[From a video script I wrote in 2019]

In a close community, there are rules of propriety that are kept regardless of whether they are written.  The reputation of a person prevents the person from acting too selfishly.  In a city, where the population is too large for the ordinary interactions to result in personal reputation changes, there is less incentive to act unselfishly, and there is less incentive against acting selfishly.  
You can’t change this by simply being a “good person” in a city.  And, if you act as a nice person in a city or if you expect reciprocity in your business dealings in cities, you will most likely be taken advantage of.  The pressure of cities towards selfishness means the majority of people in the city will be acting much more selfishly than they would in small communities.

This is reflected in game theory.  In the prisoner’s dilemma game, [https://www.nature.com/scitable/knowledge/library/game-theory-evolutionary-stable-strategies-and-the-25953132/ figure 2], when prisoners are randomly paired, not expecting to encounter each other again, it is most beneficial to rat the other prisoner out [figure 2b].  When the prisoners are paired with each other frequently, it is most beneficial for each prisoner to stay silent [figure 2c].  Since, in the second case the prisoners can expect to encounter each other again, the reciprocity of staying silent works.  The situation we see in cities is much like the case of randomly paired prisoners, where it is most advantageous to be maximally selfish.
Similarly, when a society’s culture decays, you can expect the same lack of reciprocity, and the same level of selfishness throughout.  

This natural tendency for cities to promote selfishness can be combated with culture and laws.  But, instead, the elite of the day seem to wish to destroy culture and cause class warfare to prevent upward mobility; consequently, cities remain generators of selfishness.  In a selfish society, which tends to reflect selfish cities, do not be nice.  Instead of being generally nice, focus on forming groups with each other where reciprocity is present.