Lambert Adolphe Quetelet (1796-1874), the Belgian mathematician, is famous world-wide for the invention of his 'Body Mass Index' (also called the 'Quetelet Index') which remains the official measurement for obesity to this day. The method is only valid for people over the age of 20. If the result is greater then 30 then a person is officially obese.
The formula for calculating the Quetelet index (QI) is:
weight in kilograms / height in metres²
In non-metric measurements, the formula becomes:
(weight in pounds) * 703 /
height in inches²
One science was not enough, however, for Quetelet. Starting around 1830, he became heavily involved in statistics and sociology. Quetelet was convinced that probability influenced the course of human affairs more so than earlier generations had and more so than his contemporaries did. Astronomers had used the law of error to gain accurate measurement of phenomena in the physical world.
Quetelet believed the law of error could be applied to human beings. If the phenomena analyzed were part of human nature, Quetelet believed that it was possible to determine the average physical and intellectual features of a population. Through gathering the "facts of life," the behaviour of individuals could be assessed against how an "average man" would normally behave. He believed it possible to identify the underlying regularities for both normal and abnormal behaviour.
"Average man" could be known from graphically arraying the facts of life as bell shaped curves. It is commonly accepted that Quetelet was the first to come up with the term "average man".
Quetelet had come to be known as the champion of a new science, dedicated to mapping the normal physical and moral characteristics. Quetelet called it social mechanics. He published a detailed account of the new science in 1835 which he titled Sur l'homme et le developpement de ses facultés, essai d'une physique sociale. This was a lengthy account of the influence of probability over human affairs.
Quetelet thought more of "average" physical and mental qualities as real properties of particular people or races awaiting discovery and not just abstract concepts. Quetelet helped give cognitive strength to ideas of racial differences in nineteenth century European thought. His conception of "average man" is the central value about which measurements of a human trait are grouped according to the normal curve. In some of Quetelet’s later work, "average man" is presented as an ideal type, as if nature were shooting at the "average man" as a target and deviations from this target were errors. Cournot and others criticized the concept. An individual average in all dimensions might not even be biologically feasible, they argued.
In 1846 he published a book on probability and social science that demonstrated as diverse a collection of human measurements as the heights of French conscripts and the chest circumferences of Scottish soldiers could be taken as approximately normally distributed. The use of the normal curve in areas so far from astronomy and geodesy had a powerful influence on Francis Galton and may have influenced James Clark Maxwell in his formulating of the kinetic theory of gases. Quetelet believed that if the investigator took care to ensure that they had obtained accurate measurements of individuals belonging to a particular race or nationality, it would be possible to determine any unknown physical or intellectual aspect of the population under investigation.
Quetelet was the first to use the normal curve other than as an error law. His studies of the numerical consistency of crimes stimulated discussion of free will versus social determinism. He collected and analyzed statistics on crime, mortality, etc. for the government and devised improvements in census taking.
Quetelet organised the first international statistics conference in 1853. He died in Brussels on 17 February 1874.