patrick.net

Consanguineous marriage, particularly between first or second cousins, is reported to be relatively common in certain regions of Somalia, though data varies significantly by clan and region. A 2019 genetic study of Ethiopian Somalis found that 48.7% of individuals were products of first or second cousin marriages, with an average inbreeding coefficient (FROH) of 0.018, which is higher than the typical coefficient for second cousin offspring.
Regional rates in similar cultures range from 44% to 63%.

In northern Somalia, traditional marriage practices often involve contracting unions between previously non-related families to establish new alliances.
In southern Somalia, the preferred spouse is often a patrilateral parallel cousin, either real or classificatory.
While cousin marriage is not universally common across all Somali clans, it is more prevalent in certain groups, particularly those with stronger ties to Arab heritage.
Some sources indicate that it is more common in smaller clans or among specific subgroups, such as the Reer Badiya, though it is often arranged or encouraged by older family members to strengthen familial bonds.

https://search.brave.com/search?q=consanguinity+by+country+somalia&source=web&conversation=e974c1a33ec94232cbc86b&summary=1

From perplexity.ai:

Meta‑style reviews and classic studies suggest an average drop of about 2.5–3.5 IQ points for children of first cousins (about 0.15–0.25 standard deviations).

Some individual studies in highly inbred populations report larger differences (up to 8–10 points), but those settings often combine multiple generations of consanguinity and other risk factors, so they are not typical for a one‑off first‑cousin marriage.

More important than the small shift in average IQ is the increased risk at the low end:

A classic analysis estimated the risk of “mental retardation” (IQ <70 in older terminology) rising from about 1.2% with unrelated parents to about 6.2% for children of first cousins.

This is thought to be due to recessive disease alleles becoming homozygous more often when parents share ancestry at many loci.

One point I learned in too many years of biology in college is that everyone has a few defective genes making nonfunctional proteins, but normally, you also have a different gene from the other parent at the same locus (as gene locations are called).

The other gene is unlikely to also happen to be defective, so most genetic diseases require inheriting the same bad gene from both parents. This is why inbreeding causes problems.

There is an opposite effect, called "outbreeding depression" in which it's beneficial to disable a gene in a certain environment. A simple example is genes for dark skin. In sunny climates, these genes help protect the skin, but in northern climates, they result in vitamin D deficiency and a resulting increase in disease. So in northern climates, people with broken genes for melanin did better in life when they got the same broken gene from both parents via inbreeding, eventually displacing people with dark skin completely in Europe. So a bit of inbreeding helped them, and outbreeding hurt them.

Also interesting: a vast number of your genes code for the brain and for joint structure, so typical signs of inbreeding are retardation and defective joints.