Fisher's principle is a biological principle which states that for the vast majority of sexually reproducing species, the evolutionarily stable sex ratio between males and females is 1:1.

Fisher's original 1930 argument for this principle was based on the theory of parental expenditure, but that argument has been displaced by the 1967 ESS-based argument by Hamilton.

The argument, which you can find on wikipedia, is as follows: 1) Suppose male births are less common than female. 2) A newborn male then has better mating prospects than a newborn female, and therefore can expect to have more offspring. 3) Therefore parents genetically disposed to produce males tend to have more than average numbers of grandchildren born to them. 4) Therefore the genes for male-producing tendencies spread, and male births become more common. 5) As the 1:1 sex ratio is approached, the advantage associated with producing males dies away. 6) The same reasoning holds if females are substituted for males throughout. Therefore 1:1 is the equilibrium ratio.

I argue that statement 2, and therefore the principle as a whole, is no longer true for humans living in an industrialized feminist society. The specific statement which is now invalid is "A newborn male then has better mating prospects than a newborn female, and therefore can expect to have more offspring."

In a modern society, males have less mate value than females, even if males are outnumbered, since females no longer depend on males for provision or protection, which were roles that historically women depended on men for. Even in a society where women outnumber men 20 to 1, men wouldn't necessarily experience any reproductive benefits by virtue of their rarity if the functional roles of their sex are assumed by women and displaced by modern technology. Women don't need men to provide for them if they can provide for themselves, or if machines and AI can provide the resources required for survival.

In other words, in a society where women don't depend on men for survival, men will simply never be scarce, no matter how few of them there are and the reproductive value of most men will be close to zero. In this system, any gene which predisposes a person to produce sons will have less reproductive success than a gene which predisposes a person to produce daughters. Thus, Fisher's principle will be violated and the evolutionarily stable sex ratio will greatly shift to favor female births over male births.