Pareto efficiency

Imagine slicing a cake among friends. Suppose any new way of cutting it that gives someone a bigger piece must take a piece from someone else. At that point you have squeezed out all the 'free' improvements — you cannot make anyone happier without making someone else sadder. That tipping point is named after the Italian economist Vilfredo Pareto, and it captures a very minimal idea of efficiency.

An allocation is Pareto efficient (or Pareto optimal) when there is no possible change that makes at least one person better off while making nobody worse off. A change that helps someone and harms no one is called a Pareto improvement; once none remain, you are at a Pareto-efficient point. The bar is deliberately low: it says nothing about fairness. A world where one person owns everything and everyone else owns nothing is Pareto efficient, because you cannot give the poor anything without taking from the rich.

Pareto efficiency matters because it is the gentlest possible definition of 'we cannot do better' — almost everyone can agree that we should grab Pareto improvements when they exist. The famous First Welfare Theorem says that, under idealized conditions, a competitive market reaches a Pareto-efficient outcome on its own — a precise version of Adam Smith's invisible hand. But the same theorem's many assumptions (no externalities, no market power, full information) are exactly the conditions whose absence defines market failure, and Pareto's silence on fairness is why economists pair it with separate ideas about equity.