Ronald Fisher, the English biologist developed a number of ideas concerning secondary characteristics in his 1930 book, including the Fisherian runaway, which postulates that the desire for a characteristic in females combined with that characteristic in males can create a positive feedback loop or runaway where the feature becomes hugely amplified. The 1975 handicap principle extends this idea, pointing out that a peacock's tail, for instance, displays fitness by being a useless impediment that it is very hard to fake. Another Fisher idea is the sexy son hypothesis, whereby females will desire to have sons that possess the characteristic that they find sexy in order to maximize the number of grandchildren they produce.[2] An alternative hypothesis is that some of the genes that enable males to develop impressive ornaments or fighting ability may be correlated with fitness markers such as disease resistance or a more efficient metabolism.