The arguments in favour of a flat tax argue that it is more 'fair' than other forms of taxation because it theoretically treats all persons and all forms of income equally by taxing them all at the same rate. This firstly fails to explain why any arbitrary percentage at which the tax is set is necessarily the 'fair' number and thus why everyone should receive the wonderful privilege of paying that exact number and not another based upon their income, expenditure and circumstances. The effect of the tax upon different individuals in different circumstances is thus key to the tax's supposed 'fairness', and to the undermining of this argument. For example, say both Mr Smith and Mr Jones earn £50,000 a year. However Mr Smith is a young man with few assets who relies upon his personal savings to finance a future business, and Mr Jones is an old man who has already built up or inherited £500,000 in assets. There is no clear reason why it is 'fair' for them to both pay the same rate of taxation despite their vastly different circumstances and the effect that the tax would have upon each of them, as Mr Jones' built-up wealth is protected, but Mr Smith's ability to build up assets and start a business in the first place is undermined. Or take the example of a sick man and a healthy man with the same income, but one of whom thus has much higher medical costs to pay. In the status quo they can be taxed differently (by allowing for income tax deductions for healthcare expenditure), however under a flat tax they would both be taxed the same, and it this would severely harm the sick man. Therefore, it seems counter-intuitive that in a world where all individuals are not the same and rates of taxation impact in highly different ways upon individuals depending on their differing circumstances (over which they may have little control), that every individual should pay exactly the same rate of tax, as if they were all the same. That hardly seems 'fair'.

Many of the so-called 'loopholes' which a flat tax would close, by ending the exemptions given when you engage in certain kinds of expenditure under the current income tax system, are actually positive features which incentivize 'good' economic behaviour. One of the great advantages from owning a home, for example, is the resulting ability to deduct mortgage interest payments from taxes. This makes owning a home more expensive, meaning a greater number of people will be able unable or unwilling to buy homes, and will thus be forced to rent instead. This harms their long-term economic prospects, as their mortgage payments would result in them eventually owning an asset whereas rent payments bring them no return, and as self-owned homes become in less demand, the value of the homes which hundreds of thousands of people have already spent decades paying mortgages for will plummet. This would also cause great harm to the construction industry as fewer people can afford to buy new houses. Another example of a useful 'loophole' is that profits and capital gains are currently taxed differently. Under a flat tax they would both be taxed equally, representing a kind of 'double taxation' which would hit most heavily new, young venture capitalists going into high-risk industries, and so will stifle investment and innovation. Finally, the current income tax deductions we allow for charitable giving would be 'closed' with a flat tax, and hence charitable giving would become less affordable and less attractive to most taxpayers.[1] Thus, the closing of these 'loopholes' will in fact disincentivize what we consider to be beneficial economic behaviour, leading to a worse economic state for everyone.

'Regressive' means that a tax impacts upon the poor more greatly than upon the rich, and this is exactly what occurs with a flat tax. Because everyone pays the same percentage, both a rich and poor man would for example pay 10% of their income in tax. As the poor spend a greater percentage of their income on their basic necessities (such as rent and food) than the rich do, as  the rich have far more discretionary  income to spend on luxuries.[1] Therefore, the impact of a 10% tax upon a poorer person is far greater in terms of limiting their ability to buy things they may want or need than it is upon a richer person, and consequently the harm of taxing a poorer person at the same rate as a richer person is greater than the harm of taxing a richer person at a higher percentage. Even if the 'personal allowance' allows the poorest in our society to exempt their income from the flat tax (which, of course, offers no relief to the middle class, who now pay a greater percentage tax on their income), they will still be significantly worse off as a consequence of the sales component of the flat tax. This again stems from the poorest spending a greater percentage of their income on necessities, which are not currently subject to sales tax (VAT). Once these VAT 'loopholes' (such as on books, children's clothing and food) are closed, the poorest will be harmed as they have to pay out even more to obtain the necessities of life. Both These increased harms breed resentment and can lead to social disorder, as was seen in the UK in 1990 when an attempt to introduce a 'poll tax' (a form of flat tax, with everyone paying the same charge) led to severe rioting in London and caused the plan to be abandoned.[2] Therefore the regressive nature of a flat tax makes it undesirable and more harmful than current forms of taxation.