per capita

Per capita combines the Latin preposition per (through, by means of, for each) with capita, the accusative plural of caput (head). Caput is one of the most productive words in Latin: it means head in the physical sense, but also the top of something, the source of a river, the chief city of a territory, the leader of a group, the life or person of an individual (as in 'capitis deminutio' — loss of legal status, literally a diminishment of the head). Caput gives English 'capital,' 'captain,' 'chapter,' 'cattle,' 'chattel,' 'achieve' (via Old French achever, 'to bring to a head'), 'chef,' 'chief,' 'cadet,' and 'cap.' The phrase per capita means 'by heads,' 'for each head,' or 'for each person' — the head standing in for the person who possesses it.

In Roman law, per capita was contrasted with per stirpes as the two methods of distributing an estate among heirs. Per stirpes (by roots, by stocks) distributed the estate through family lines, with each branch receiving an equal share regardless of how many individuals were in that branch. Per capita distributed equally among all surviving heirs individually, regardless of their relationship to the deceased. If a man died leaving two children and three grandchildren (from a predeceased third child), per stirpes would give each child a third and divide the remaining third among the three grandchildren; per capita would give each of the five survivors an equal one-fifth share. Roman inheritance law allowed testators to choose between the methods, and the choice reflected different theories of the proper relationship between family branches and individual persons.

The per capita calculation — dividing a total by the number of persons to find the per-person amount — has become the fundamental unit of comparative economic measurement. Gross domestic product per capita is the standard measure of a country's economic output per person, used to compare living standards across countries and over time. Per capita income, per capita debt, per capita CO2 emissions, per capita healthcare spending — the calculation is applied to virtually every quantity that can be meaningfully divided by population. The World Bank, the IMF, the United Nations, and every national statistical agency use per capita as a fundamental analytical category. The Roman head-count for estate division became the basis for international development economics.

The per capita approach to measurement embeds a specific assumption that is worth making explicit: it assumes that the quantity being measured is, or should be, equally distributed among persons, and that the per-person average is therefore a meaningful description of individual circumstances. This assumption is challenged by economists and social scientists who note that averages conceal distributions — a country with very high per capita GDP may have enormous inequality, making the average a poor description of what most people actually experience. The Gini coefficient, the poverty rate, and the median income all exist in part to supplement and correct the limitations of per capita averages. The Roman lawyers who debated per capita versus per stirpes were arguing about a similar problem: how do you divide something among persons when the persons themselves stand in different relationships to the thing being divided?