Rodrigo Labouriau draws on Danish data to show smaller towns and closer birth places correlate with more children irrespective of income and education.
We are in pursuit of something that is inherited but not genetic and which is able to stabilize a population of animals so that it has a chance of not going extinct, but the same mechanism will kill babies, kill all the babies, if we are ignorant of how it works. That is an extreme statement, but I shall be backing it up. Even if it has a low chance of being true, it merits the focus of attention of scientists far more than many other well-funded fields. And far from being a low chance, the probability is as close to absolute certainty as anything I know.
The next data set comes from Denmark. Rodrigo Labouriau and António Amorim compared fertility in Denmark with something they call “marital radius” (how far apart a couple were born). This is what they found:
Number of children on the vertical axis. Marital radius on the horizontal.
Fig. 23 6
On the face of it, it does not match the Sibly curve nor the Iceland experience. Initially, the two published the Genetics article referred to below. There they mentioned that once the size of the town where the couple lived and their marital radius were taken into account, there was no effect of wealth or education on the number of children they had – no effect at all. Roll that around in your head for a bit. If there were an element of choice involved, it seems hard to imagine that education, income or the combination would fail to influence the choice. I cannot think of anything else that would influence choice that you could demonstrate objectively. If you interviewed them, and it would be a lot of interviews, I truly suspect you’d have things said like, “That’s the number we wanted,” or “It was all we could afford,” or “Who wants to bring children into a world that’s in this kind of shape?” or “Making babies is the way we spend our evenings.” I suspect, and it is outside my comfort zone to speak of it, that in our human nature, if we do something, we are disposed to rationalize it, to tell ourselves that it was what we meant to do.
I have a bunch of anecdotes in support. When I was young in diagnostic radiology, we would often do something called a UGI series, upper gastro-intestinal. The patient would swallow some suspended barium sulfate and the radiologist would use a fluoroscope to watch it pour into the stomach and on into the small intestine. Of course, occasionally the patient would get bored and start up a conversation. The most critical part of the study was to visualize the duodenal bulb, the first inch or two of bowel; that’s were a certain kind of ulcer frequently occurs. And of course, the patient would, more often than chance would dictate, begin to speak just as the bulb was filling. Was he acting under free choice, or was he responding to something about his inner workings? Let me give a resounding “maybe” on that one.
But on the face of it, it seems that the only choice made is “at the altar” – choice of mate rather than in bed.
What they published in that original paper was only families with short marital radius. They got inbreeding depression, starkly evident. I hope I was not impertinent, but I wrote to Dr. Labouriau and suggested they look at greater marital radii, expecting fertility to fall off again. They did so and have the result you see above. This was most gracious of them, putting effort into something proposed by an unknown stranger. If I had a chance to relive this moment, I would say, “And this time, when you graph it out, square the number of children. What you are doing is estimating average kinship or a series of populations. Populations are distributed over areas, not along lines.” Had they done so, it would have bunched the curve up on the left and stretched it out on the right, and the curve would now have resembled the two curves we have already seen. The “optics” would have been a bit better.
Examining their graph now, at marital radius fifty kilometers or less we see the inbreeding depression we knew all along had to be there and which first caught their eye. This is your chance to see inbreeding depression in humans.
At marital radius one hundred kilometers or greater there is a linear decrease in fertility as distance increases. Extrapolating a straight line in a Cartesian, linear, plane is straight forward; if the line is descending, has a negative slope, eventually that line must reach zero. It is clear that if you heedlessly select mates from a great distance (marry very distant kin; all humans are kin to some degree) for enough generations the population will die, while marrying closer kin judiciously would let the population continue to flourish. We already suspected as much from the Iceland data. However, from this alone, it looks like it could happen in a single generation, say if you married at thousands of kilometers away. This will prove not to be the case.
That much is the heart of this chapter. The rest is more like fun, I hope, but a bit open-ended.
Going back to the graph, it is generally accepted that an average of 2.1 children per couple give zero population growth, neither positive nor negative. The is most desirable; a consistently positive growth will ultimately outrun any finite environment that might support it. This seems nowadays to get more press than it deserves. We are near the end of a global population explosion. For a time, resources will inevitably be on the stretch, but that period of time will be finite. On the other hand, permanent negative growth must lead to population collapse followed most likely by the collapse of civilization followed by continued population collapse. This is not an attractive outcome.
