How Christianity reversed the Roman demographic decline & restarted cognitive evolution

DNA from human remains is shedding light on the cognitive evolution of ancient Romans. What did Christian Rome do right that Pagan Rome did wrong?

Written by Peter Frost.

DNA evidence shows a rise, fall, and renewed rise in mean cognitive ability in ancient Rome, correlating with different eras. Various factors like natural selection, lifestyle, and socio-political influences like Christianity may be responsible. The Imperial era saw a decline, possibly due to reduced natural selection and social factors like increased slavery and lower reproductive rates among the upper classes. Late Antiquity, marked by the rise of Christianity, showed an increase in cognitive ability, possibly due to biocultural changes in marriage and reproduction. The findings raise questions about the long-term impact of socio-political conditions on cognitive evolution.

DNA from human remains is shedding light on the cognitive evolution of ancient Romans. After rising from the Neolithic to the time of the Republic, mean cognitive ability fell during the Imperial era. It then rose again during Late Antiquity. What did Christian Rome do right that Pagan Rome did wrong?

The Roman Empire was anything but eugenic. We actually have concrete data on this point, specifically DNA from human remains. Ancient DNA can reveal not only the physical appearance of long-dead people but also their level of intelligence or “cognitive ability.” We have in fact identified thousands of genes whose alleles are associated with variation in that trait (Lee et al., 2018). By checking which alleles are present and at what frequencies, we can make an estimate of cognitive ability that strongly correlates with performance on standardized tests (r = 0.8) and, on a group level, with mean population IQ (r = 0.9) (Piffer, 2019).

A team led by Davide Piffer looked at DNA from 127 individuals who lived in central Italy over a period extending from the Neolithic to contemporary times. The team found that mean cognitive ability rose from the Neolithic to the time of the Roman Republic. It then fell during the time of the Empire. Finally, a renewed upward trend began during Late Antiquity and continued into medieval times (Piffer et al., 2023).

Rise, fall, and renewed rise of mean cognitive ability in central Italy (Piffer et al, 2023, Figure 1).

Another research team, led by Michael Woodley of Menie, found a steady rise in mean cognitive ability among Europeans and Central Asians from 4,560 to 1,210 years ago. That study used DNA from 102 individuals, mainly from the early Bronze to early Iron ages, with none from the Imperial Roman era (Woodley of Menie et al., 2017).

In sum, mean cognitive ability steadily rose once humans began to farm and congregate in villages and towns, probably because their minds had to satisfy a growing range of cognitive demands. Then during the Roman Empire, this trend reversed throughout the Mediterranean. Why?

Relaxation of natural selection? Or rise of Christianity?

One explanation is that life was so comfortable that cognitive ability became much less important for survival and reproduction. It therefore regressed because natural selection no longer favored the more intelligent over the less intelligent. Then with the onset of Late Antiquity, life took a turn for the worse, and natural selection began to operate again.

Was the Imperial era a time of relaxed natural selection? That view is inconsistent with estimates of life expectancy: “Almost all ancient historians now accept the view, originally propounded by Keith Hopkins in 1966, that for the general population, average Roman life expectancy at birth is likely to have lain in a range from 20 to 30 years” (Frier, 2001, p. 144). Natural selection seems to have had plenty of room to do its work.

And did life take a turn for the worse during Late Antiquity – the period from 300 to 700 CE? Using skeletal data, Jongman et al. (2019) concluded that physical health actually improved after 300 and worsened only after 550. That trend is consistent with the historical record. It was not the abdication of Romulus Augustulus in 476 that brought an end to Roman civilization. In fact, his barbarian successors, Odoacer and Theodoric, were good rulers. It was really the Gothic War (535-554) and the Plague of Justinian (541-549) that destroyed Roman civilization, particularly on the Italian peninsula.

Late Antiquity did, however, have one constant: Christianity. The Christian faith was legalized throughout the Empire in 313 CE and became the State religion in 380 CE. Is it a coincidence that the same period saw an increase in mean cognitive ability? The Church intervened extensively in human reproduction, particularly by formalizing marriage and by vilifying divorce and extramarital relationships. Could those biocultural changes explain why mean cognitive ability rose after 300 CE?

To test the two proposed explanations—hard times versus Christianity—we could split the Late Antiquity data into two sub-periods: the years 300 to 550 and the years 550 to 700. If mean cognitive ability rose only during the second half, we could attribute the rise to hard times. Otherwise, Christianity would be the likelier cause.

