to pay more total tax than they would as two single persons. In the United States, this is called the marriage penalty.
   The US tax code allows taxpayers to claim deductions (such as charitable contributions, mortgage interest, or payments for state taxes) on their income. Taxpayers can choose either an automatic standard deduction or else can choose to itemize their deductions. Two single people filing separate returns can each choose the deduction policy that benefits them more, but a married couple filing a single return will both be forced to use the same method. For example, if one person has no significant deductions, the person can take the standard deduction ($6,100 as of 2013). A different person, who has, for example, $10,000 in charitable contributions, would be better off itemizing his deductions since the standard deduction is $6,100 (single, 2013 tax year).
   In the most extreme case, two single people who each earned $400,000 would each pay a marginal tax rate of 35%; but if those same two people filed as "Married, filing jointly" then their combined income would be exactly the same (2 * $400,000 = $800,000), yet $350,000 of that income would be taxed as the higher 39.6% rate, resulting in a marriage penalty of $32,119 in extra taxes ($16,100 for the 39.6% bracket alone, plus the remainder is due to the higher phase out of the lower brackets.) Using the formulas for 2016 income, if both persons have a taxable income X greater than $415,050 then as singles each would pay 0.396X−$43830.05, whereas if they were married filing jointly they would pay 2(0.396X)−$54333.70, so they lose 2($43830.05)−$54333.70 or $33,326.40.
   Individuals are eligible for a reduced rate of federal income tax on capital gains and qualifying dividends. The tax rate and some deductions are different for individuals depending on filing status. Married individuals may compute tax as a couple or separately. Single individuals may be eligible for reduced tax rates if they are head of a household in which they live with a dependent.
   A person who is married on the last day of the tax year generally must use either the Married Filing Jointly or Married Filing Separately filing status. The exception is a taxpayer who is considered unmarried for Head of Household filing purposes and actually qualifies as Head of Household.
  
  To be considered unmarried, a taxpayer must meet all the following tests:
  
  BULLET::::1. The taxpayer must file a separate return.
  
  BULLET::::2. The taxpayer paid more than half the cost of keeping up the home for the tax year.
   In some couples, the greater earner may benefit from filing as married, while the lesser earner from not being married. For example, consider two single people, one with an income of $100,000 (and therefore paying a marginal rate of 28%) and the other with no income (and therefore paying no income tax). By being married and filing jointly, the $100,000 earner reduces his/her bracket to the 25% rate, receiving a "marriage bonus" for a net tax savings of $364, while the nonearner goes from the 10% bracket to the 25% bracket on the first dollars earned upon entering the workforce.
   Conversely, when progressive tax is levied on the individual with no consideration for the partnership, dual-income couples fare much better than single-income couples with similar household incomes. The effect can be increased when the welfare system treats the same income as a shared income thereby denying welfare access to the non-earning spouse. Such systems apply in Australia and Canada, for example.
  
  Section::::Post-marital residence.
   Proposals to "raise the cap" will continue to place an extra burden on 2-earner families where each partner has earned income (not capital gains or other property-based income that is exempt from the tax).
   In a family of two earners with even slightly different earnings rates, the lesser earner pays more taxes on marriage because s/he will incur a "marriage penalty" and the greater earner receives a "marriage bonus" because of "stacking effect" problems. Progressivity in earned income, passive income and capital gains rates, together with fictional income splitting for tax measurement purposes, creates this problem, which also means that the two-earner families subsidize sole breadwinner families. The tax system subsidizes a family, whether married or not, where the uncompensated work of the home is done by one partner, and all earnings are held by the other partner. Because of income splitting and certain progressive benefit distribution to dependent spouses through Social Security and Medicare, sole breadwinner families at the same levels of income as two-earner families pay substantially lower taxes. All two-earner families thus have to subsidize sole breadwinner families through the "marriage bonus" given to sole breadwinners and these social programs which distribute extra benefits to the sole breadwinner; in Social Security, for example, a sole breadwinner and nonearning spouse pay 1× taxes for 2× (or more benefits), while a marriage of two earners pays 2x taxes for 2× benefits. Over time, this puts accelerating pressure on a family to have only one partner with earned income and the other do unpaid work of the family.
   The marriage penalty can be even worse in cases where one spouse is not a citizen or resident of the United States . Although that spouse cannot be required by US law to pay US taxes, since the US person is still required by law to file taxes on worldwide income, two choices are left. The US person may either file as 'Married Filing Separately' (or 'Head of Household' if they have at least one qualifying person who is not their spouse) or try to convince their spouse to voluntarily pay US income taxes on their income by filing a joint return. The former requires using the 'Married Filing Separately' or 'Head of Household' tax brackets, which are less beneficial than 'Married Filing Jointly'. The latter allows that person to use the more favorable 'Married Filing Jointly' tax brackets but requires paying tax on the non-US person's income, which would not be required for two otherwise identical single people.
   The GRAPHICAL plateau range for Married Filing Jointly continues for five thousand dollars longer than does the plateau for the other filing statuses and thus MFJ can be advantageous for some income ranges. Single, Head of Household, and Qualifying Widow(er) are all equally valid and eligible filing statuses for claiming EITC. The only disqualifying status is Married Filing Separately. However, a couple can file as Married Filing Jointly even if they lived apart for the entire year if legally married and both agree.
  
  Section::::Impact.:Impact on health.
   BULLET::::- Increasing marriage penalty relief by raising the income threshold at which the EIC begins to phase out for married couples to $5,000 above the amount for unmarried filers, thereby giving MFJ filers a longer plateau. The combined plateau and phase-out range for married filing jointly is still not double that for single filers, and thus there still is a marriage penalty, just less than there used to be.
   For example, using 2009 figures, a filer with $358,800 of income not only gets zero exemption, but is also taxed on an "additional" $35,475 that was never actually earned (see "Line 29 — Alternative Minimum Taxable Income" in 2009 Instructions for Form 6251 or "Line 28 — Alternative Minimum Taxable Income" in 2010 Instructions for Form 6251). This prevents a married couple with dissimilar incomes from benefiting by filing separate returns so that the lower earner gets the benefit of some exemption amount that would be phased out if they filed jointly. When filing separately, each spouse in effect not only has their own exemption phased out, but is also taxed on a second exemption too, on the presumption that the other spouse could be claiming that on their own separate MFS return.
   The marriage penalty in the United States refers to the higher taxes required from some married couples with both partners earning income that would not be required by two otherwise identical single people with exactly the same incomes. There is also a marriage bonus that applies in other cases. Multiple factors are involved, but in general, in the current U.S. system, single-income married couples usually benefit from filing as a married couple (similar to so-called income splitting), while dual-income married couples are often penalized. The percentage of couples affected has varied over the years, depending on shifts in tax rates.
   In 2015, Portugal abolished the mandatory joint taxation of a family, establishing separate taxation for married (or "de facto" unions) taxpayers as the norm, with an option being available for joint taxation.
   The National Bureau of Economic Research reported a study showing "that two earner couples—the horses that pull our economic plow—pay for the second job with taxes that are far beyond the well known marriage penalty." The effective tax rates for these families are significant factors in the currently (2012) regressive tax system in the United States that President Barack Obama, U.S. Representative and former Presidential candidate Ron Paul, investor Warren Buffett, and investor George Soros and others have highlighted and criticized, although none has focused fully on the implications of the U.S. system being built around subsidizing sole breadwinners in federal taxes and benefits, including the relation to the federal debt.
  