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  • On Progenitors and Population

    Published Date : August 1987
     As we go back through time, the number of our ancestors doubles each generation.  The cumulative effect of this is impressive.  In round numbers, each of us had a thousand ancestors ten generations ago, a million twenty generations ago, and a billion thirty generations ago.  The numbers increase so fast that at some point in quite recent time we find we have as many ancestors as there were people alive.

    The writer was born in England in 1924.  England is a useful place for a population study because its boundaries have remained unchanged, and in the six hundred odd years since the Black Death the growth of population has been steady.  If we take a generation as twenty years - twenty-five doesn’t change the answer significantly - then about twenty-one generations before mine, that is, about the year 1500, a member of my generation had about 3.2 million forebears.  As close as one can tell this was the entire population of England at that time.  However - and this is a big however - just one generation before 1500, we had once again doubled the number of ancestors, but the population was less than half this number.

    How can this be?  The genealogist in me would be delighted to claim kinship to Shakespeare, and even to Henry VIII, by a blanket claim to all inhabitants of England at that time, but the arithmetitian in me tells me something is amiss.  A check of the best population estimates of the day show an increase from about 2.5 million in 1400 as the country struggled back from the ravages of the Black Death, to about 4.2 million in 1600.  This amounts to a net growth of six percent per generation during that period.  The growth of the number of ancestors in the other direction along the time scale, however, is at one hundred percent per generation.  The lines must inevitably cross.  We can argue about generation lengths and estimates of the population but inaccuracy of estimates would only serve to move the date of progenitor!population equality a few years forward or back in time.  It would not answer the basic question.  Similarly, the minor immigration or emigration in or out of England would have only slight, but similar, effect. Clearly the answer has to be in the rate at which the numbers of our ancestors grow as we move back through the generations.

    One influence on the number of progenitors would be an intertwining of limbs of the family tree caused by marriage between cousins-first, second and so on.  Now, marriage to a first cousin is not common today, but seems to have occurred with greater frequency in earlier days.  If 18th-century Virginia is a measure, it has been said to be a safe assumption that “Anyone in the same social class within five miles is kinfolk”.  The limited access to potential mates can be assumed to have had its effect on marriage between cousins, but surely not to the drastic degree we seem to require?  A closer look at the effect of such marriages gives some interesting answers.

    Visualize your family tree going back twenty generations, doubling the number in each generation.  Now, if your parents were first cousins, you have a set of common great-grandparents and only six great-grandparents instead of eight.  This is a 25% reduction in number, and it persists through each generation until by twenty generations back we have only 750,000 ancestors instead of a million, a substantial reduction.  If second cousins marry, the effect is similar, occurring one generation farther back in time, and therefore with half the effect.  Third and fourth cousin marriages would similarly have one quarter and one eighth the effect of first cousins.

    Now most of us would not know a fourth or fifth cousin if we met one, or in the case of non-genealogists, married one.  But the cumulative effect of marriages to near or distant relatives may answer our question.

    But how many of us (royalty excepted) can count more than two or three first cousin marriages in our family tree?  If we assume that in each and every generation one in eight of our ancestors married a first cousm. the effect would be to reduce from 2.0 to 1.6 the multiplier by which each generation grows.  But this only serves in the case examined to move back the equality date to around 1400.  In order to get down to numbers of ancestors which move the equality date to a suitably obscure period of history, when other factors could be assumed to come into play, we must assume that one in four of our ancestors before our grandparents’ time married first cousins.  This means that on average everyone in our tree had one set out of their four sets of greatgrandparents who were first cousins, or two sets out of four who were second cousins, or, (and here we have the general answer), four sets out of four who married third cousins.


    We can thus propound Whimsy’s First Law of Genealogy:

    “Mankind marries on average a third cousm.

    If the reader finds this as hard to swallow as does the writer, perhaps he can offer an alternative answer.  The question must have arisen before.  Even Whimsy’s First Law gets us into trouble by about 1200 A.D., but perhaps a Second Law will clarify that in due course.  In the meantime it makes for one further intriguing thought.  If we accept the popularly held notion that inbreeding produces inferior physical and mental capabilities in the offspring, is it only a coincidence that the time at which the population was increasing fast enough to permit a wider choice of mate, the period beginning about 1400, was the start of the English Renaissance?


    Editor’s Note: “Whimsy’s First Law of Genealogy” also hovers in the background of several NEXUS “Notable Kin” articles and of the mariner and hometowns series by feature writer Philip S. Thayer.  See especially “Nantucket Soup” (NEXUS 3[1986]: 26-27) and “Braintree, Massachusetts, in 1790” (NEXUS 4(1987]: 128-30). For a further introduction to the fascinating topic of long term genealogical inbreeding, see B.S. Bramwell’s “Frequency of Cousin Intermarriage” in the Genealogists’ Magazine 8 (1938-39): 305-16. English population studies since the Black Plague are an area of study also of local historian and NEXUS contributor Dr. George Redmonds (see NEXUS 4: 127-28).  Further discussion on this lively topic of disappearing and cross-bred ancestors is welcome.

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