Behavioural variation in 172 small-scale societies indicates that social learning is the main mode of human adaptation

Abstract

The behavioural variation among human societies is vast and unmatched in the animal world. It is unclear whether this variation is due to variation in the ecological environment or to differences in cultural traditions. Underlying this debate is a more fundamental question: is the richness of humans' behavioural repertoire due to non-cultural mechanisms, such as causal reasoning, inventiveness, reaction norms, trial-and-error learning and evoked culture, or is it due to the population-level dynamics of cultural transmission? Here, we measure the relative contribution of environment and cultural history in explaining the behavioural variation of 172 Native American tribes at the time of European contact. We find that the effect of cultural history is typically larger than that of environment. Behaviours also persist over millennia within cultural lineages. This indicates that human behaviour is not predominantly determined by single-generation adaptive responses, contra theories that emphasize non-cultural mechanisms as determinants of human behaviour. Rather, the main mode of human adaptation is social learning mechanisms that operate over multiple generations.

Keywords: cognitive niche hypothesis; cultural evolution; cultural inertia; evoked culture; human behavioural ecology; social learning.

Figure 1.

Cultural history has a larger effect than ecology on the presence or absence of traits in a tribe. Boxplots of (a) C and E, and (b) P, E and S, for 457 behavioural traits. Boxes show the median, 25th and 75th percentiles, error bars show 1.5 IQD.

Figure 2.

Cultural history has a larger effect than ecology in most categories of traits. Boxplots of the scaled ratios given by (a) (tan⁻¹C/E)/45, (b) (tan⁻¹P/E)/45, (c) (tan⁻¹P/S)/45 and (d) (tan⁻¹S/E)/45. The tan⁻¹ scaling transforms the ratios C/E, P/E and S/E, which can range from 0 to ∞, into numbers that range between 0° and 90°. This gives the slope of the line from the origin (0,0) to the coordinate (C, E), (P, E) or (S, E), respectively, which when divided by 45° yields a number between 0 and 2. A number greater than 1 (to the right of the vertical black line) means that the effect of the numerator is larger than that of the denominator, and vice versa.

Figure 3.

The effect of cultural phylogeny decreases with time, but can be detected over thousands of years. Boxplots of P using language phylogenies of different time depth. Time Depth 8 corresponds to the distribution of P when phylogeny is captured using all eight levels of the language classification, from Level 1 (phylum, the oldest level) to Level 8 (language, the most recent level). Time Depth 7 corresponds to the distribution of P when cultural history is captured using Levels 1 to 7, and so on. The x-axis is truncated for visibility.