The studies that speak about a limited increase in recessive hereditary diseases in marriages between direct first cousins generally mention an increase of approximately 1.7% to 2.8%, meaning less than 3% above the normal baseline already present within the general population, as mentioned in a reference paper published in the journal “Genetics in Medicine”.

This specifically relates to autosomal recessive hereditary disorders.

As for other illnesses and conditions, some studies observed a slight increase in certain cases, while others did not observe such an increase. However, in all scenarios, the discussion still revolves around a limited statistical increase.

When it is said that the increase ranges from 1.7% to 2.8%, this also means that more than 97% of children born from cousin marriages are born free from these disorders.

The question therefore arises: is it from legislative wisdom to forbid a broad and socially beneficial category of marriage because of a limited statistical increase?

The Principle Cannot Be Applied Consistently

One should ask: should Islam be expected to forbid every action that may increase the possibility of a particular illness or harm?

If this principle were applied consistently, then marriage after the age of 40 would also need to be forbidden, since the likelihood of chromosomal disorders such as Down syndrome increases after that age.

This study shows that down syndrome risk rises with advanced maternal age because many cases are linked to chromosomal errors connected to maternal meiosis – https://pubmed.ncbi.nlm.nih.gov/8644722/

This study shows when the number of births to older mothers increases, the expected prevalence of Down syndrome also increases, because the risk is age related https://pmc.ncbi.nlm.nih.gov/articles/PMC3746264/ 

This study shows that not only does the likelihood of down syndrome occur in childrne born from women 35+ but it also found that advanced maternal age is associated with higher risk of fetal aneuploidy, including trisomy 21, trisomy 18, trisomy 13, and sex chromosome abnormalities.

Outside the matter of pregnancy, if legislative wisdom required forbidding everything that carries even a small possibility of harm, then the professions of medicine and nursing would also need to be forbidden because of the risk of infection.

Driving cars and flying in aircraft would need to be forbidden because of the possibility of accidents.

Sports and physical labour would likewise need to be forbidden because of the possibility of injury.

So must the Sharia prohibit all of these matters before its wisdom can be accepted?

The Sharia establishes its rulings upon predominant benefit and predominant harm, while leaving a broad sphere of permissible matters within which people conduct their lives.

Where Would the Limit Be Set?

If it is argued that marriage between direct first cousins increases hereditary disorders by a certain percentage, while marriage between more distant relatives carries a lower percentage, then where exactly should the line be drawn?

At what precise percentage should the Sharia prohibit marriage in order to be considered wise and protective of human welfare?

Should the limit be 2%? Or 1%? Or half a percent?

Marriage in general carries the possibility that children may be born with illnesses or disabilities.

Does legislative wisdom therefore require the banning of marriage altogether because of these possibilities?

The Presence of Modern Genetic Testing

Today, with the availability of medical testing, both relatives and non relatives are able to undergo genetic screening and genetic counselling before marriage. Such testing can significantly reveal the likelihood of hereditary disorders appearing in their children.

Should the Sharia therefore be expected to forbid something and then later permit it based upon the invention of a modern medical test?

The Sharia establishes broad and enduring principles suitable for every time and place. Its rulings are not built upon temporary technological developments or medical procedures that did not exist at the time of revelation.

The Sharia established the original ruling in this matter as permissibility, while also encouraging people to seek beneficial knowledge and take appropriate means. Among those means today are modern genetic testing and counselling.

In other words, the Sharia establishes permissibility as the original principle while encouraging wisdom, caution, and consideration of benefit and harm.

Social and Human Benefits

The permissibility of cousin marriage also takes into account social realities and human needs.

Completely closing the door to cousin marriage would historically have caused hardship and conflict with human reality in many societies.

Many tribes and communities lived in geographical or social isolation, such as nomadic groups, rural populations, and remote regions, where internal marriage often became a practical necessity.

In addition, such marriages can carry important human and social benefits that a purely material perspective may overlook.

A man may marry his widowed or orphaned relative in order to protect her, support her children, and preserve her dignity. Such social benefits may outweigh a limited increase in hereditary risk.

Likewise, close interaction between relatives can naturally create emotional attachment.

Had the Sharia entirely forbidden such marriages, this could have opened the door to unlawful relationships and increased desire toward what had been prohibited.

Islam therefore widened the circle of what is lawful in this matter so that human instincts are regulated through lawful means rather than suppressed in a manner that leads to corruption.

Whoever wishes to marry his relative may do so through lawful and honourable means.

This reflects the wisdom of the Sharia in regulating human nature rather than denying it.

