Welcome to the forum, DannyITR.
This seems to be a little trick Sid plays on really big civilizations.
At emperor level, you start with 2 born-content people (the dark blue shirts in the city display) in every city. After those first two, each additional citizen is born unhappy (the black shirts).
At emperor level, you can build 12 cities with no ill effects. But then, the born-contents start to disappear. When you have 24 cities, each city will only have one born-content. By the time you have built 36 cities, there are no born-content citizens anywhere. About now the cure for cancer starts looking real good.
When you go beyond 36 cities, you start adding those red-shirted citizens.
Each 12 cities you build adds another red shirt in every city.
The good news about these guys is they respond well to luxuries. Two diamonds makes one go from red (very unhappy) to light blue (happy). The bad news is it's twice as hard to make them content. A cathedral only makes makes two of them content.
For Sid's next trick on the big civilizations, watch what happens when you discover future tech 65.