The Black Plague and European Intelligence

In this post, I want to detail some evidence as to how the ‘Black’ plague or bubonic plague caused a change in European genetic makeup, particularly that concerning intelligence. A few years ago, a study made mainstream news showing that Europeans have less susceptibility to HIV, and generally have stronger immune systems than before, because of the black plague (Laayouni et al., 2014). Now, that finding is not actually the focus of this post, but it allows the ideas within the post to become more realistic.

The Black Plague lasted just a few years and in that time, it killed about 30-60 percent of Europe’s population (Benedictow, 2005). By doing so, it likely took out entire classes of people, which may have had positive effects on the following gene pool. Hence, the story of European success can be partially explained.

This is not to say that the entirety of European genetic change can be to the Black Plague, which would be very likely false. The European shift from bad conditions to great conditions did not happen in only 1300 and 1400, though it did appear to begin there. However, the entirety of European success can be predicted from continuous genetic change, which, itself, may have been sparked from the Black Plague.

The story may follow like this: the Black Plague selectively wipes out the bottom half of the population. Because of this, the bell curve essentially shifts. This is elaborated more on in Section 3 This genetic change is self-fulfilling, causing the population to exponentially become better over time. It’s a very simple idea, which has a good amount of evidence to support it.

Table of Contents:
1: The Selectivity of the Black Plague
2: Population Replacement
3: A Rough Estimate
4: Ripple Effect
5: Conclusions

1: The Selectivity of the Black Plague

The position might be held that the Black Plague was so bad that everyone was affected – the rich, the poor, the kings, the peasants, etc. But, this does not hold with what evidence we have so far. One example of this come from DeWitte and Wood (2008). They conducted a large study on nearly 500 skeletons from the peak of the Black Death. In the paper, they find that the people most affected by the Black Plague were those who already had poor health conditions, or had higher frailty. While it is not guaranteed, it is a likely assumption that these people were predominantly poorer people, or lower in social class relative the rest of the population. The upper classes had more resources to afford higher-quality medical care, and they were likely given higher priority in medical care on the basis of class alone. Yes, medical care was poor in general, but that doesn’t mean it was equally poor for everyone.

A review by Cavigelli and Chaudhry (2012) finds that social class is robustly related to mortality and immune system, which, would imply strong lower-class selectivity if consistent during the Middle Ages.

More directly, Benedictow (2005) states,

“This means that they overwhelmingly registered the better-off adult men of the population, who for reasons of age, gender and economic status had lower mortality rates in plague epidemics than the general population. According to the extant complete registers of all households, the rent or tax-paying classes constituted about half the population both in the towns and in the countryside, the other half were too poor. Registers that yield information on both halves of the populations indicate that mortality among the poor was 5-6 per cent higher. This means that in the majority of cases when registers only record the better-off half of the adult male population, mortality among the adult male population as a whole can be deduced by adding 2.5-3 per cent.”

The next piece of evidence is more a function of non-selectivity of the plague. I have reviewed some evidence of a disparity in rural and urban average IQs, favoring urban people elsewhere (see Jeffrey, 2019). This probably wasn’t different during the Middle Ages, as most rural workers were low-skill workers. Rural citizens also had lower life expectancies in the Medieval ages (see Kowaleski, 2014). The previous source also details how higher-status people in Medieval societies tended to live longer. Additionally, it has been empirically shown that during the plague, most of the deaths were actually in the countryside (see Benedictow, 2005)

The poet, Guillaume de Machaut describes the destruction of the rural economy after the Black Death:
For many have certainly
Heard it commonly said
How in one thousand three hundred and forty nine
Out of one hundred there remained but nine.
Thus it happened that for lack of people
Many a splendid farm was left untilled,
No one plowed the fields
Bound the cereals and took in the grapes,
Some gave triple salary
But not for one denier was twenty [enough]
Since so many were dead…

This is not to say that urban and rural areas were not disproportionately affected at all. For example, the size of a city prior to the plague is strongly predictive (r=0.97) of the duration that the city endured the plague (Olea and Christakos, 2005). But, it is clear that rural areas were drained of people, which possibly had a eugenic effect.

Overall, there is significant evidence the lower classes were disproportionately affected by the plague.

