Researchers investigated further into why and how humans speak so many languages across the globe, and why they are spread out unevenly. Using two different path analyses, a Stationary Path analysis and a GWPath, researchers tested the effect of eight different factors on language diversity. Out of the eight variables (river density, topographic complexity, ecoregion richness, temperature and precipitation constancy, climate change velocity, population density, and carrying capacity with group size limits), population density, carrying capacity with group size limit, and ecoregion richness had the strongest direct effects. Overall, the study revealed the role of multiple different mechanisms in shaping language richness patterns. The GWPath showed that not only does the most important predictor of language diversity vary over space, but predictors can also vary in the direction of their effects in different regions. They conclude that there is no universal predictor of language richness.

The authors investigate the relationship between linguistic diversity and various environmental and spatial variables associated with biodiversity. Most of these variables predict linguistic diversity variably across different continents, and more so within Africa and extended Asia (Asia, the Pacific, and Australia) than within Europe and the Americas. This divide is theorized to be a result of differences in demography and impact of colonialism between the two global regions. However, two environmental factors, landscape roughness and density of river systems, are found to be significant predictors across all global regions. The authors suggest that, as in processes of speciation, rough terrain and watercourses both create physical barriers between which languages can develop in isolation while, in the case of river junctions, also providing transportation routes whereby hybrid languages can occasionally manifest.

Inter-group variation is greater in humans than in any other animal, and scholars continue to debate the cause of this diversity. Two competing explanatory models of human variation emphasize either (1) ecological differences and "evoked" culture or (2) population-level effects of cultural transmission. The former emphasizes mechanisms that operate within a single generation, while the latter emphasizes cumulative cultural history operating over many generations. To test these competing models, the authors measured the relative power of ecological variables as compared to culture history to predict behavioral variation in 172 western North American tribes. Culture history is subdivided into culture phylogeny (based on language phylogeny) and spatial distance.

In the field of bioacoustics, the Acoustic Adaptation theory suggests that variation in vocalization across different species can be accounted for by the acoustic properties of different habitats. Here, the researchers test consonant- and vowel-heaviness in human languages against various environmental variables in order to examine the theory's potential application to our own species. The authors identify a significant negative correlation between consonant heaviness and temperature, precipitation, and tree cover, and some positive correlation with rugosity and elevation as their most important findings, while acknowledging the potentially influential roles of migration and demographic factors in producing this relationship.

Prior research by Oswalt (1973, 1976) and Torrence (1983, 2001) has suggested that risk of resource failure is a significant predictor of toolkit complexity and diversity among hunter-gatherers. In this paper, the same relationship is tested among small-scale herding and farming groups. However, no significant correlation is discovered between any measure of resource risk and any measure of toolkit complexity. The researchers suggest that this absence may be the result of greater reliance on non-technological diversification methods among farmers (i.e. spatial diversification, mixed farming, intercropping), or of other unaccounted-for sources of risk (i.e. intergroup raiding and warfare).

This paper poses and tests relationships between local spatial variability of rainfall and property rights, as well as the degree of hierarchy, across 41 different tribal societies in Sudan. The authors find that there is a significant negative relationship between rainfall variability and degree of hierarchy. They also find that in the presence of local rainfall variability in Sudan, members of a tribe are more likely to establish common property institutions that allow access to all members.

The article discusses the role of land ownership in natural resource management and social-ecological resilience, and explores the factors that determine ownership norms in human societies. The study tests long-standing theories from ecology, economics, and anthropology regarding the potential drivers of land ownership, including resource defensibility, subsistence strategies, population pressure, political complexity, and cultural transmission mechanisms. Using cultural and environmental data from 102 societies, the study found an increased probability of land ownership in mountainous environments and societies with higher population densities. The study also found support for the idea that neighboring societies might influence land ownership. However, there was less support for variables associated with subsistence strategies and political complexity.

The second and third parts of this book classify the economic systems of foraging and agricultural societies in the SCCS based on correlations between their institutions of property an distribution. These economic types are then examined for relationships with other social, political, demographic, and environmental factors in order to draw tentative conclusions regarding the origins of the Agricultural and Industrial Revolutions. The fourth part of the book uses cross-national data to examine similar associations in industrial/service economies, and is not included here.

Finger millet, pearl millet, and sorghum are important drought-tolerant crops used as primary staple crops in drylands. Despite their long history, they have received less attention in recent years and their production has decreased. New models show that annual precipitation is not the only factor affecting their agriculture, but other variables like growing cycle duration, soil water-holding capacity, and soil nutrient availability also play a crucial role, as evidenced by ethnographic data. The traditional cultivation practices of these crops can be a sustainable response to the increasing aridity levels worldwide and can contribute to the resilience of dryland agriculture.

Using multiple logistic regression, the researchers compare the relative strength of predictors of land ownership across 102 societies. The analysis finds significant predictive power in factors such as neighbors' property system, population density, and geography.