Why does a son receive twice the share of a daughter in faraid?

This ruling comes directly from the Qur'an: a son receives two shares to a daughter's one share when they inherit together as residuary heirs (asabah). The wisdom explained by scholars is that a man's financial responsibility is heavier: he must provide maintenance for his wife, children and sometimes his parents and sisters, while a woman's property remains her own. If the deceased leaves only daughters, one daughter receives 1/2, while two or more share 2/3. This is not a measure of human worth, but a distribution balanced against the burden of responsibility. Parents who wish to help a daughter may give a hibah fairly during their lifetime. See the faraid guide.