Even our behavior and emotions seem to have been shaped by a prankster. Why do we crave the very foods that are bad for us but have less desire for pure grains and vegetables? Why do we keep eating when we know we are too fat? And why is our willpower so weak in its attempts to restrain our desires? Why are male and female sexual responses so uncoordinated, instead of being shaped for maximum mutual satisfaction? Why are so many of us constantly anxious, spending our lives, as Mark Twain said, "suffering from tragedies that never occur"? Finally, why do we find happiness so elusive, with the achievement of each long-pursued goal yielding not contentment, but only a new desire for something still less attainable? The design of our bodies is simultaneously extraordinarily precise and unbelievably slipshod. It is as if the best engineers in the universe took every seventh day off and turned the work over to bumbling amateurs.