- The image of the pyramid represents a very small elite, very large poor populations, and a middle class that, while central in the pyramid, is not necessarily large. These are typically emerging countries with rising middle classes.
- The balloon-shaped graph shows a central and rising middle class, with a small privileged class and reduced poor categories. The image is one of common progress, driven by the dynamism of the middle class. The typical illustration is that of Western societies with an important middle class during the post-war boom. Most OECD countries look like this hot air balloon, even if it is now climbing at a slower rate, and, in some countries, the pod may come off.
- The hourglass is a metaphor for the erosion of the middle class, dislocated by the enrichment of its upper stratum and the impoverishment of its lower class. This could be due to the new industrial revolution, the crushing of hierarchies, and the splitting of production between high and low end. If the image of the hot air balloon indicates a collective elevation, that of the hourglass indicates a progressive decrepitude, associated with a dualisation. It is the promise of social advancement that is missing.
- The diamond image is more a project than a reality, that of a balanced society, with a very large middle class, and a very small elite and poor population. This is the Glorious Thirty’s dream project for France.
Evolution of the middle class: expansion in the developing world, crisis in the developed world
The global trend is undoubtedly towards emerging countries characterised by a large middle class, while the opposite phenomenon can be observed in wealthy countries. Among the many intervals used to define the middle class, the OECD used the one between $10 and $100 per day (in purchasing power parity) in its 2010 work. These studies reveal a global middle class, which accounted for a quarter of the world’s population in 2009, expected to represent 59% in 2030. Europe, which accounted for a third of the total in 2009, is expected to have only 14% by 2030, while 66% of the world's middle class is expected to be Asian by that date.