Why do sons inherit more than daughters in Islam?

Mostly because the Quran places the duty of financial support on men, while a woman's money stays her own. A son usually receives twice a daughter's share, but he is also responsible for supporting his wife, children and often his parents. Women's shares were a major advance when revealed, and in several cases women inherit the same as men.

Women were given a guaranteed share

In many societies of that time, women received nothing from the family estate. The Quran changed that:

لِّلرِّجَالِ نَصِيبٌ مِّمَّا تَرَكَ ٱلْوَٰلِدَانِ وَٱلْأَقْرَبُونَ وَلِلنِّسَآءِ نَصِيبٌ مِّمَّا تَرَكَ ٱلْوَٰلِدَانِ وَٱلْأَقْرَبُونَ مِمَّا قَلَّ مِنْهُ أَوْ كَثُرَ ۚ نَصِيبًا مَّفْرُوضًا "For men there is a share in what their parents and close relatives leave, and for women there is a share in what their parents and close relatives leave, whether it is little or much. These are obligatory shares." Quran 4:7

The 2:1 rule

For children, the Quran says:

يُوصِيكُمُ ٱللَّهُ فِىٓ أَوْلَـٰدِكُمْ ۖ لِلذَّكَرِ مِثْلُ حَظِّ ٱلْأُنثَيَيْنِ "Allah commands you regarding your children: the share of the male will be twice that of the female." Quran 4:11

This applies to sons and daughters when both inherit. For the full picture of the shares, see How does Islamic inheritance work?

Responsibilities explain the difference

Scholars explain the 2:1 ratio through who carries financial duties. Men are tasked with supporting their families (Quran 4:34). A husband must provide for his wife and children even if she is rich. A woman, by contrast:

Receives a dowry from her husband (Quran 4:4), which is hers to keep.
Has no obligation to spend her own money on the household.
Keeps her inheritance and her earnings as her own property (Quran 4:32).

So a brother's larger share often goes to supporting others, while a sister's smaller share stays entirely with her. To see how this works in marriage, see Does a Muslim wife have to share her salary?

Women do not always inherit less

The 2:1 ratio is only one case. There are situations where women get the same as men. For example, the Quran gives a mother and a father each one sixth when their child leaves children (Quran 4:11), and a deceased person's brothers and sisters from the mother's side share equally (Quran 4:12).

If it still feels unfair

Many people, Muslim and non-Muslim, find this rule hard to accept at first. It is fine to ask questions. Muslims accept it as divine guidance with wisdom, even where we do not see every detail. A parent can also give gifts during their lifetime, and scholars differ about whether these should be equal or follow the 2:1 ratio, so ask an imam. For Islam's broader teaching on women, see What does the Quran say about women?

Key points

The Quran guaranteed women a share when many societies gave them none (Quran 4:7).
A son's share is twice a daughter's (Quran 4:11).
Men carry the financial duties; women's money stays their own.
In several cases women inherit the same as men.

Tip: If you are writing a will, ask your imam about fairness in lifetime gifts to your children. It prevents misunderstandings later.