IIn July 1838, when considering the prospects of marriage, Charles Darwin took a sheet of paper and made a list of pros and cons. Advantages included the possibility of children, companionship (marriage was “better than a dog”) and having someone to look after the house. The disadvantages were the “terrible loss of time”, potential arguments and financial burdens on a wife. Among the benefits of a bachelor’s degree, he wrote that “no [being] forced to visit relatives.” This problem, however, was easily dealt with. After some flirtations, he settled on Miss Emma Wedgewood, the daughter of his favorite uncle and thus his first cousin.
Although he was the father of evolution and genetics, Darwin was not a great social innovator. Bourgeois Victorians regularly sought out their mates at family gatherings. In the long perspective of human history, cousin associations do not dominate the landscape of our cultural consciousness. Apart from royalty, who had an avoidably small dating pool, family marriages are seen as outliers. Or maybe we think so. A new study of the common inhabitants of the ancient Aegean shows something quite different.
An international team, led by researchers 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 Neolithic and Middle Bronze Age inhabitants of the Aegean Sea (17th-12th centuries BC) and noticed an interesting result: more than half of the people who lived on these islands married their cousin. The results were published in open access last week in the prestigious journal Natural ecology and evolution.
Professor Philipp Stockhammer, a lead author on the study and an archaeologist at the Max Planck Institute, told CNN that the study was important for what it revealed about the social structures of the communities that lived on the island. “We managed to construct the first family board for the Mediterranean. We can see who lived together in this house by looking at who was buried out in the courtyard. We could see, for example, that the three sons lived as adults in this house. One of the spouses took his sister and a child with him. It is a very complex group of people living together.”
According to the article, the high rates of “relative endogamy” (cross-cousin associations) are “unprecedented in the global ancient DNA record.” Stockhammer explained: “People have studied thousands of ancestral genomes, and there is hardly any evidence for societies in the past with cousin marriages. From a historical perspective, this is truly unprecedented.”
If you’re thinking to yourself “well they’re on an island, who else are they going to marry”, you’re not alone. But the researchers who conducted the study concluded that “small population size was probably not an important factor … cross-sibling unions were practiced in different geographical contexts – on islands of different sizes as well as the Greek mainland and are not evident anywhere 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.
Part of what 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 married in antiquity is in Hellenistic Egypt. From 322-30 BCE, Egypt was ruled by the Greco-Macedonian Ptolemaic dynasty, the descendants of one of Alexander the Great’s generals. Early in this period, the Ptolemies established a practice of incestuous marriage, marrying siblings with siblings and cousins (or half-cousins) with cousins.
What is strange about this is that Greek intellectuals are known to have abhorred incest and saw it as a loss of self-control and debauchery. In a sinister rendering of the bloody transfer of power from Ptolemy VI to Ptolemy VIII, a writer from a third century AD Apparently, after the death of Ptolemy VI, Ptolemy VIII, the king of Cyrene, was offered the throne and the hand of his sister, Queen Dowager Cleopatra II (not the famous Cleopatra, one of many others). There had been a Ptolemy VII (offspring of Ptolemy VI and Cleopatra II) who had planned to marry his own mother. But on the wedding day, Ptolemy VIII broke into the party, slaughtered his nephew, “and entered his sister’s bed, still dripping with her son.” And you thought your relatives behaved badly at your wedding.
Given that Greek literature—and society in general—regards incest as one of the greatest taboos (it didn’t go well for Oedipus, after all), there has been proactive scholarly debate as to why the Ptolemies engaged in it. One of the main explanations is that they were influenced by the local culture. According to Diodorus, the Egyptians had made a law that allowed brothers and sisters to marry, just as the Egyptian gods Osiris and Isis had done. This was allegedly the reason why the pharaohs married their sisters. Now, it’s worth pointing out two things: First, Egyptian pharaohs didn’t marry their sisters as often as popular mythology claims they did. Second, the Greeks had their own married sibling gods (hello, Zeus and Hera). Despite this and although other Greek families who had moved to Egypt also married their cousins, there is a tendency to blame the Egyptians for Ptolemaic incest.
Although the “Greeks” (if we can really use the term that early) of the Aegean islands lived hundreds of years before and were socio-economically removed from the Ptolemies, this new study shows us that Greeks married their cousins long before the Ptolemies settled in Egypt. While anthropological studies of elite Egyptian cemeteries (3600-3000 BC) reveal that ancient Egyptians also practiced endogamy, they were clearly not alone. The Ptolemies may have thought of their behavior as influenced by existing traditions, or they may have been colonial xenophobes, the point being that the Egyptians should not take all the blame.
