I'm happy to announce the publication of a new paper of mine in The Journal of Artificial Societies and Social Simulation (JASSS). JASSS is an open access journal focusing on the use of computer simulations as a tool for understanding social systems. I love publishing papers that anyone can get to at any time, without requiring logins, subscriptions, or fees. I wish every paper could be open access, and I wish I had the funds at my disposal to throw money at journals like JASSS to support such efforts. If I ever do have the money, that's the direction I'm going to throw it.
My paper, titled "A Model-Based Analysis of the Minimum Size of Demographically-Viable Hunter-Gatherer Populations" will appear in Volume 20, Issue 4 (due to be released on October 31). You can read the paper online here. It should be available in pdf form soon.
If you really want to get into the nitty gritty, the raw model code and an explanation are available here.
The focus of the paper using an agent-based model to revisit the question of how large human groups have to be to be demographically viable (i.e., able to survive over the course of many generations). This is a key question for understanding the size and structure of ethnographically- and archaeologically-known hunter-gatherer social systems as well as fleshing out scenarios of hunter-gatherer groups colonizing empty landscapes. Here is the abstract:
"A non-spatial agent-based model is used to explore how marriage behaviors and fertility affect the minimum population size required for hunter-gatherer systems to be demographically viable. The model incorporates representations of person- and household-level constraints and behaviors affecting marriage, reproduction, and mortality. Results suggest that, under a variety of circumstances, a stable population size of about 150 persons is demographically viable in the sense that it is largely immune from extinction through normal stochastic perturbations in mortality, fertility, and sex ratio. Less restrictive marriage rules enhance the viability of small populations by making it possible to capitalize on a greater proportion of the finite female reproductive span and compensate for random fluctuations in the balance of males and females."
My paper, titled "A Model-Based Analysis of the Minimum Size of Demographically-Viable Hunter-Gatherer Populations" will appear in Volume 20, Issue 4 (due to be released on October 31). You can read the paper online here. It should be available in pdf form soon.
If you really want to get into the nitty gritty, the raw model code and an explanation are available here.
The focus of the paper using an agent-based model to revisit the question of how large human groups have to be to be demographically viable (i.e., able to survive over the course of many generations). This is a key question for understanding the size and structure of ethnographically- and archaeologically-known hunter-gatherer social systems as well as fleshing out scenarios of hunter-gatherer groups colonizing empty landscapes. Here is the abstract:
"A non-spatial agent-based model is used to explore how marriage behaviors and fertility affect the minimum population size required for hunter-gatherer systems to be demographically viable. The model incorporates representations of person- and household-level constraints and behaviors affecting marriage, reproduction, and mortality. Results suggest that, under a variety of circumstances, a stable population size of about 150 persons is demographically viable in the sense that it is largely immune from extinction through normal stochastic perturbations in mortality, fertility, and sex ratio. Less restrictive marriage rules enhance the viability of small populations by making it possible to capitalize on a greater proportion of the finite female reproductive span and compensate for random fluctuations in the balance of males and females."
My main finding is that, under the varying conditions I investigate with my model, I find no support for the often-repeated idea that a society of about 500 persons is required to ensure demographic viability. Students of American anthropological archaeology -- especially, I suspect, those of us that went to the University of Michigan or were taught by U of M alums -- will immediately recognize the "magic number 500" as a concept that emerged from the research of Joseph Birdsell and (later) H. Martin Wobst. As I discuss in the paper, I think neither Birdsell nor Wobst intended the number 500 to take on the meaning that it did -- it became a kind of shorthand gloss for setting a general lower boundary on the size of hunter-gatherer social systems.
My modeling results suggest that the number of people required for demographic viability can be safely pushed down south of 200. In over 67,000 model runs (under varying conditions of mortality, fertility, and marriage rules) where the mean population exceeded 150 people, the population went extinct only nine times. I'd take those odds.
All the modeling was done under conditions with no logistical constraints to identifying and obtaining marriage partners: no spatial component to interaction, no impediments to the flow of information. Logically, putting the model systems in space and dispersing the populations across a social/physical landscape would have the ultimate effect of raising the population size required for demographic viability. Would it double or triple it, though? I highly doubt it. But the great thing about modeling is that we don't have to be satisfied to simply suppose things -- we can model the problem. Understanding that less than 200 people are required for demographic viability assuming no interaction issues, we can then unpack the issue to ask why hunter-gatherer societies are often much larger. What role does the structure of mobility play? What about the need to maintain a geographically-extensive social fabric to buffer large-scale environmental variability? Here are a couple of paragraphs from my conclusion:
My modeling results suggest that the number of people required for demographic viability can be safely pushed down south of 200. In over 67,000 model runs (under varying conditions of mortality, fertility, and marriage rules) where the mean population exceeded 150 people, the population went extinct only nine times. I'd take those odds.
All the modeling was done under conditions with no logistical constraints to identifying and obtaining marriage partners: no spatial component to interaction, no impediments to the flow of information. Logically, putting the model systems in space and dispersing the populations across a social/physical landscape would have the ultimate effect of raising the population size required for demographic viability. Would it double or triple it, though? I highly doubt it. But the great thing about modeling is that we don't have to be satisfied to simply suppose things -- we can model the problem. Understanding that less than 200 people are required for demographic viability assuming no interaction issues, we can then unpack the issue to ask why hunter-gatherer societies are often much larger. What role does the structure of mobility play? What about the need to maintain a geographically-extensive social fabric to buffer large-scale environmental variability? Here are a couple of paragraphs from my conclusion:
"My results are broadly consistent with those from two other models (Moore 2001 and Wobst 1974) that have considered questions of demographic viability and accord reasonably well with the empirical data we have that documents the existence of hunter-gatherer social systems appreciably smaller than 500 persons (see Birdsell 1953, Figure 9; Moffett 2013). Factors other than stochasticity in mortality, fertility, and sex ratio (e.g., environmental variability of spatial components of interaction behaviors) presumably influence the size of actual hunter-gatherer social systems and encourage them to exceed the minimum size threshold required for demographic viability. If we accept that a population of 150 is a reasonable baseline estimate for the population size sufficient to ensure demographic viability over long spans of time, we might then reasonably reconsider our explanations for why some hunter-gatherer social systems exceed this minimum. If there is a downward pressure that encourages hunter-gatherer social systems to be as small as possible, it seems likely that something other than demographic viability (in the sense of the term as used here) constitutes the limiting factor when social systems encompass significantly more than 150 people. Understanding how other factors might relate to the minimum and maximum size of hunter-gatherer populations will require further work."
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