2: Population Replacement

While it took a long time for Europe to fully restore its population, the rate of population growth was actually exacerbated by the plague. In Table 1 below, I have recreated (I excluded some unnecessary rows) a table of data from Urlanis (1941):

Table 1
European Population Dynamics, 1000-1500
YearTotal European Population, millionsAbsolute growth per century, %Average growth per year, %
100056.4
110062.110.10.10
1200689.50.09
125072.915.70.14
130078.70.15
135070.7-0.8-0.21
140078.10.20
14508316.10.12
150090.70.18

Before the Black Death, a large rise in births took place, but they were cancelled out by famine and war at the time (Herlihy, 1997). So, when the plague took place and especially when wages soared afterwards (Britannica, 2019), it should be no surprise if more people had access to food post-Plague. This gives some explanation to the rise in absolute population growth in the generations after the Black Plague’s prime era.

An important aspect of Medieval fertility is who was actually having the children. Unlike today, fertility during the Middle Ages was eugenic, meaning the highest social classes were having more surviving children than the lower classes (Clark and Hamilton, 2006; Clark, 2009). This is going to be particularly important in deriving an estimate of the effect of this event.

3: A Rough Estimate

This section is technically the least scientific because I have to make arbitrary assumptions or guesses. But, whether or not my rough, official estimate is correct is not the issue, because with all the information we have so far, we can predict there was certainly some permanent increase in European intelligence. As we will see at the end of this section, this does not have to be large to have massive consequences.

How The Curves Shift

The first objection to the importance of my theory is the shape of the distribution after the Black Plague. When we measure the distribution of traits within a population, they generally form what is called a normal distribution or a bell curve. When the Black Plague hit, we know that the bottom half of society was cut off, but it is not likely the trait distribution automatically re-aligned itself (to a normal distribution).

On its own, this isn’t a large burden to the theory presented here. But, it doesn’t matter anyways, because fertility was eugenic in Medieval Europe. So, after x generations, the populations likely clustered around a new mean, therefore shifting the normality of the distribution (back to normal).

Was Feudal Society Meritocratic?

One major limitation I saw to this theory was that Medieval society was generally less meritocratic towards intelligence and social class than we are today. A large part of Medieval culture was assigning people towards specific castes based on divinity, which also had some hereditary aspect to them. There was also a good deal of idolization of the warrior spirit.

That said, they weren’t entirely non-meritocratic – some elements of intelligence-valuing certainly happened, even if they weren’t intentional. The first would be the praise of a priestly caste in Medieval Europe. I read a good deal of perennial traditionalist literature around a year ago and I specifically remember a quote from Rene Guenon (1927). He said,

“If however the intellectual elite were effectually constituted and its supremacy recognized, this would be enough to restore everything to order, for spiritual power is in no way based on numbers, whose law is that of matter; besides-and this is a point of great importance-in ancient times, and especially in the Middle Ages, the natural bent of Westerners for action did not prevent them from recognizing the superiority of contemplation, or in other words, of pure intelligence. “

And so with this in mind, and the fact that priests, who were highly valued, had to learn how to read the Bible, intelligence was likely indirectly valued and pushed towards the upper social classes.

The other point I want to make is that height was particularly valued in Ancient civilization. The reason for this is that being tall was seen as being closer to the gods. Height also shares a genetic correlation with intelligence, even when controlling for assortative mating, so there appears to be an intrinsic link between the two which can be attributed to pleiotropy (Keller et al., 2013). There are probably other links similar to this which we could show, but these points should be enough to show there was some inherent meritocracy in Medieval Europe.

An Estimate and Its Implications

Because I am about to make a lot of arbitrary assumptions, I will preface this by saying these are all personal estimates and by no means hold scientific weight. This is my belief based on the information we have so far. So, a disagreement with this estimate (and with the following assumptions) is entirely natural. As I will explain after the estimate, any increase is going to make a major difference.

So, the first assumption is that we can cut off the bottom half of the normal distribution because of the plague. This assumption is based off two details: the amount of people killed by the plague is still debated, though I find the 60% estimate to be more convincing, and the plague was not perfectly selective. This assumption also makes an estimate easier

The second assumption is going to be how far the distribution moved in the following generations. My (very arbitrary) assumption is that the average IQ moved 2 SDs after multiple generations before correcting for extra details. It had to have a substantial boost because of the eugenic selection at the time and the sheer amount of lower class people stripped away from the civilization.

