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Runaway sexual selection posits that extreme male traits (such as the male peacock's tail, or the huge antlers of the now-extinct Irish Elk) can evolve through a process in which the male trait and the female preference for that trait become genetically linked. The male trait does not necessarily have to be beneficial to the male in any way, all it has to do is to attract females.

When males with the trait mate with females that prefer the trait, the trait AND the preference for the trait are inherited by all their offspring, whether male or female. In a "runaway" process, fueled by the positive-feedback-loop of ever-exaggerated male traits and ever-increasing female preference, the male trait becomes more and more extreme, until the natural-selection disadvantages of the trait (such as a tail so large that the male peacock cannot move) outweigh the sexual-selection advantages.