Physical health over time in the Roman Empire, as shown by skeletal data (Jongman et al., 2019, Figure 4).

Disconnect between material success and reproductive success?

If Christianity caused mean cognitive ability to rise, why did the Paganism of Imperial times cause it to fall?

In short, because material success was not being translated into reproductive success. The upper classes were no longer passing on their higher cognitive ability to the next generation, either because they were having fewer children or because they were having them with concubines of lower cognitive ability. This explanation is not wholly at odds with the dolce vita one: the pursuit of a comfortable life had become less dependent on having a large family.

Rome’s cognitive decline thus had three causes:

• A decline in fertility and family formation among the upper classes (Caldwell, 2004; Hopkins, 1965; Roetzel, 2000, p. 234; Sullivan, 2009, pp. 27-28, 35-38).

• An increase in female hypergamy, driven by freed slaves, with a corresponding decrease in the reproductive importance of upper-class women (Perry, 2013).

• An increase in the slave population, particularly foreign slaves (Harris, 1999), with a corresponding disruption of local cognitive evolution. Surplus individuals from the upper classes could no longer move down into lower-class niches and replace the lower classes, as happened in late medieval and post-medieval England (Clark, 2007; 2009; 2023). Such niches were deemed fit only for slaves (Frost, 2022).

At first thought, the cognitive decline during Imperial times seems counterintuitive. This was when Rome was advancing its frontiers and building roads, aqueducts, and fortifications. Much less had been achieved during the time of the Republic. How do we explain the apparent contradiction?

The answer is that the Empire could mobilize far more human resources to achieve its ends. Centralized power can do a lot more with a large population than with a small one. Economies of scale arise, and labor can be put to specialized uses. In particular, the “smart fraction” of society, even if shrinking, can be put to better uses. Quantity has a quality all its own.

Did the Imperial era favor martial values?

The Empire may have been less than ideal for maintaining cognitive ability, but surely its rulers believed in virile, warrior values? And surely they helped maintain those values in the population?

Yes and no. The Roman Empire, like almost all states, was created through violence and maintained through violence. This point was made by Augustine in the fifth century:

… what are kingdoms except great robber bands? For what are robber bands except little kingdoms? The band also is a group of men governed by the orders of a leader, bound by a social compact, and its booty is divided according to a law agreed upon. If by repeatedly adding desperate men this plague grows to the point where it holds territory and establishes a fixed seat, seizes cities and subdues peoples, then it more conspicuously assumes the name of kingdom … (Augustine. De civitate dei 4.4)

Though created through violence, the Roman State outlawed the use of violence by everyone but itself. That monopoly was challenged whenever individuals acted violently to achieve their own ends. Such people, called bandits, made themselves adversaries of the State and when caught were punished by being thrown to the beasts, burned alive, or crucified (Frost, 2010).

The result was a social contradiction. Whereas the State could achieve its ends violently, the people had to achieve theirs peacefully. What was legitimate for one was illegitimate for the other.

The Pax Romana certainly benefited Roman society in the short term. By removing aggressive males from the population, it made life more liveable and spurred economic activity, especially long-distance trade. In the long term, however, this social pacification led to genetic pacification. The desire to use violence, even to protect oneself and one’s kin, was steadily removed from the gene pool and replaced with a more docile and submissive mindset (Frost, 2010).

Keep in mind that male aggressiveness is moderately to strongly heritable. A meta-analysis of 51 twin and adoption studies found a heritability of 40%(Rhee and Waldman, 2002). A later twin study found a heritability of 96%, the subjects being 9–10 year-olds from diverse ethnic backgrounds (Baker et al., 2007). This higher estimate reflects the closer ages of the subjects and the use of a panel of evaluators to rate each of them. According to another twin study, heritability is 40% when the twins have different evaluators and 69% when they have the same evaluator (Barker et al., 2009). Finally, a recent review paper concluded that about half the variance in aggressive behavior is genetic in origin (Veroude et al., 2015)

The Pax Romana thus pacified the Roman population not only socially but also genetically. One notable outcome was that people became less willing to enlist than they had been in earlier generations, and many would pay gold or cut off their thumbs to evade military service (Swain and Edwards, 2004, pp. 156-157; Williams and Friell, 1994, p. 37). From the fourth century onward, the army suffered from a lack of recruits and became increasingly dependent on foreign mercenaries (Southern and Dixon, 1996, pp. 67-72). A new kind of Roman was emerging, one less interested in violence and more submissive to authority. In fact, the new Roman was coming to see aggressive conduct as wrong, even wicked.

