In July 1838, as he considered the prospect of marriage, Charles Darwin, took a sheet of paper and made a list of pros and cons. Pros included the possibility of children, companionship (marriage was “better than a dog”), and having someone to take care of the house. The drawbacks involved the “terrible loss of time,” potential quarrels, and financial burdens of a wife. Among the advantages of bachelordom, he wrote was “not [being] forced to visit relatives.” This problem, however, was easily dealt with. After some flirtations, he settled upon Miss Emma Wedgewood, the daughter of his favorite uncle and, thus, his first cousin.
Though he was the father of evolution and genetics, Darwin was not a great social innovator. Bourgeois Victorians regularly sought their mates at family gatherings. In the long view of human history, however, cousin-unions do not dominate the landscape of our cultural consciousness. Except for royals, who had an avoidably small dating pool, intra-family marriages are seen as outliers. Or so we might think. A new study of the ordinary inhabitants of the ancient Aegean shows something quite different.
An international team, led by scientists from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, produced a scientific study of the genetics of people from a number of Greek islands. The team analyzed more than a hundred samples of genomes from inhabitants from the Neolithic and Middle Bronze age Aegean (17-12th centuries B.C.) and noticed an interesting result: more than half the people who lived on these islands married their cousins. The results were published open access last week in the prestigious journal Nature Ecology & Evolution.
Professor Philipp Stockhammer, a lead author on the study and archeologist at the Max Planck Institute, told CNN that the study was significant for what it revealed about social structures of the communities who lived on the Island. “We managed to construct the first family pedigree for the Mediterranean. We can see who lived together in this house from looking at who was buried outside in the courtyard. We could see, for example, that the three sons lived as adults in this house. One of the marriage partners brought her sister and a child. It’s a very complex group of people living together.”
According to the article the high rates of “consanguineous endogamy” (cross-cousin unions) are “unprecedented in the global ancient DNA record.” Stockhammer explained, “People have studied thousands of ancestral genomes and there’s hardly any evidence for societies in the past of cousin-cousin marriage. From a historical perspective this really is outstanding.”
If you’re thinking to yourself “well they are on an island, who else are they are going to marry” then you’re not alone. But the scientists who conducted the study concluded that “small population size was probably not a major reason… cross-cousin unions were practiced in different geographic contexts—on islands of different sizes as well as the Greek mainland and are not evident at some places during the second millennium.” On Crete, one of the islands included in the study, people had more options but they still seem to have kept things in the family.
One of the things that is interesting about this study is how it disrupts conventional narratives about marital practices among the ancient Greeks. The one place Greeks (or at least Greek ex-pats) are known to have intermarried in antiquity is in Hellenistic Egypt. From 322-30 B.C., Egypt was ruled by the Greco-Macedonian Ptolemaic dynasty, the descendants of one of Alexander the Great’s generals. Early on in this period the Ptolemies established a practice of incestuous marriage, marrying siblings to siblings and cousins (or half-cousins) to cousins.
What’s strange about this is that Greek intellectuals are known to have abhorred incest and saw it a loss of self-control and debauchery. In luridly relaying the bloody transfer of power from Ptolemy VI to Ptolemy the VIII, one third-century writer laid the sensationalism on thick. Apparently, after the death of Ptolemy VI, Ptolemy VIII, the king of Cyrene, was offered the throne and the hand of his sister, the widowed Queen Cleopatra II (not the famous Cleopatra, one of many others). There had been a Ptolemy VII (the progeny of Ptolemy VI and Cleopatra II) who had planned to wed his own mother. But on the wedding day Ptolemy VIII burst into the party, slaughtered his nephew, “and entered his sister’s bed still dripping with the gore of her son.” And you thought your relatives behaved badly at your wedding.
Given that Greek literature—and societies in general—see incest as one of the greatest taboos (things did not turn out well for Oedipus, after all) there has been a protracted scholarly debate about why the Ptolemies engaged in it. One of the primary explanations is that they were influenced by the local culture. According to Diodorus, the Egyptians had made a law permitting brothers and sisters to marry, just as the Egyptian deities Osiris and Isis had done. This, allegedly, was why the pharaohs married their sisters. Now, it’s worth pointing out two things: First, Egyptian pharaohs did not marry their sisters as frequently as popular mythology maintains they did. Second, the Greeks had their own married sibling deities (hello, Zeus and Hera). Despite this and even though other Greek families who had moved to Egypt were also marrying their cousins, there is a tendency to blame the Egyptians for Ptolemaic incest.
Though the “Greeks” (if we can really use the term this early) of the Aegean islands lived hundreds of years beforehand and were socioeconomically removed from the Ptolemies, this new study show us that Greeks were marrying their cousins long before the Ptolemies settled in Egypt. While anthropological study of elite Egyptian cemeteries (3600-3000 B.C.) reveals that ancient Egyptians also practiced endogamy, they clearly weren’t alone. The Ptolemies may have thought of their behavior as influenced by preexisting traditions or they may have been colonial xenophobes, the point is, Egyptians shouldn’t take all the blame.
