What is the psychological basis for people coming together during a disaster or major event? Something I have witnessed several times, and got me wondering, what is the psychological basis for people coming together in response to a disaster or a major event?  Be it large scale or local.

An example is one that I was right in the middle of (and at one stage was in very serious danger) - the Queensland floods of 2011, which is encapsulated nicely in this video-editorial "Hope Lives Here". Now, a couple of points from the editorial - the narrator states that as the city of Brisbane has grown, people have become more distant and are not likely to even know their neighbours' names.

This is obviously not just pertinent to the example above, but is repeated in many places.

However, when push comes to shove, and when a community of strangers are faced with a catastrophe, destruction, loss and are basically 'forced into a corner' - this is when the community comes together. Following the example, a 'Mud Army' started up of people helping strangers get back on their feet (I was involved in that, despite just coming out of surgery a few weeks before).

When the danger passes, life goes back to normal (generally) and we all seem to go our separate ways.

What is the psychological basis (more in depth than the linked editorial) for why we come together in a disaster or major event?
This article might be helpful. In particular, humans need social connections (according to Maslow's hierarchy of needs). Stress initiates people to come together. It was thought that women would come together during disaster and men would fight (i.e. the fight and flight response). However men also come together during disaster.

A distinction must be made between acute and chronic stress. Acute stress (e.g. once in a life time natural disasters) promote social bonding and connection. Chronic stress (e.g. smoking, ruminating a lot, etc.), however, does not seem to produce the same effect. When the acute stress is removed, there is no imminent need for social bonding with strangers.

From a biological basis, it seems that oxytocin might play a key role. In particular, oxytocin is released as a reaction to a social experience. So if a people see others in distress they may release oxytocin which promotes prosocial behavior.