As can be seen, from the graph, the line of fertility against marital radius crosses the 2.1 children per couple twice. To my eye one of these crossings is at about 20 kilometers, or about 12 miles. A young man will stroll at some 3 miles an hour, so if he wants to date a possible mate it would be best were she fairly nearby. There is some general rule that people will mutter if they have to walk (or drive or cycle) regularly for more than an hour. Would I, at a younger age, walk 4 miles for a date? I did it many times. So did my friends, those without cars. Of course, the line crosses the 2.1 level at about 250 kilometers, but even I never walked that far for a date.
I have an old map from a book called Camden’s Britannica published in 1695. It is odd that the title sounds like an encyclopedia, but the first English language encyclopedia I can find was the first Encyclopedia Britannica in 1768. At all events, it is a very old map, and I surmise it was copied from the Domesday Book, the most exhaustive inventory any country has ever made and was put together not long after the Norman Conquest. My map covers only a bit of Dorset. The map was evidently put together for land owners rather than travelers. There are no roads marked. But what they did was circle the amount of land that would support 100 farms or a “hund.” At a glance, a hund is about 6 miles across, so a young man need only walk an hour to visit a sweetheart within his hund; this is much more plausible than about 80 hours. If the countryside of Denmark was traditionally half as densely populated as that of England, then – area being the square of diameter – then our lad must walk only a quarter as far or 3 hours rather than 12. It’s not a great match, but we can see that it is in the ball park.
If, by strange event, either of these strategies were to be adopted and supported with social pressure, which do you suppose would be best? The 155-mile tactic has a couple of advantages. It offers each individual a much wider choice of possible mate. It also does not fall as badly afoul of what seems to be a grisly horror of inbreeding. I didn’t invent the phrase, might have been Freud, but I smell the fear everywhere.
Advantages of the shorter distance would be that it would be easier to keep track of, and it would tend to put couples together who share tastes and enthusiasm. My speculation is that between the two … “plans” seems too strong a word, maybe “theoretical options” … there is no difference in outcomes as far as the health of the offspring. Inbreeding depression is epigenetic, not genetic, and depends on probably only two mechanisms. If the mechanisms are satisfied as it were, the outcome should be fine either way.
This leaves us with the question: just how distant kin are those couples sharing a 6 or a 155-mile marital radius. Here I must confess to disappointment. Denmark is traditionally an agricultural society, while in Iceland, traditionally, a sheep herding and fishing society. Danes would thus be more sedentary while Icelanders more likely to move about. Each of the two scientists in question chose a method eminently adapted to the nationalities they studied.
I had hoped the two would see how well their work dovetailed, and they would work out just what was safe, and how people could get there. I wished all in vain. I am not sure they even exchanged pleasantries. The irony to me is that when I was presenting my poster in the Australian meeting I’ve described, I talked to a lot of people. One of the things I’d mention, if I could hang onto one of them long enough, was that the whole question could be laid out with the help of the Icelandic genealogy. When that was, in fact, done, they mentioned that they had received a clue from somebody I do not think I spoke with. So, it looks like the idea went from me to another and thence, perhaps, directly to Helgason. Of course, he had no way of crediting me, which is fine. Passing ideas along is what talking to people is all about.
Now, if I might clear my throat first, there is the notch. Somewhere between marital radius of 50 and 100, there is a distinctive fall, even if subtle, in fertility; my sketch exaggerates it a little. As I said, piling up data and scrutinizing it is a recurrent theme. I have looked at that notch for years. I cannot give you what seems to me an intuitively satisfying explanation. I have mentioned I am a bit skeptical of the highest marital radius fertility experience being a strictly straight line. This would be most understandable; the data points look thin and noisy.
But in the area of the notch, there is a lot of action. And the statistician to whom we owe this most illuminating graph seems to have been sure enough of his ground to introduce it although it kind of spoils his nice symmetry. Oh, we shall be seeing notches ad nauseum in data to be shown later. I am not clever enough to figure out whether this notch is the equivalent of all those others. There is a correlation, but correlation is not causation. On the other hand, if you have the patience to glance over my computer simulations, disgustingly noisy as they are, I think you might agree that, even the notch is modeled by the computer. And the computer program has no hidden processes (even though the computer and operating system are opaque to me). Then I shall make bold to assert that all the damage done by inbreeding depression is mirrored exactly by outbreeding depression.
Logically, I would expect now to stamp my foot and injure it, screaming, “Fertility is totally fixed by kinship issues and nothing else. War, famine, plague, social programs and natural disaster happen but in comparison are trivial in the long run.” Alas, it is not so simple. As we shall see later, there is evidence that folic acid in standard clinical dosage can reduce fertility, too.