From a biological point of view it becomes clear that first cousin marriage is not recommended because close relatives have a higher than normal consanguinity which means an increased chance of sharing genes for recessive traits. With this high amount of shared DNA, you have a higher risk of birth defects in a baby. Even if cousin marriages are not performed, you can still have such genetic defects in populations where there is a restricted social structure.

In Pakistan, where there has been cousin marriage for generations, and according to professor Anne-Marie Nybo Andersen from South Danish University, the current rate is 70%,[5] one study estimated infant mortality at 12.7 percent for married double first cousins, 7.9 percent for first cousins, 9.2 percent for first cousins once removed/double second cousins, 6.9 percent for second cousins, and 5.1 percent among non-consanguineous progeny. Among double first cousin progeny, 41.2 percent of pre-reproductive deaths were associated with the expression of detrimental recessive genes, with equivalent values of 26.0, 14.9, and 8.1 percent for first cousins, first cousins once removed/double second cousins, and second cousins respectively.

A BBC report discussed Pakistanis in the United Kingdom, 55% of whom marry a first cousin. Given the high rate of such marriages, many children come from repeat generations of first-cousin marriages. The report states that these children are 13 times more likely than the general population to produce children with genetic disorders, and one in ten children of first-cousin marriages in Birmingham either dies in infancy or develops a serious disability.[6]

The BBC also states that Pakistani-Britons, who account for some 3% of all births in the UK, produce "just under a third" of all British children with genetic illnesses. Published studies show that mean perinatal mortality in the Pakistani community of 15.7 per thousand significantly exceeds that in the indigenous population and all other ethnic groups in Britain. Congenital anomalies account for 41 percent of all British Pakistani infant deaths.[7][8][9][10]

Worldwide, it has been estimated that almost half of all Muslims are inbred:

A rough estimate shows that close to half of all Muslims in the world are inbred: In Pakistan, 70 percent of all marriages are between first cousins (so-called "consanguinity") and in Turkey the amount is between 25-30 percent.[11]

Statistical research on Arabic countries shows that up to 34 percent of all marriages in Algiers are consanguine (blood related), 46 percent in Bahrain, 33 percent in Egypt, 80 percent in Nubia (southern area in Egypt), 60 percent in Iraq, 64 percent in Jordan, 64 percent in Kuwait, 42 percent in Lebanon, 48 percent in Libya, 47 percent in Mauritania, 54 percent in Qatar, 67 percent in Saudi Arabia, 63 percent in Sudan, 40 percent in Syria, 39 percent in Tunisia, 54 percent in the United Arabic Emirates and 45 percent in Yemen.[12][13]

The British geneticist, Professor Steve Jones, giving The John Maddox Lecture at the 2011 Hay Festival had stated in relation to Muslim inbreeding, "It is common in the Islamic world to marry your brother’s daughter, which is actually [genetically] closer than marrying your cousin."[14]

https://wikiislam.net/wiki/Cousin_Marriage_in_Islam#Science_and_Statistics