The extended family
We make a count of our relations.
One of our consultants remembers reading, many years ago, the assertion that any two people in the Western world are, on the average, seventh cousins. We’ve tried to find the source of this statement, looking through many old papers (and some new ones), to no avail. We don’t know how the calculation was done or what was meant by “the Western world.” So lately we decided to work out for ourselves what degree of relationship we can expect of otherwise perfect strangers.
Suppose each pair of parents has, on the average, n children. Each child would have, on the average, n – 1 siblings, and 2(n – 1) uncles and aunts, resulting in 2n(n – 1) first cousins. Putting in numbers, if n is 5, each person has forty first cousins. To some people nowadays that might sound like a lot, but for an old-fashioned farming community it’s not in the least surprising. Keeping with five-child families and continuing the process, we find 40,000 fourth cousins and 40 million seventh cousins. The latter is well below the population of the Western world in recent centuries, but it’s still pretty impressive. And the numbers we get are very sensitive to the assumed number of children. Only going to six, we get over a hundred thousand fourth cousins and nearly 180 million seventh cousins. Our half-remembered statement now looks quite reasonable.
Of course our number n hides an awful lot of variation and subtlety. It’s not the average number of children any woman has in life, but those who grow up and have children themselves. It has to be adjusted (for instance) for those who die childless or prematurely, as well as for those who remarry and have more children with a different partner. Old-fashioned farming families might have ten or a dozen children; for them an n of five or six is probably a reasonable (if conservative) estimate. But of course it also ignores geography.
Before about 1850, people married within rather restricted areas. Counting one’s own village, the nearest and next-nearest, very few people would come up with a population of tens of thousands (and the number of possible partners would be smaller). We’re forced to the possibly uncomfortable conclusion that the majority of marriages in the old days were between people no more distantly related than fourth cousins, and probably not even that. “The average degree of relationship in the Western world” becomes an almost meaningless concept.
Did it make a difference? We don’t have the background in genetics to say anything definite. We’re aware of no legal prohibition, or even discouragement, of marriages more distant than first cousins. Even those were permitted among the nobility (with, however, sometimes unfortunate results). Third- and fourth-cousin marriages might in fact be reasonably safe. We suspect, however, that the few people who did travel long distances, sailors and soldiers and long-haul merchants, may have had much more to do with maintaining the genetic health of our ancestors than is generally realized.
Nowadays, of course, most people live in cities with large populations, even by fourth-cousin standards, and many find partners even further afield. We know of one American Jew of eventual European extraction with two children by a Malay partner. Another, the daughter of an American of Irish lineage and a Filipino, has married an Estonian and now has two children. And, as demonstrated by these examples, the size of families is much smaller than in the old farming days. As the world shrinks and we get closer to everyone, they become ever more distant cousins.
