It's a small act of altruism that changes not only their world view but the space-time continuum.

The theory also holds that the act of altruism should be reciprocated if the balance of needs reverse.

Hence, any act of altruism would be directed primarily towards kin.

The study revealed that nearly three quarters (73 percent) of programs contained at least one act of altruism and on average viewers saw around three acts of altruism an hour.

Nonetheless, based on their actions, we can perhaps speculate on the motives that underpin those small acts of altruism whose rarity we all bemoan.

Taking all this trouble to build such a sympathetic environment was not done out of pure altruism but to improve the organisation.

Does 'the giver's high' negate the concept of pure altruism?

History suggests that there a many reasons why the wealthy endow schools - such as enshrining their good name, giving something back to their community or pure altruism.

I offer this plan to you out of pure altruism (though this may be a paradox too far) because the system would have been disastrous for me.

Nobody pretends that the retailers' attempts to green themselves are pure altruism.

Reciprocal altruism shows that such income plays an important social role.

So alongside reciprocal altruism there evolved ways of identifying people who would - if you did a favour for them - do a favour back.

What other human behaviour has its origins in reciprocal altruism?

Surely, cooperation and reciprocal altruism - so central for the evolution of our societies - would be better served if we removed the obstacles for cooperation between groups by speaking fewer languages?

We pass on our genes as individuals, but we survive only as groups, which depend on habits of reciprocal altruism and, at times, self-sacrifice.