Anthropologists debate why it is that people marry close relatives. In the case of the new study of Bronze Age inhabitants of the Aegean islands, researchers believe that marital practices were influenced by 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 live in the same place for a longer period of time. Genetics is local, so the less movement, the less genetic variation. Or put another way, the smaller your dating pool, the more likely you are to marry someone you share a grandmother with.
But there are other factors that come into play as well. In his classic and exceptionally entertaining book Incest and influence social anthropologist Adam Kuper explained the economic benefits of marrying one’s cousins. Among seventeenth-century aristocrats in England, cousin marriages between heiresses and a paternal cousin were popular because they “retained her estate in her father’s family.” Any viewer of Downton Abbey is familiar with the problem. Cousin marriage exploded among the bourgeoisie in the long nineteenth century, Kupfer writes, 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. Naturalists and mediciners, including, of course, Darwin himself, were increasingly interested in (and concerned about) heredity, so it made sense to marry outside the bloodline but within the clan. Readers of Jane Austen know how frequent and random such battles can be: Mr. Knightley is the brother of Emma’s brother-in-law and Elinor of Reason and feelings marries her brother-in-law, Edward Ferrars.
Almost all human societies have some sort of ban on sexual relations between family members. According to the nineteenth-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 towards those with whom we share a lot of genetic material. Sigmund Freud, on the other hand, rather provocatively argued that it is only social taboos that prevent us from indulging in incestuous appetites (left to our own devices we would jump over our siblings’ bones)
While Freud’s theories enjoyed great popularity in the twentieth century, recent work has confirmed many aspects of Westermarck’s hypothesis. Unrelated individuals who live together as children, for example, show reduced sexual interest in each other (this is called the Westermarck effect or reverse sibling imprinting). Studies in Lebanon, for example, have shown that cousins are less likely to marry each other if they were raised together.
Putting aside the question of incest between siblings (or, shudder, parents), do these theories have much relevance to cousins? Even today, cross-cousin marriage is remarkably common and far from taboo. Anthropologist Jonathan Marks, the author of Stories about the ex-monkeys, told me that “first cousins remain the most preferred spousal partner, still representing 10-15% of marriages globally.” You may be on the receiving end of jokes, but you shouldn’t expect a higher rate of infant mortality. Cross-cousin marriages do not appear to cause genetic problems, Marks added, unless practiced repeatedly across generations. If there is nothing biologically problematic about cross-cousin unions, where does the mild sense of ickiness that comes with it come from?
The answer lies in religion. Leviticus 18:6-18 prohibits sexual intercourse between certain close family members (“close relatives”). Cousins are not mentioned – but the differences between siblings, half-siblings and cousins are unclear in the Bible – and there is plenty of cousin marriage. Lured into a messy love triangle, for example, Jacob marries not one, but two of his cousins in Genesis 29. As Kupfer points out in her book, the Bible stipulates that women are not allowed to marry their nephews, but is silent on matters of men and their nieces. So also, the apostle Paul rejects the relationship between stepson and stepmother, but says nothing about cousins.
It was with the introduction of Christian laws and a series of church councils that cousins were seen as a problem. The Synod of Trullan (692 AD) attempted to produce an ecclesiastical common law in the wake of Muslim invasion, extending earlier prohibitions against incest to include cousins. The eighth-century Byzantine legal collection, the Ecloga, punished marriage with second cousins by flogging. The Council of Trent forbade marriage between first and second cousins, but magnanimously allowed unions with identical third cousins (ie someone with whom you share great-great-grandparents). 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 outlook of Darwin and his ilk. The Book of Leviticus forbids relationships between a man and a woman who had been married to that man’s father, brother or son (meaning mothers, stepmothers, sisters-in-law and daughters-in-law). According to the principle that a woman becomes her husband’s flesh when she marries, the Catholic Church ruled that in-laws are also incest. If you become “one flesh” with your spouse, their sister is now your sister. This was the technicality by which Henry VIII attempted to annul his marriage to Catherine of Aragon.
In religious circles, incest is not only about blood ties. Spiritual relationships can be just as difficult. The helm, an eighteenth-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 is not 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.