The Base Matters More Than the Rate

Every tax has a rate and a base. Public argument is almost entirely about the first and almost all of the revenue is decided by the second, which is why comparing headline rates across countries tells you very little about how much anybody collects.

The base is the thing being taxed, and its definition is where the work happens. A corporate tax applies to profit, but profit is not a fact in the world like a temperature. It is a legal construct: revenues minus whatever the law says may be deducted. Change the depreciation schedule, the treatment of interest, the rules on carrying losses forward, and the same underlying business reports a different number. Two countries can share a headline rate of twenty-five per cent and collect proportions of national income that differ by a factor of two.

Beyond definition there is mobility. A base that can move, wait, or relabel itself will do at least one of those when taxed. Financial income can be realised in a different year. Reported profit can be assigned to a different jurisdiction through a licence fee. A skilled worker can emigrate. A plot of land can do none of these, and neither, mostly, can consumption: people who live somewhere buy things there.

Frank Ramsey formalised the efficiency consequence in 1927. Tax inelastic things more heavily, because doing so changes behaviour least, and behaviour changed by a tax is the deadweight loss: value that does not appear anywhere, neither in private hands nor in the treasury. It is not transferred; it simply does not happen.

The rule is correct and it is not a guide to policy, which is worth being clear about. The most inelastic consumer purchases are the ones people cannot do without: heating, staple food, medicine. Following Ramsey strictly would tax them hardest, for the precise reason that the people buying them have no alternative. Efficiency and fairness are separate criteria, the rule addresses only the first, and no democratic tax system follows it closely. What it provides is not a recommendation but an accounting of what a given choice costs.

The clearest case of a base that cannot escape is land. Its supply is fixed regardless of the tax rate, so the owner cannot respond by producing less of it, and a tax on its value therefore has no deadweight loss at all. Economists have agreed on this across every political division for two centuries. It is collected at serious scale almost nowhere, which is a useful measurement of how much tax design is actually driven by efficiency arguments as against by who owns what.

The practical lesson for reading any tax proposal is to find the base before looking at the rate. A large rate on a base that has been defined into near-nonexistence is theatre, and a modest rate on a broad base that cannot move is money. This is also why revenue forecasts so often disappoint: they are produced by multiplying a rate by the current size of a base, and the base does not stay still while the multiplication is performed.