Turning The Tables On Christians

Some Christians who are not well versed in the bible may mock the fact that cousin marriage is permissible in Islam. However little do they know, not only is there no prohibition of cousin marriage in the bible, but Jacob married his first cousins, Leah and Rachel.

Yaʿqub (Jacob) عليه السلام married Rachel and Leah, his cousins

Rachel was the daughter of Laban, and Laban was the brother of Yaʿqub’s mother.

Genesis 29:10 says:

“When Jacob saw Rachel daughter of his uncle Laban, and Laban’s sheep, he went over and rolled the stone away from the mouth of the well and watered his uncle’s sheep.”

Genesis 29:16 also says:

“Now Laban had two daughters; the name of the older was Leah, and the name of the younger was Rachel.”

So Leah and Rachel were the daughters of Yaʿqub’s maternal uncle. That makes them his first cousins.

Verse proving he married them

Genesis 29:28 says:

“Jacob did so. He finished the week with Leah, and then Laban gave him his daughter Rachel to be his wife.”

Genesis 29:30 says:

“Jacob made love to Rachel also, and his love for Rachel was greater than his love for Leah.”

So this is a clear case of Jacob marrying his first cousins, Leah and Rachel.

Permissibility Does Not Mean It Is  Obligated

The Sharia did not command cousin marriage. It merely permitted it.

There is a significant difference between permissibility and obligation.

In fact, some fuqaha preferred marrying outside one’s close family in order to avoid weakening the offspring.

Abu Saʿid ibn Yunus said: Al Husayn ibn Muhammad ibn al Dahhak al Farisi narrated to us. I heard al Muzani say: I heard Imam al Shafiʿi say:

“Whenever the women of a household do not marry men outside of their own family, and their men do not marry women outside of their own family, foolishness appears among their children.” (Adab al Shafiʿi, pp. 133–134; Hilyat al Awliyaʾ, 9/125; Al Intiqaʾ, p. 98; Manaqib al Shafiʿi by al Bayhaqi, 2/201.)

Siyar Aʿlam al Nubalaʾ 10/42–43

In “al Bayan”, it is narrated from al Shafiʿi that he said: “If a man marries within his close relatives, foolishness will most likely appear among his children.”

I say: Reality bears witness to this.

Al Qadi Husayn also mentioned another narration with a similar meaning:

“Marry from outside your close relatives, so that your children do not become weak.”

Meaning: so that weakness does not affect the child.

I did not find, in this subject, anything in the books of hadith that could support this except what I found in “Gharib al Hadith” by Ibrahim al Harbi, through the narration of ʿAbd Allah ibn al Muʾammal, from Ibn Abi Mulaykah, who said: “ʿUmar رضي الله عنه said to the family of al Saʾib: ‘You have become weak, so marry from outsiders.’”

Al Harbi said: “The meaning is: marry women from outside the family.”

Al Badr al Munir fi Takhrij al Ahadith wa al Athar al Waqiʿah fi al Sharh al Kabir by Ibn al Mulaqqin 7/500

Illness and Divine Decree

The birth of a child with a defect, whether in cousin marriages or non cousin marriages, though painful and difficult, still falls within the decree and wisdom of Allah.

The obligation in the Sharia is to care for such a child, fulfil his rights, and show patience and responsibility.

This is from the trials through which Allah tests His servants:

لِيَبْلُوَكُمْ أَيُّكُمْ أَحْسَنُ عَمَلًا

“So that He may test which of you is best in deeds.” (Hud: 7)

Human beings live in a universe of trials and tests.

The Most Important Point

Most importantly, whoever builds certainty in Islam upon strong and sound foundations will not have his faith shaken by doubts, even if he does not possess a detailed answer to every individual question.

He returns unclear matters back to the clear and established foundations.

After reflecting upon this issue carefully, it becomes apparent that cousin marriage is not truly a valid doubt in the first place.

When a person’s certainty becomes shaken by the smallest question, this often indicates weakness in the underlying foundations of certainty itself. The slightest uncertainty then becomes capable of shaking the entire structure.

For this reason, even if one doubt is removed, another may soon appear if the foundations themselves are weak.

Part of the failure of many modern educational systems is that they do not build strong certainty, nor do they teach sound methods of thinking and reasoning.

As a result, instead of scientific knowledge increasing faith and conviction, it sometimes becomes a source of doubts for young people who lack the intellectual and spiritual tools needed to process such questions correctly.

For this reason, students, especially university students, should devote attention to beneficial knowledge that strengthens certainty, develops sound thinking, and teaches how to return unclear matters back to clear foundations.

They should know that Islam, built upon great and established certainties, possesses answers to these questions even if a person has not yet personally discovered them.