A hen or a cock?

Is it possible to guess by the egg what will hatch: a hen or a cock?

This question has burned in scholars' minds since Aristotle's times. In spite of a great deal of research, there is no reliable method to determine the sex of a future chicken by the egg. But amateur poultry keepers think that if a year-old cock is introduced to four-year-old hens, the female share of the get will be 80%. If an old cock is allowed to copulate with young females, the majority of the get will be cocks.