1. The Introduction is a bit long. Conversely, I would like to see more information on the dataset in the Method section, particularly the following:
- The 127 individuals were from Rome and central Italy.
- Ancestry in the dataset varied over time:
- During the Imperial period, ancestry shifted toward the eastern Mediterranean. Very few individuals were primarily of western European ancestry
- During Late Antiquity, with the move of the capital from Rome to Byzantium, there was a sharp decline in eastern Mediterranean ancestry and an overall decline in Rome's population. There was a corresponding larger proportion of individuals of central European origin, which may also reflect settlement by Goths and Lombards.
- During the Medieval period, ancestry further shifted toward central and northern Europe.
The authors should also provide dates for the different periods: Pre Iron Age ??; Republic 320 - 27 BCE; Imperial Rome 27 - 300 CE; Late Antiquity 300 - 700 CE; Medieval 700 - ??; Contemporary Italy ??. This point is important. For instance, the term "Late Antiquity" is defined differently by different authors.
2. The authors seem to feel that "harsh conditions" select for increased cognitive ability. This is not true if the harsh conditions occur randomly. If they occur over a predictable annual cycle, you will get strong selection for planning and for cognitive solutions over a long timeframe. Otherwise, you simply get selection for fatalism and fetishism. In the Canadian Arctic, the degradation of environmental conditions during the Little Ice Age led to increased production of religious objects by the Dorset people, but not to new technologies.
This is, incidentally, a frequent criticism of the "cold winter theory." Why don't we see strong selection for cognitive ability in desert regions? Again, it is not the harshness of the environment that selects for cognitive ability. It is the predictableness of harsh environmental conditions. Only then do you get selection for planning and forward thinking. This was the case in northern Eurasia during the last ice age. Food and fuel were available during predictable intervals of time. That situation led, among other things, to the development of untended devices, such as pits, traps, weirs, and nets, and the digging of storage pits to refrigerate perishables, either meat for year-round consumption or bones for winter fuel.
Frost, P. (2019). The Original Industrial Revolution. Did Cold Winters select for Cognitive Ability? Psych 1(1): 166-181. https://doi.org/10.3390/psych1010012
3. The authors rely heavily on secondary and tertiary sources. Why not cite the original sources (e.g., Ibn Khaldun)?
4. Did the Late Antique Little Ice Age cause the Justinian plague? I don't think so, but what does it matter? If you burden your main argument with too many dubious sub-arguments, many readers will dismiss the main argument. This is a recurring problem I see with your paper. You can't resist the temptation to throw in lots of controversial ideas, and those controversies tend to overwhelm the main argument.
5. Why is there no mention of the rise of Christianity? The Western Church played a major role in reversing cognitive decline, specifically by supporting the formation of monogamous families, by discouraging slavery, at least during the long period from 500 to 1500 AD, and by creating the peace, order, and stability that allowed the middle class to expand and become dominant. At the very least, you should address the Schulz et al. (2019) paper.
Frost, P. (2022). When did Europe pull ahead? And why? Peter Frost's Newsletter. November 21. https://peterfrost.substack.com/p/when-did-europe-pull-ahead-and-why
Schulz, J.F., D. Bahrami-Rad, J.P. Beauchamp, and J. Henrich. (2019). The Church, intensive kinship, and global psychological variation. Science 366(707): 1-12. https://doi.org/10.1126/science.aau5141