Through most of our prehistory, humans lived under conditions that would have strongly favoured the evolution of all four kinds of altruism. We lived in villages, or earlier in discrete roving bands like baboons, partially isolated from neighbouring bands or villages. Most of your fellow band members would have been kin, more closely related to you than members of other bands – plenty of opportunities for kin altruism to evolve. And, whether kin or not, you would tend to meet the same individuals again and again throughout your life – ideal conditions for the evolution of reciprocal altruism. Those are also the ideal conditions for building a reputation for altruism, and the very same ideal conditions for advertising conspicuous generosity. By any or all of the four routes, genetic tendencies towards altruism would have been favoured in early humans. It is easy to see why our prehistoric ancestors would have been good to their own in-group but bad – to the point of xenophobia – towards other groups. But why – now that most of us live in big cities where we are no longer surrounded by kin, and where every day we meet individuals whom we are never going to meet again – why are we still so good to each other, even sometimes to others who might be thought to belong to an out-group?

It is important not to mis-state the reach of natural selection. Selection does not favour the evolution of a cognitive awareness of what is good for your genes. That awareness had to wait for the twentieth century to reach a cognitive level, and even now full understanding is confined to a minority of scientific specialists.

What natural selection favours is rules of thumb, which work in practice to promote the genes that built them. Rules of thumb, by their nature, sometimes misfire. In a bird’s brain, the rule ‘Look after small squawking things in your nest, and drop food into their red gapes’ typically has the effect of preserving the genes that built the rule, because the squawking, gaping objects in an adult bird’s nest are normally its own offspring. The rule misfires if another baby bird somehow gets into the nest, a circumstance that is positively engineered by cuckoos. Could it be that our Good Samaritan urges are misfirings, analogous to the misfiring of a reed warbler’s parental instincts when it works itself to the bone for a young cuckoo? An even closer analogy is the human urge to adopt a child. I must rush to add that ‘misfiring’ is intended only in a strictly Darwinian sense. It carries no suggestion of the pejorative.

The ‘mistake’ or ‘by-product’ idea, which I am espousing, works like this. Natural selection, in ancestral times when we lived in small and stable bands like baboons, programmed into our brains altruistic urges, alongside sexual urges, hunger urges, xenophobic urges and so on. An intelligent couple can read their Darwin and know that the ultimate reason for their sexual urges is procreation. They know that the woman cannot conceive because she is on the pill. Yet they find that their sexual desire is in no way diminished by the knowledge. Sexual desire is sexual desire and its force, in an individual’s psychology, is independent of the ultimate Darwinian pressure that drove it. It is a strong urge which exists independently of its ultimate rationale.

I am suggesting that the same is true of the urge to kindness – to altruism, to generosity, to empathy, to pity. In ancestral times, we had the opportunity to be altruistic only towards close kin and potential reciprocators. Nowadays that restriction is no longer there, but the rule of thumb persists. Why would it not? It is just like sexual desire. We can no more help ourselves feeling pity when we see a weeping unfortunate (who is unrelated and unable to reciprocate) than we can help ourselves feeling lust for a member of the opposite sex (who may be infertile or otherwise unable to reproduce). Both are misfirings, Darwinian mistakes: blessed, precious mistakes. Do not, for one moment, think of such Darwinizing as demeaning or reductive of the noble emotions of compassion and generosity.

Nor of sexual desire. Sexual desire, when channelled through the conduits of linguistic culture, emerges as great poetry and drama: John Donne’s love poems, say, or Romeo and Juliet. And of course the same thing happens with the misfired redirection of kin- and reciprocation-based compassion. Mercy to a debtor is, when seen out of context, as un-Darwinian as adopting someone else’s child:

The quality of mercy is not strained.
It droppeth as the gentle rain from heaven
Upon the place beneath.

Sexual lust is the driving force behind a large proportion of human ambition and struggle, and much of it constitutes a misfiring. There is no reason why the same should not be true of the lust to be generous and compassionate, if this is the misfired consequence of ancestral village life. The best way for natural selection to build in both kinds of lust in ancestral times was to install rules of thumb in the brain. Those rules still influence us today, even where circumstances make them inappropriate to their original functions.

Such rules of thumb influence us still, not in a Calvinistically deterministic way but filtered through the civilizing influences of literature and custom, law and tradition – and, of course, religion. Just as the primitive brain rule of sexual lust passes through the filter of civilization to emerge in the love scenes of Romeo and Juliet, so primitive brain rules of us-versus-them vendetta emerge in the form of the running battles between Capulets and Montagues; while primitive brain rules of altruism and empathy end up in the misfiring that cheers us in the chastened reconciliation of Shakespeare’s final scene.

* I was mortified to read in the Guardian (‘Animal Instincts’, 27 May The Selfish Gene is the favourite book of Jeff Skilling, CEO of the infamous Enron Corporation, and that he derived inspiration of a Social Darwinist character from it. The Guardian journalist Richard Conniff gives a good explanation of the misunderstanding: http://money.guardian.co.uk/workweekly/story/0,,1783900,00.html. I have tried to forestall similar misunderstandings in my new preface to the thirtiethanniversary edition of The Selfish Gene, just brought out by Oxford University Press.

For full text of The God Delusion go to: http://uath.org/download/literature/RichardDawkinsGodDelusion.pdf