My friend was amazed by this act of altruism, partly because - as he put it - ‘such acts of charity were not so common in my part of the world where people are not so kind to animals in general.’ But he was also amazed at his own reaction to the act. He was filled with a deep sense of peace. A strange quietness filled his surroundings, and he felt completely free of worry or anxiety. The sense of peace and joy remained with him for a few days, and, in his words, ‘I believe it is still there to some degree.’

This is a powerful example of an experience which most of us are familiar with, even though psychologists haven’t paid much attention to it so far. It’s the fantastic warm, elevated feeling we get when we witness acts of kindness. Even the most simple altruistic acts might give you a touch of this: a passer-by giving his packed lunch to a homeless man, a stranger offering to help a blind person cross the road, or a subway passenger giving up his seat for a old lady. Or when we see seeming acts of kindness amongst animals - for example, perhaps you could watch this short video now, and observe your response.

In this way, witnessing altruistic acts can be a source of what Abraham Maslow called ‘peak experiences’ - those moments of awe, wonder and a sense of ‘rightness’ which make us feel immensely grateful to be alive.