In relation to marrying first-cousins in what is referred to as consanguineous marriages, then it is an issue often raised against the Muslims. In many Muslim cultures, it is not uncommon for a man to marry his first cousin. However, a number of scientific reports highlight that marrying first cousins leads to children being born with birth defects [https://www.bbc.com/news/uk-england-leeds-23183102]. Not to mention, it is illegal to do so in a few countries such as China, South Korea, Russia and 24/50 states of the US.
How do we reconcile the data that shows the harms of marrying a first cousin and the permissibliity of doing so in Islam? We can reconcile it in the following manner:
The data on the harms of marrying one’s first cousin are not conclusive, rather there are studies that prove that the harms are quite minuscule. In the most recent wide-scale study published by Columbia University (2018), researches found that marrying one’s first cousin does not necessarily harm their children [http://science.sciencemag.org/content/360/6385/171]. Data scientists did a quantitative study of 13 million individuals using family trees found on genealogy profiles. They concluded that children born from married cousins had an estimated 4-7% likelihood of birth defects compared to 3-4% of children born to parents who are distantly related. This percentage goes up if these children marry their first cousins and so forth. So marrying first cousins alone is not necessarily harmful, but doing so continuously may be. There are studies on both sides of the argument discussing the potential harm of this practice.
Islamically, it is permissible to marry a first cousin as Allah says: “All others [i.e. women who are not mentioned in the previous verse] are lawful, provided you seek (them in marriage) with Mahr (bridal money given by the husband to his wife at the time of marriage) from your property” [4:23]
The Prophet (ﷺ) married Zaynab bint Jahsh (may Allah be pleased with her) and she was his cousin [Siyar ‘Alaam an-Nubala (2/211)]
And the Prophet (ﷺ) married off his daugther Fatimah (may Allah be pleased with her) to her cousin Ali bin Abi Taalib (may Allah be pleased with him) [al-Bukhari no. 441]. Likewise, others from the Companions (may Allah be pleased with them) did so.
In fact, it has been mentioned that some of the notable Westerners married their first cousins, such as Charles Darwin and Albert Einstein.
While it is permissible in Islam to marry one’s first cousin, is not a Sunnah per se nor an emphasized practice. I am not aware of any evidence that says that a Muslim should marry their first cousin and that is is recommended over marrying those who are unrelated. And so, preferring and encouraging to marry first cousins is more of a cultural practice than an Islamic one.
Taking the potential harm into consideration, there is no contradiction between what some studies prove and what Islam permits. This is because some of the earlier Scholars understood that continuous inbreeding can cause birth defects and thus cautioned against the practice.
Imam ash-Shafi’ee (may Allah have mercy upon him) said:
لَيْسَ مِنْ قَوْمٍ لا يُخْرِجُونَ نِسَاءَهُمْ إِلَى رِجَالِ غَيْرِهِمْ فِي التَّزْوِيجِ ، وَلا رِجَالَهُمْ إِلَى نِسَاءِ غَيْرِهِمْ فِي التَّزْوِيجِ ، إِلا جَاءَ أَوْلادُهُمْ حَمْقَى
“There is not a group of people who do not bring forth their women to marry men other than their own men, nor do their men go forth to marry women other than their own, except their children will be fools (i.e. born with a mental defect)” [Adaab ash-Shaafi’ee wa Manaaqibuhu (pg. 99) with a Saheeh chain of narration]
Imam al-Maawardi (may Allah have mercy upon him) said: “They (i.e. the Arab) used to choose-for the likes of this situation-marrying those who are distantly related and are strangers (to each other). They saw that this is more conducive for childbirth and excellent for the creation (of a newborn).They used to avoid marrying family and relatives. They saw that it was harmful for the creation of the child and its birth” [Adab ad-Dunya wad-Deen (pg. 173)]
In sum, it is permissible to marry first cousins in Islam and there is little harm with that. In terms of continuous inbreeding of cousins, then it is from the Adab (etiquette) of those before us to avoid this out of fear of birth defects. It should be noted;however, that this is simply an Adab and not a prohibition. Everything is in the Hand of Allah, the Exalted and Sublime.
And Allah Knows Best
Faisal bin Abdul Qaadir bin Hassan