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allows people to produce surplus food and thereby avoid starvation in times of drought or scarcity. Everywhere agriculture has been invented—and it has been invented many times independently around the world—population sizes have increased. But what makes agriculture interesting is that people’s health actually declined after it was introduced. Skeletons dug up from some of the first societies to acquire agriculture, for example in parts of Turkey and Egypt, show that people got smaller, their bones were less sturdy, they often had skeletal deformities from the hard labor of grinding corn, and they lived shorter lives. Does this mean that agriculture is a set of exploitative memes? Probably not. On average, people who adopted agriculture left more surviving offspring behind. Agriculture has been good for our genes, and this is almost certainly why it has all but replaced hunting-and-gathering all around the world. Natural selection does not maximize happiness or even well-being, but rather long-term reproductive success.
    There is another reason we can expect to have good defenses against exploitative memes or ideas. Richard Dawkins and John Krebs pointed this out some years ago in the context of biological predators and their prey. It goes by the name “the life-dinner principle,” a variant of one of Aesop’s Fables . Aesop knew that when a fox chases a rabbit, the rabbit is running for its life but the fox is only chasing its dinner. This tells us that we can expect natural selection to act more strongly on rabbits to evade foxes than on foxes to catch rabbits. A fox that fails to catch a rabbit can look for another dinner. But there are no more lives for the rabbit that gets caught. An ant that evades a brain fluke saves its life, but the brain fluke can look for another ant to infect. People with psychological defenses against the idea of celibacy preserve their chances of reproducing, but celibacy as an idea can always seek out another mind to infect.
    The life-dinner principle gives us a way to understand which of two sets of replicators is likely to “win” when one of them sets out to use the other for its own gain. Winning doesn’t necessarily mean driving the loser to extinction, just that on average the winner’s adaptations are not bested by the loser’s. One rough way to predict the winner in any given arms race is to ask if one side stands to lose more than the other. The life-dinner principle tells us to bet on the one winning which has most to lose if it doesn’t. Genes for running fast in rabbits have more to lose if they are not fast enough than genes for running fast do in foxes. A fox gene that is not fast enough to catch this particular rabbit can always seek out another. Genes in our bodies might have more to lose if manipulated by a celibacy or suicide meme than the meme has in failing to infect our minds. The memes can always find other brains.
    Still, why don’t manipulative memes just win outright? They can always evolve new ways to exploit us faster than our genes can evolve psychological defenses to deflect them. But can they? The same could be said of most biological parasites and yet they don’t always win. For example, the rapid rate of evolution of the influenza virus is why you are advised to get vaccinated every year, but even if you don’t, there is a good chance you won’t be infected. In spite of their prodigious abilities to evolve, the viruses don’t always win because each of us carries inside our bodies a miniature Darwinian evolutionary system that is evolving in real time. This is our immune system, and it protects us by generating an effectively infinite variety of different immune cells, each one capable of recognizing a different kind of attacker. Natural selection acting inside our bodies favors those immune cells good at defending us, encouraging more copies of them to be made, and allowing the others to fade away. This evolutionary process is going on inside