Anthropologists debate why it is that people marry close relatives. In the case of the new study of Bronze age occupants of the Aegean islands, scientists think that marital practices were affected by the food supply. Local agriculture centered on the production of grapes and olives, and these were crops that required sustained cultivation over a period of decades. This would have forced people to stay in the same place over a longer period. Genetics are local so the less movement, the less genetic variation. Or, put differently, the smaller your dating pool, the more likely you are to marry someone with whom you share a grandma.
But there are other factors at work as well. In his classic and extraordinarily entertaining book Incest and Influence social anthropologist Adam Kuper explained the financial advantages of marrying one’s cousins. Among 17th-century aristocrats in England, cousin marriages between heiresses and a paternal cousin were popular because they “kept her estate in her father’s family.” Any viewer of Downton Abbey is familiar with the problem. Cousin-marriage exploded among the bourgeoisie of the 19th century, writes Kupfer, as a means of distinguishing a new class of gentlemen from middle-class shopkeepers. As a strategy it cemented kinship groups and helped propel these groups to prosperity, influence, and prestige.
For the Victorian bourgeoisie many of the matches that cemented family ties were with in-laws. Natural scientists and medics, including, of course, Darwin himself, were becoming increasingly interested in (and worried about) heredity, so it made sense to marry outside the bloodline but inside the clan. Readers of Jane Austen know how frequent and fortuitous such matches could be: Mr. Knightley is the brother of Emma’s brother-in-law and Elinor of Sense and Sensibility marries her brother-in-law, Edward Ferrars.
Almost every human society has some kind of prohibition against sexual relations between family members. According to the 19th-century Finnish sociologist Edward Westermarck, these taboos exist because the offspring of first- and second-degree unions have a higher risk of mortality. From an evolutionary perspective we developed sexual aversions to those with whom we share a lot of genetic material. Sigmund Freud, by contrast, quite provocatively claimed that it is only social taboos that prevent us from indulging in incestuous appetites (left to our own devices we’d jump our siblings’ bones).
While Freud’s theories enjoyed great popularity in the 20th century, more recent work has confirmed many aspects of Westermarck’s hypothesis. Unrelated individuals who live together as children, for example, exhibit decreased sexual interest in one another (it’s called the Westermarck effect or reverse sibling imprinting). Studies in Lebanon, for example, have shown that cousins are less likely to marry if they are raised together.
Putting aside the question of incest between siblings (or, shudder, parents), do these theories have much relevance for cousins? Even today, cross-cousin marriages are remarkably common and far from taboo. Anthropologist Jonathan Marks, the author of Tales of the Ex-Apes, told me that “first cousin is still the most widely preferred spousal partner, still representing 10 percent to 15 percent of marriages globally.” You might be on the receiving end of jokes, but you shouldn’t expect a higher incidence of infant mortality. Cross-cousin marriages do not seem to produce genetic problems, added Marks, unless repeatedly practiced across generations. If there’s nothing biologically problematic about cross-cousin unions then where does the mild sense of ickiness that accompanies it come from?
The answer lies with religion. Leviticus 18:6-18 prohibits sexual intercourse between certain close family members (“near of kin”). Cousins aren’t mentioned—but the distinctions between siblings, half-siblings, and cousins are blurry in the Bible—and there are plenty of cousin marriages. Tricked into a messy love triangle, for example, Jacob marries not one, but two of his first cousins in Genesis 29. As Kupfer points out in his book, the Bible stipulates that women are not permitted to marry their nephews but is silent on the question of men and their nieces. So too, the apostle Paul disapproves of stepson-stepmother relations but says nothing about cousins.
It was with the introduction of Christian legal codes and a succession of Church councils that cousins came to be seen as a problem. Scrambling to produce an ecclesiastical common law in the aftermath of Muslim invasion, the Trullan synod (A.D. 692) extended earlier prohibitions on incest to include cousins. The eighth-century Byzantine legal compilation the Ecloga punished marriage to second cousins with flogging. The Council of Trent outlawed marriages with first and second cousins, but magnanimously permitted unions with one’s third cousins (i.e., someone with whom you share a great-great-grandparent). If they had been around, DNA testing companies could have made a fortune validating legitimate marriages.
Where Christianity finds itself in a pickle is with exactly the kind of in-law unions that solidified the prospects of Darwin and his ilk. Leviticus bans relations between a man and a woman who had been married to that man’s father, brother, or son (So, mothers, stepmothers, sisters-in-law, and daughters-in-law). Under the principle that a woman becomes the flesh of her husband when she marries, the Catholic Church stipulated that in-law relationships are also incest. If you become “one flesh” with your spouse, then their sister is now your sister. This was the technicality on which Henry VIII sought to annul his marriage to Catherine of Aragon.
In religious circles, incest isn’t only about blood ties. Spiritual relationships can be just as tricky. The Rudder, an 18th-century collection of Orthodox Christian legal texts by Nicodemus of the Holy Mountain, prohibits Christians from marrying the children of their godparents on the grounds that they are spiritual siblings. So now you have that to worry about.
It is interesting to note that Darwin is not the only field-shaking scientist to have married his cousin. Albert Einstein’s second wife, Elsa Löwenthal, was his maternal cousin. This isn’t to say that smart people marry their cousins, but it does mean that you have fewer family members to divide your time between. And not having in-law problems probably frees up some time for research.