Children of cousins who marry are significantly likelier to suffer from psychosis, depression and/or anxiety, a vast Irish study has found. The results were reported Wednesday in JAMA Psychiatry.
There's reason religion and law tend to steer people away from consanguinity, meaning, marriage between related people. There's a satirical Facebook community page in Hebrew called "When Daddy and Mommy are Cousins," and it isn't about granny's cooking tips. Users upload links to news reports about stupid or mind-boggling things people do, such as the woman seeking advice about a cheating ex, or the four British men who joined ISIS complaining about England stripping them of their citizenship because that could expose them to brutality.
The inference isn’t that consanguinity induces identification with ISIS. It's that procreating with family increases the probability that negative recessive traits lurking in the family will be expressed. The Irish study sought to clarify whether children born to first cousins are at heightened risk of psychosis and/or mood disorders too, and found they very much are.
First-cousin marriages are perfectly legal in Europe and most of the United States. Cousin marriages do statistically have more birth defects among their progeny than do unrelated couples, though it bears qualifying that problem of kissing cousins is less acute when the couple are the first in the family to do so. The trouble chiefly arises when cousin-cousin marriage is the norm in the family or society.
Recessive traits accrue, as demonstrated by an Egyptian study dating from 2013, which described how "stillbirths, child deaths and recurrent abortions were significantly increased among consanguineous parents" compared with non-related parents.
The Irish paper reported in JAMA, "Consanguineous Marriage and the Psychopathology of Progeny," looked at well over a quarter-million people – specifically, 363,960 people born in Northern Ireland from 1971 to 1986; nationwide data on prescription medication; and deaths records.
The relationship between the parents was ascertained by asking them.
- Venomous shrew that survived the dinosaurs is sequenced, just in time
- Volcanic blast drove Icelandic pagans to Christianity
- How earthquake warning systems are like missile alerts: Seconds that could save your life
Psychosis was "identified" by receipt of antipsychotic medications; mood disorders were "identified" by receipt of antidepressant or anti-anxiety drugs.
All in the family genetics
The result is not a happy one. After full adjustment for factors known to be associated with poor mental health, the researchers in the Irish study calculated that children of first-cousin consanguineous parents were more than 3 times as likely to be prescribed antidepressant or anxiolytic medications. They were more than twice as likely to be in receipt of antipsychotic medication compared with children of nonrelated parents.
So the probability of mental difficulties has to be added to the probability of physical ones. The most startling statistic in the 2013 Egyptian study was that almost 93% of the children born with limb abnormalities turned out to have related parents. Its conclusion was that public health education and genetic counseling are highly recommended in the community.
Israel is all too versed in the genetic price of inbreeding. The entire Ashkenazi (European Jewish) population alive today is believed to have descended from a tiny group of just 350 people, roughly speaking, who lived 600 to 800 years ago. The bottleneck occurred 25 to 30 generations ago, according to the Columbia University study – and Ashkenazim today are associated with no less than 19 genetic disorders.
Marriage within the Ashkenazi community isn't as high-risk as marrying a first or even second cousin, but before the rabbi sings the nuptial blessings, Ashkenazi Israelis typically undergo genetic testing for at least some inheritable diseases, such as the deadly Tay-Sachs. Just in case.
In Israel, as in Egypt next door, consanguineous relations have not been shunned. A 1991 study reported that 43% of marriages in the Arab Muslim community (excluding Bedouins) were consanguineous; 67% of the Bedouin marriages; 32% of Arab Christian marriages and 54% among the Druze. All those figures have been declining since then.
Meanwhile, the 1991 rates in the Jewish community were a fraction of those figures, the highest (over 9%) being reported among Jews originating in Iran.
Not coincidentally, the Bedouin community also suffers high rates of congenital malformation, mental deficiencies and infant mortality. Of 1,290 Bedouin women who delivered at Soroka, the only hospital in Israel's south where most Bedouin live, from November 2009 to January 2010 – nearly half (45%) were involved in consanguineous marriage; and most of those marriages involved first cousins.
The study on the Negev Bedouins also urged better education and counseling, to lower the infant mortality rate among the Bedouins, which was found to be the highest among all groups in Israel: 13.6 per 1,000 in 2010 compared with the national figure of 3.7 per 1,000.
For the sake of comparison, the average in the United Kingdom was about 4.3 (2010 data, since which time it's dropped to 3.9) and in Ireland, it's 3.6.