In-group Quotes

[In-group exclusivism has] killed more human beings and destroyed more cities and villages than all the epidemics, hurricanes, storms, floods, earthquakes, and volcanic eruptions taken together. It has brought upon mankind more suffering than any other catastrophe. (Pitirim Sorokin)
Religious ritual invariably offers a sacrificial display of commitment to supernatural agents that is materially costly and emotionally convincing. Such ritual conveys willingness to cooperate with a community of believers by signaling an open-ended promise to help others whenever there is true need. These expensive and sincere displays underscore belief that the gods are always vigilant and will never allow society to suffer those who cheat on their promises. Such a deep devotion to the in-group habitually generates intolerance toward out-groups. This, in turn, leads to constant rivalry and unending development of new and syncretic religious forms.
Scott Atran
This breaking of the limitations of hospitality to a small in-group, of offering it to the broadest possible in-group, and saying, you determine who your guest is, might be taken as the key message of Christianity.
Ivan Illich
Dividing into teams doesn't necessarily mean denigrating others. Studies of groupishness have generally found that groups increase in-group love far more than they increase out-group hostility.
Jonathan Haidt
I did say that in-group, authority and purity are necessary for the maintenance of order, but I would never give them a blanket endorsement.
Jonathan Haidt
Let me give you another, the way in which group membership can entrain language-usages that are self-deceptive. You can divide people into in-groups or out-groups, or use naturally occurring in-groups and out-groups, and if someone's a member of your in-group and they do something nice, you give a general description of it – "he's a generous person".
Robert Trivers