Adolphe Quetelet

The Average Man · Astronomer & Statistician · 1796–1874

The Brussels astronomer who turned the mathematics of measurement error onto human beings, invented the average man, and made the regularity of social numbers an argument that society has laws.

Who was Quetelet?

Adolphe Quetelet was born at Ghent in 1796 and took the first doctorate in science awarded by the new University of Ghent, on conic sections. He wrote poetry, translated, and painted before mathematics settled the matter. In 1823 he persuaded the government to fund an observatory at Brussels and was sent to Paris to learn how such places are run. He came back with more than instruments. Laplace and Fourier had shown him the theory of errors: how repeated measurements of the same quantity scatter around the true value in a regular and predictable shape. It occurred to him that the same shape appears where there is no true value being measured at all. The chest circumferences of five thousand Scottish soldiers fall into it. So do the heights of French conscripts. From that he built a programme. In 1835 he published Sur l'homme et le développement de ses facultés, essai de physique sociale, and its central figure is l'homme moyen, the average man — a type about whom a population is arranged, with individual departures from him treated in the same way as errors of measurement. He showed that the yearly numbers of marriages, of suicides and of crimes are remarkably steady, including the numbers committed by particular means, and argued that this steadiness cannot be explained by individual choice and therefore points to laws of society. He called the enterprise social physics. He organised the first International Statistical Congress in 1853, advised governments across Europe on the conduct of censuses, and devised the index of weight divided by the square of height that is still in use. The average man was attacked in his own lifetime, most sharply by Cournot, on the ground that the average of a set of individuals need correspond to no possible individual — the average of the sides of a set of right triangles is not itself a right triangle. Quetelet never fully met the objection. His curve also passed to Francis Galton, who used it in the service of eugenics, a direction Quetelet would not have chosen and cannot be entirely absolved of preparing.

Major achievements

• Founded and directed the Royal Observatory of Brussels
• Showed that human measurements fall into the same regular curve as measurement errors
• Published Sur l'homme in 1835, founding the statistical study of society
• Demonstrated the year-on-year steadiness of social numbers such as crimes and marriages
• Organised the first International Statistical Congress in 1853

Life in brief

• 1796 — Born at Ghent: The first doctorate in science from the new university.
• 1823 — Paris: Sent to learn observatory practice; returns with the theory of errors.
• 1828 — The Brussels Observatory: Founded on his persuasion, and directed by him for life.
• 1835 — Sur l'homme: Social physics, and the average man.
• 1846 — The Belgian census: Conducted on his plan, and a model for others.
• 1853 — The first statistical congress: Organised at Brussels, the beginning of international comparability.
• 1874 — Died at Brussels: After a stroke had limited his work for nearly two decades.

Explore the life of Quetelet — five chapters

1. An Observatory, and a Trip to Paris — Where the method came from
2. Five Thousand Scottish Soldiers — The same curve, with nothing being measured
3. The Average Man — 1835, and the objection he could not answer
4. The Budget of the Scaffold — Crimes counted, and a claim about society
5. What Galton Did Next — A curve handed on

Read the five-chapter life of Adolphe Quetelet →

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