Every tax does the same two things: it raises revenue and it distorts behaviour. The art of tax policy is getting the first with as little of the second — and as little unfairness and administrative cost — as possible. This course is about the design choices and the collection machinery that determine the outcome. The pure theory of incidence and optimal taxation is developed in the Public Finance course; here we build on it toward practice, starting with the criteria every tax must answer to.
The four criteria
What makes a tax good
Going back to Adam Smith's four maxims, a tax system is judged on: • Revenue adequacy — does it raise enough, reliably, to fund the state? • Efficiency (neutrality) — does it distort economic choices as little as possible per shilling raised? (The deadweight-loss minimisation of the Public Finance course.) • Equity — is it fair, both vertically (those with more pay more) and horizontally (equals are treated equally)? • Administrability (simplicity) — can it actually be collected at reasonable cost to the state and the taxpayer, and is it hard to evade? These pull against each other. The most efficient tax (a lump-sum poll tax) is grossly inequitable; the most finely equitable tax can be unadministrable. Tax design is the management of these trade-offs, not the maximisation of any one.
The developing-country tax-ratio gap
Rich countries collect 35–45% of GDP in tax; many African economies collect 12–18%. This gap is the central fact of tax policy in the region, and it is not mainly about rates — rates are often similar. It is about base and reach: a large informal economy the state cannot see, a narrow formal employment base for income tax, weak administration, and historically a reliance on a few easy-to-tax handles (the border, a few large firms, formal-sector wages).
The 15%-of-GDP threshold
Gaspar, Jaramillo, and Wingender (IMF, 2016) find evidence of a tipping point around a tax-to-GDP ratio of 12–15%: states that cross it tend to see accelerated growth and state-building, as if reaching a minimum fiscal capacity unlocks the ability to provide the public goods (order, infrastructure, courts) that sustain development. Below it, the state cannot do enough to escape the low-capacity trap (the Besley-Persson logic of the Governance course). Raising the tax ratio toward and past 15% is therefore not just a revenue goal — it is a development goal.
Capacity versus effort
Two countries with the same tax ratio may be in very different positions. Tax capacity is the revenue a country could raise given its economic structure (income level, openness, sectoral composition, urbanisation); tax effort is how much of that capacity it actually realises. A low ratio driven by low capacity (a poor, agrarian, informal economy) calls for a different response — build the base and the administration — than a low ratio driven by low effort (a country that could tax more but chooses not to, perhaps because it is funded by resource rents or aid, the Governance-course point). Diagnose which before prescribing.
The structure of African tax systems
Compared with rich countries, African tax systems lean more on indirect taxes (VAT, excise, fuel levy) and trade taxes, and less on personal income tax — a direct consequence of the narrow formal base. A long-running structural shift has been the move from trade taxes (easy to collect at the border, but distorting and shrinking as tariffs fall under WTO and AfCFTA commitments) toward domestic consumption taxes, especially VAT. Understanding a country's tax mix — and where the next reliable shilling can come from — is the practical starting point of every reform.
Exercise
A finance ministry proposes a new 1.5% levy on the value of all mobile-money transfers, attractive because mobile-money flows are large, digital, and easy to tap. (1) Evaluate the proposal against each of the four criteria. (2) Identify the central tension between two of the criteria in this case. (3) The country's tax ratio is 14% and it is trying to cross the 15% threshold. Does that change your assessment of whether to proceed? (4) Distinguish whether this is better understood as raising tax effort or expanding tax capacity.