Peer review of: Scale and information-processing thresholds in Holocene social evolution (2020)

Reviewers' Comments: 
 
Reviewer #1: 
Remarks to the Author: 
This is an excellent paper that deserves publication in Nature Communications with 
only minor changes. The authors expand upon work recently published in PNAS that 
used PCA to argue that change in population (social scale) was the major driver of 
human cultural evolution. This paper develops that finding and looks at other 
components manifest in the underlying data and their interaction with population. 
The authors find that capacity for information processing is also a key driver of 
cultural evolution, and argue that this finding helps to explain the interesting 
pattern of punctuated equilibrium that many scholars have identified in cultural 
evolutionary sequences. The authors also provide a brief counter-argument to an 
article on the evolution of moralizing high gods that recently appeared in Nature. 
 
The paper's content and findings are novel and will be of interest to a broad 
audience of scholars in the historical and evolutionary sciences. This paper fits 
within the interdisciplinary focus of Nature Communications, but might be more 
appropriately placed in Nature Human Behavior or Nature Ecology and Evolution 
(where a commentary on the importance of studying cultural evolution appeared in 
one of the early issues). In any case, this is an important paper and is certainly of 
the quality and impact to be published in Nature Communications. 
 
The data used by the authors is based on the Seshat databank and have already 
proven themselves to be useful in exploring human cultural evolution. They are 
robust data, valid and reliable, and completely appropriate to the author's purpose. 
The analyses performed on these data are clear, appropriate, and well-described. 
The interpretation made of those findings are reasonable and clearly presented. 
 
I am a bit puzzled about the extensive analyses of bimodality presented in the 
supplementary materials, as they seem unnecessary in terms of supporting the 
specific analyses presented in the paper. They are quite interesting, however, and I 
would urge the authors to consider publishing them separately. 
 
I also do not think that the short critique of the moralizing high gods paper by 
Whitehouse et al. is necessary. The section does not add much to the paper's core 
analyses or argument, and to me only distracts from the paper's overall flow. I 
would suggest leaving it out. 
 
The authors do a good job in the short space they have discussing relevant 
literature, but I would suggest they give a nod to Greg Johnson and to Krisztina 
Kosse who built the foundation upon which the general "scalar stress" interpretation 

of the relationship between population, information, and cultural change was built. 
 
In sum, this is an excellent paper with potentially broad impact and appeal and 
deserves publication. 
 
Peter Neal Peregrine 
 
 
 
Reviewer #2: 
Remarks to the Author: 
This manuscript addresses the fundamental question: What is the relationship 
between the increase in size of a polity and the increase in its information 
processing capabilities? Is an overall increase in the social complexity of a polity 
(e.g. number of levels of hierarchy) driven first by an increase in polity size, or first 
by an increase in information processing capabilities and institutions such as 
money? For the first time, this manuscript places these in temporal order when 
considering data from a large number of polities around the word. By building on a 
previous Principle Components Analysis conducted by Turchin and colleagues, which 
considers 9 complexity characteristics for 200 polities over time, the manuscript 
delineates between the two alternatives by considering (graphically) the 
relationship between the first and second principle components. The conclusions are 
non-trivial but also intuitive. They are that polities tend to first increase in size, 
until they reach a size threshold limit. Polities then need to increase their 
information processing capabilities to grow further. Finally, a limit in the amount 
that information processing capabilities can increase is reached, which is only 
crossed by further increases in size. 
 
I find the analysis sound and am supportive of publication. I have minor changes to 
suggest: 
 
1. Presumably the process of increase in size, then in information processing, then 
in size again is cyclic, with the second growth in size presumably allowing for a 
second increase in information processing capabilities. Would you expect to see this 
is then dataset was extended up to the 21st century? The industrial revolution could 
be ideal to consider in this context. However, your results also suggest there may 
be a sink point around a value of 2 for PC1, which would suggest the process 
cannot continue? 
 
2. On page 2 it would be good to clarify exactly how many polities are in the Seshat 
database. 285 polities are used for the analysis, but what percentage of the 
database is this? 

 
3. Page 3: "This is followed by a second region where polities become more spread 
apart; we can speak of them taking different paths in their growth in scale" -- I 
think this should be "different paths in their growth in information processing 
capabilities". 
 
4. Page 5 mentions error bars. These should be shown on the graphs. 
 
5. Page 5 states that growth in scale may be constrained by unsophisticated money 
or writing. It is could also be constrained by "texts", which has a relatively high 
loading on PC2. 
 
6. In the caption of Figure 3 it would be good to stress that the gradients of the 
lines are the key aspects that the reader should focus on. 
 
7. On page 10 I find the claim that small-scale societies are relatively more diverse 
than large-scale societies surprising. Although you do provide a reference, it would 
be good to expand on the argument for this if possible. 
 
8. When talking about institutions, be careful not to anthropomorphise too much. 
Under the definition of institutions you cite, an institution is defined by a political 
interaction that sets the rules for a subsequent economic interaction. This is 
different to the notion of institutions as actors, e.g. a university as an institution. In 
this sense, institutions themselves would not seek to improve mechanisms for 
information storage and retrieval. Rather, the actors in the political interaction (e.g. 
chiefs) would seek to do this, so they can better shape the rules of the economic 
interaction to benefit themselves. 
 
 
9. SI page 1: "at most one NGA controlling the polity at any time", the orders of 
NGA and polity should be switched here.