The Average Person and Social Physics - Quetelet's Contribution to Social Science

The Average Person
Adolphe Quetelet (Ghent, 1796 - Brussels, 1874) was working with averages observed in the statistics of his country. He studied phenomena such as crime, suicide and marriage and framed the many variables in measurable terms. His goal was to understand the statistical principles underlying these phenomena. Quetelet had harsh opponents, who found that his approach challenged free will.

Quetelet also studied the prevalence of obesity. He gave us the Quetelet index, nowadays better known as the body mass index (BMI). BMI is easy to calculate (weight in kg divided by the square of height in meters) and provides a quick indication of possible health risks associated with being under or overweight. The index is largely independent of gender and age in adults, and it's widely used in medical practice as an initial screening tool. After Quetelet's lifetime, populations from Asia and Oceania turned out to have a different fat percentage than Western populations for the same BMI.

Furthermore, working with averages is problematic when people don't correspond to the average. For instance, BMI doesn't take into account differences in body type, such as the ratio of muscle, bone and fat tissue. E.g. a bodybuilder can have a high BMI without being overweight or having a high fat percentage. In the elderly, bone mass decreases and people become smaller, which complicates BMI interpretation. But Quetelet developed his index to obtain statistics about groups of people and not to make individual health diagnoses. Despite its limitations, BMI remains a popular tool due to its simplicity and usefulness in everyday medical use. 

Social Physics
Quetelet believed that the regularities observed in social phenomena could be considered social laws, like the laws of physics. Quetelet’s idea of the average person was central to his vision of a 'social physics'. By studying large groups of people, Quetelet proposed, we could identify regularities that resemble physical facts. He wrote that these social regularities, while variable and influenced by numerous factors, still point towards underlying principles that can potentially be as precise as the laws of physics.

In 1835, Quetelet published his best known book, 'Sur l'homme et le développement ses facultés, ou Essai de physique sociale. In it, he wrote about:

- The physical and moral development of humans;

- The influence of different factors on birth rates, such as age, profession, and living conditions

- Mortality rates and their relation to factors like climate, profession, and social conditions;

- The impact of seasons and times of day on birth and death rates;
- The effects of civil and religious institutions on population dynamics;
- The importance of studying population growth and its relation to economic prosperity.

Unfortunately, the complexity and variability inherent in social phenomena, like obesity,  suicide and crime, make it challenging to establish social laws with the same degree of certainty as physical laws. Changing social conditions and the difficulty in isolating specific influences contribute to this complexity. Despite these challenges, Quetelet laid the foundation for the quantitative study of social life.

Book fragment 'Sur l'homme et le développement ses facultés', p. 8-13:

"The consistency with which the same crimes are committed annually in the same order and yield the same proportions of punishments is one of the most remarkable facts that court statistics reveal. (...) We can predict in advance how many individuals will stain their hands with the blood of their fellow humans, how many counterfeiters there will be, and how many poisoners, almost in the same way we can predict the births and deaths that will occur. Society carries within itself the seeds of all crimes that will be committed, and simultaneously the means necessary for their development. It is, in a sense, society that prepares these crimes, and the perpetrator is merely the instrument that executes them.

Each social state presupposes a certain number and order of the resulting offenses as a necessary consequence of its organization. This realization, which might seem discouraging at first glance, is instead comforting upon closer examination because it shows the possibility of improving people by modifying their institutions, habits, and, in general, everything that affects their way of being. Essentially, it presents us with an extension of a law well known to all philosophers concerned with society in its physical relationships: as long as the same causes exist, we must expect the return of the same effects. What might have led us to believe this was not the case with moral phenomena is the excessive influence always generally attributed to humans in all matters concerning their actions. A remarkable fact in the history of science is that the more enlightenment develops, the more we have seen the power attributed to humans shrink—a speck of dust floating unnoticed in space, an earthquake, or a flood is enough to wipe out an entire people or destroy the work of twenty centuries in an instant. Conversely, when humans seem more surrendered to their actions, we see them paying a regular tribute to nature in births and deaths each year. (...)

By observing the masses, moral phenomena thus seem to fall within the order of physical phenomena, leading us to accept as a fundamental principle in this kind of research that the larger the number of individuals we observe, the more individual peculiarities, both physical and moral, fade, and the series of general facts that sustain and maintain society prevail. It is only a few [people], endowed with superior genius, who can bring about a perceptible action on the social system; and even then, this action often takes a long time to fully manifest its effects.

If the changing action of humans were immediately transmitted to the social system, any form of prediction would be impossible, and we would search in vain for lessons in the past for the future. But this is not the case: when active causes have been able to establish themselves, they exert a noticeable action long after we have attempted to combat and destroy them; therefore, we cannot take enough care to highlight them and develop the most effective means to modify them in a useful way. This reaction of humans upon themselves is one of their noblest attributes; it is the most beautiful field in which one's activity can unfold. As a member of the social body, they subject themselves at every moment to the necessity of causes and regularly pay tribute to them; but as a human, using all the energy of their intellectual faculties, they somehow control these causes and modify their effects, striving for a better state."