This was both a cause and an effect of the spread of Christianity. The latter proved to be a better fit for the new Roman.

Lessons for us today

Can we learn from Rome’s example? Yes, but the lessons are mostly negative. The Empire owed its existence to qualities of mind and behavior that existed previously—not only during the time of the Republic but also earlier. That mental and behavioral capital was run down during the Imperial era.

There were intellectuals and even emperors who understood what was happening, but little could be done without abandoning Imperial ambitions and returning to the sobriety of former times. Successful individuals were acquiring material possessions, rather than starting families. As for sexual desire, it could be satisfied in less cumbersome ways … A new sexual norm had become established, and few would challenge it.

Some emperors did, but they encountered resistance from the very people whose lineages were dying out (Sullivan, 2009). In 27 BCE, Augustus Caesar condemned childlessness in a speech to Rome’s equestrian class:

For you are committing murder in not begetting in the first place those who ought to be your descendants; you are committing sacrilege in putting an end to the names and honours of your ancestors; and you are guilty of impiety in that you are abolishing your families, which were instituted by the gods, and destroying the greatest of offerings to them — human life. (Cassius Dio. Roman History, 56.5.2)

Nor could Rome’s leaders do much about other long-term problems, like the declining quality of conscripts. The only workable solution was to hire foreign mercenaries, who could see close-up how weak and vulnerable the Romans really were.

Another long-term problem was dependence on slavery. Slaves had become essential to the economy, particularly in agriculture. That dependence was an Achilles’ heel: agricultural slaves generally had high time preference and if left on their own would produce only enough for their own needs, plus a small surplus (Harper, 2011, pp. 176, 250-254). They thus had to be coerced to produce enough for the entire market. Whenever that coercion weakened, during times of unrest or invasion, production fell sharply. The Romans knew this. In fact, everyone knew this.

But the benefits of slavery were too seductive. It wasn’t just the monetary benefits. Slave-owners enjoyed having access to young nubile women, without marital obligations. Slaves understood this weakness, as exemplified by a character from a fifth-century comedy: “We all steal and no one turns traitor, for we are all in it together. We watch out for the master and divert him, for slaves and slave-girls are in alliance” (Harper, 2011, p. 251).

There thus arose a population of alumni—children of the master and slave girls or simply emancipated children for whom the master had developed affection. If the master had no children by his wife, he could leave his property to his alumni (Rawson, 1986, 2014, pp. 173-179). Upper-class men weren’t simply having fewer children by their wives; they were also having more by slave girls. Elite couples were no longer passing on their mental and behavioral characteristics to future generations.

Banquet scene, fresco from Pompeii (Casa dei Casti Amanti).

Rome could be saved only by a moral revolution. When Constantine the Great legalized Christianity, he saw it as a means to revitalize and preserve the Empire. Was he himself a Christian? It is doubtful. His edicts, given below, are more consistent with a pagan mindset:

• 314 CE. Apprehended slaves trying to run back to barbarian lands should either have their feet cut off or be sent into the mine.

• 315 CE. Slaves and freedmen guilty of kidnapping are thrown to the animals and freeborn guilty of kidnapping are condemned to the gladiatorial games with no possibility for resistance.

• 326 CE. Because of their lowly status, barmaids are outside of adultery laws.

• 326 CE. Women who become involved with or marry slaves are to be executed and the slaves burned.

• 332 CE. [Edict for] setting torture as the proper way to ascertain truth from slaves. (Thompson, 2008)

Christianity failed to save the Empire and may even have hastened the eventual collapse. It did, however, lay the basis for a new civilization that would not only reverse the demographic decline of Imperial times but also restart cognitive evolution. It did so in two ways:

1. By favoring monogamy and, conversely, by limiting male polygyny and female hypergamy; and

2. By favoring the peace, order, and stability that allowed entrepreneurial individuals to exploit a range of new cognitive tasks, thereby ensuring their success both economically and demographically.

Thus arose Christian Europe – a civilisation that emerged long before the conquest of the Americas, the invention of printing, the Atlantic slave trade, and the Protestant Reformation. The root cause was a new societal model of social and biological reproduction (Frost, 2022).

Peter Frost has a PhD in anthropology from Université Laval. His main research interest is the role of sexual selection in shaping highly visible human traits, notably skin color, hair color, and eye color. Other research interests include gene-culture coevolution. Find his Newsletter here.

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Human evolution didn't slow down. It accelerated!

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