God's utility function is a phrase coined by Richard Dawkins in his book River Out of Eden. "God's utility function" is the third chapter in this book. Dawkins uses this phrase to expound the Gene-centered view of evolution by equating the phrase to the meaning of life or the purpose of life. This is the why question about life which philosophers and theologians have been pondering in vain for ages, and is a counterpart to the how question about nature which engineers have been able to resolve successfully.

Dawkins first recounts how Charles Darwin lost his faith in religion by quoting his words, "I cannot persuade myself that a beneficent and omnipotent God would have designedly created the Ichneumonidae with the express intention of their feeding within the living bodies of Caterpillars." We ask why a caterpillar should suffer such cruel punishment. We ask why digger wasps couldn't first kill caterpillars to save them from a prolonged and agonizing torture. We ask why a child should die an untimely death. And we ask why we should all grow old and die.

Dawkins rephrases the word purpose in terms of what economists call a utility function, meaning "that which is maximized". Engineers often investigate the intended purpose (or utility function) of a piece of equipment using reverse engineering. Dawkins uses this technique to reverse-engineer the purpose in the mind of the Divine Engineer of Nature, or the Utility Function of God.

According to Dawkins, it is a mistake to assume that an ecosystem or a species as a whole exists for a purpose. In fact, it is wrong to suppose that individual organisms lead a meaningful life either. In nature, only genes have a utility function – to perpetuate their own existence with indifference to great sufferings inflicted upon the organisms they build, exploit and discard. As hinted at in chapter one, genes are the supreme lords of the natural world.

An ecosystem, a biome, a savannah or a forest as a whole does not possess a utility function. This is evident when one examines interactions between creatures in these systems. If a savannah had a utility function, cheetahs would have had no need to waste energy running as fast as a Lamborghini and antelopes would have found it unnecessary to waste resources trying to escape a gruesome death. If a forest had a utility function, trees in it would not have evolved to ridiculously heights in an attempt to out compete other species for sunlight. If everybody would just sit down and have a civilized discourse, benefits to the whole system can be maximized while energy spent is minimized. It seems that every species has its own interest before that of the community.

However, if a species as a whole had a utility function, the sex ratio for animals with a harem system (such as the elephant seals) would not have stayed at the common 50:50 ratio. In such harem systems, a few males monopolize all mating opportunities while the majority of males remain bachelors. This does not make economic sense; a factory with 10 lathes would not hire 100 employees and let them fight out which lucky 10 can work the lathes for a given day. The factory should fire 90 employees, and so should the species reduce the number of males. But this is not what happens in nature. Since mating always involves one male and one female, statistically a son and a daughter should yield equal reproductive success to a parent in evolutionary time. Thus a 50:50 ratio of offspring makes sense for an individual parent seal, according to the parental expenditure theory of Sir Ronald Fisher. Again, it seems that every individual has its own interest before that of the species

But, individual organisms are not masters of themselves. If an organism had a utility function, it would have chosen to remain young forever and not to die of old age. Mayflies would very much prefer to have guts so they would not starve to death within hours of emerging from water and completing copulation. Pacific Salmon would rather not die a certain death just days after their first spawning. But this is not what happens in nature. Women lose calcium to babies during pregnancy and in milk production, a lesser form of sacrifice for the sake of their children.

All above dilemmas can be resolved, if one thinks of utility functions from the perspective of DNA and genes. As long as an organism survives its childhood and manages to reproduce thus passing its genes down to the next generation, what happens to the parent organism afterwards does not really bother genes. Because an organism is always at the danger of dying from accidents (a waste of investment), it pays for the genes to build an organism which pools almost all its resources to produce offspring as early as possible. Thus we accumulate damages to our body as we age and harbor late-onset diseases such as Huntington's disease which have minimum impact on the evolutionary success of our gene overlords.

Genes are pitilessly indifferent to who or what gets hurt, so long as DNA is passed on. And Dawkins wrote at the end:

During the minute it takes me to compose this sentence, thousands of animals are being eaten alive; others are running for their lives, whimpering with fear; others are being slowly devoured from within by rasping parasites; thousands of all kinds are dying from starvation, thirst and disease. It must be so. If there is ever a time of plenty, this very fact will automatically lead to an increase in population until the natural state of starvation and misery is restored.

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