Fig. 1: The two main ways of defining cumulative culture (CC).

The line drawings (culminating in “stars”) represent cultural traits increasing in efficiency/complexity. a The process-focused definition describes CC as a gradual increase in the efficiency/complexity of a cultural trait; the product of such a process is called a cumulative cultural product, regardless of whether the product can be re-innovated by a single “naive” individual (illustrated by the dashed line). Examples for process-focused CC are the increased flight distance of paper planes over transmission chains of human adults (Caldwell and Millen, 2009) or the improved food-washing behaviours in Japanese macaques (Schofield et al., 2017, both examples lie possibly below the dashed line, i.e., both can potentially be re-innovated by naive individuals) or a bow and arrow (above the dashed line, probably too complex to be re-innovated from scratch by a single human). The area below the dashed line is equivalent to Mesoudi and Thornton’s (2018) core criteria CC, the area above is equivalent to their extended criteria. Core criteria CC is characterised by increases in the learnability of a trait or changes towards a fixed, local optimum (e.g., artificial languages becoming more easily learned (Kirby et al., 2008), pigeon flying routes increasingly approaching optimum (Sasaki and Biro, 2017)), while extended criteria CC is open-ended (e.g., many technological products such as ever-improving computers). b In the product-focused definition a process/product is labelled as cumulative only when the product of the process is beyond what any “naive” individual could reinnovate from scratch (i.e., it needs to lies above the dashed line; e.g., bow and arrow). Here, cultural traits that may be individually innovated (such as the paper planes or the food-washing behaviours) are not CC (labelled by some instead as latent solutions (Tennie et al., 2009)). For product-focused researchers, CC is inherently open-ended (Tennie et al., 2018, like Mesoudi and Thornton’s (2018) extended criteria of CC). Here, increases in the learnability of a trait or changes towards local optima of the trait resulting in products that remain within what naive individuals can re-innovate do not constitute CC, but have been called step-wise traditions (Tennie et al., 2009). Note that the term CC only applies to the level of a species or population, but not to the level of an individual. The labels that relate to the level of the individual are those introduced by Lev Vygotsky: the “Zone of Actual Development” (describing what an individual is already capable of doing by themselves) and the “Zone of Proximal Development” (describing what an individual can acquire through social learning). For further discussion on how the Vygotsky’s concepts relate to CC, see Reindl et al., 2018).