The final assumption is going to be a relief for those who think the prior number is way too high. First of all, regression to the mean probably happened for this sample. We are discussing the eugenic effect of outliers who are sensitive to RTTM, and IQ is made up of 50% additive genetic variance. Also, this society was certainly less meritocratic than modern society. Because of both of these pieces of information, I have multiplied the 2 SD increase in IQ by 0.25 = 0.5 SD increase in mean IQ after a few generations of eugenic reproduction.

What does this mean in perspective? A 0.5 SD increase in IQ is 7.5 points which is sizable on its own, but the main effect we are interested in is at the tails of the distribution – the extremes of civilization. Let’s say the avg. IQ of Europe was 85 prior to the Black Plague (not an entirely terrible estimate). About how many people would have an IQ of 160 or more (geniuses)? The percentage of people 5 standard deviations above the mean is 0.000000287% (yes, that rare). An increase in 0.5 SDs means that the reach for an IQ of 160 is somewhat shorter. The percentage of people who are 4.5 SDs above the mean is 0.00000340. This means that, if I am correct in my estimate, geniuses would be nearly 12 times more prevalent a few generations after the plague (and remember the group was repopulating).

If the increase was smaller, the large increase at the means is still going to be large, and therefore the plague is going to have an inevitably large, eugenic effect which likely rippled into multiple factors that allowed Europe to rise.

4: Ripple Effect

There are surely more effects we could list here but here are some of the major ripples caused by the Plague:

The largest ripple effect of the Black Plague was that it was responsible for reforestation, which in turn led to the Little Ice Age (Ravilious, 2006). The Little Ice Age largely decreased the temperature of Europe, which in turn, had to have pressed extreme selective pressure on Europe. The colder temperatures of the Little Ice Age caused greater group selection which is linked to increasing general mental ability (see Woodley et al., 2017). This played a large part in putting the West into power and creating the Industrial Revolution.

The idea that the Black Death sparked a change in European civilization is not new to any extent. Multiple books have been devoted to this. One which may be of interest is The Black Death and the Transformation of the West by David Herlihy (1997) which shows that the Black Death was instrumental in developing the modern university.

This makes sense, especially with the centralization of population into urban areas and the increase in population we showed before. A paper by Michael Kremer (1993) discusses the effect that population growth has on society. Essentially, when populations grow, there is more room for innovation, they then grow in GDP, and because of that will grow in population again (an exponential, self-fulfilling loop). This loop is exacerbated by the destruction of rural community, pushing towards greater proportions of universities in particular.

Murray (2002) shows that the greater the population density, the greater the amount of universities, and finally, the greater the amount of innovation in an area. Another important finding from Murray is the rise of European innovation in the arts and sciences. Sometime between 1200 and 1600, it is clear that European innovation was beginning to make a rise above everyone else:

Finally, the Plague played a major part in the religion of Europeans through a ripple effect. Slavicek (1998), a very important source on the effect of the Black Plague, describes how the Church was much more economically stressed. Because of this, they resorted to selling dead bodies and indulgences during the Plague. People began to lose faith in the Church, reducing religiosity for one, but more importantly, leading to the rise of Protestantism a bit over a century later. Protestantism is undoubtedly a major factor in the development of the West.

And so in these ways, we can see that the Plague caused an important ripple in European history which allowed it to become a major power, particularly combined with other evolutionary factors.

5: Conclusions

I have shortly explained a theory for European success through the ripple of the Black Plague. This does not interfere with other genetic explanations for European success such as Cold Winters Theory or the European Revolution, but rather they add on to them. This theory simply purports the following:

1: The Black Plague was highly selective of the lower classes in Medieval Europe
2: Following the Black Plague, the upper class began to repopulate Europe, and due to the eugenic fertility, the new population was far different from the pre-Plague population
3: There remained some meritocracy towards intelligence and social class in Medieval Europe, eugenic fertility was taking place, and so we can expect the new population was much more intelligent
4: This intelligence caused a ripple effect, pushing other major events to happen which exponentially advanced European civilization.

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