Abstract
The breakthrough study of Dean et al. (Science 335:1114–1118, 2012) claimed that imitation, teaching, and prosociality were crucial for cumulative cultural learning. None of their child participants solved the final stage of their puzzlebox without social support, but it was not directly tested whether the solution was beyond the reach of individual children. We provide this missing asocial control condition, showing that children can reach the final stage of the puzzlebox without social support. We interpret these findings in the light of current understanding of cumulative culture: there are currently conflicting definitions of cumulative culture, which we argue can lead to drastically different interpretations of (these) experimental results. We conclude that the Dean et al. (Science 335:1114–1118, 2012) puzzlebox fulfils a process-focused definition, but does not fulfil the (frequently used) product-focused definition. Accordingly, the precise role of social support for the apparent taxonomic distribution of cumulative culture and its ontogeny warrants further testing.
Introduction
A potentially defining feature of humans is the ability to produce cumulative culture (CC), a key factor differentiating us from non-humans (Richerson and Boyd, 2005; Tennie et al., 2009; Tomasello et al., 1993; Whiten, 2017, but see Claidière et al., 2014; Jesmer et al., 2018; Sasaki and Biro, 2017; Schofield et al., 2017), and a phenomenon resulting from a nexus of capacities that are believed to be more developed in humans compared to other species, such as language, prosociality or perspective-taking (Tomasello, 2019). Currently, there is no widely agreed-upon definition of CC, a situation which—as we show below—has major implications for both the design and interpretation of experimental findings. Yet, with an increasing number of experimental studies on the social and cognitive processes underlying CC in humans and non-human animals (Caldwell and Millen, 2008; Caldwell et al., 2012; Claidière et al., 2014; Dean et al., 2012; Derex et al., 2013, 2019; Derex and Boyd, 2015; Fay et al., 2019; Jesmer et al., 2018; Mcguigan et al., 2017; Reindl et al., 2017; Sasaki and Biro, 2017; Schofield et al., 2017; Wasielewski, 2014; Zwirner and Thornton, 2015), researchers have highlighted the need for conceptual refinements and clarifications (Caldwell et al., 2016; Charbonneau, 2018; Mesoudi and Thornton, 2018; Miton and Charbonneau, 2018; Reindl et al., 2017; Schofield et al., 2017). One factor that has contributed to misunderstandings and disagreements regarding what constitutes CC, and to what extent it can be found in non-human animals, is the fact that the term “cumulative culture” is used to refer to both cultural products (i.e., behaviour or products of behaviour) and processes (i.e., cumulative cultural learning, cumulative cultural evolution; Tennie et al., 2018; see Mesoudi and Thornton, 2018, for an overview of current definitions of CC)Footnote 1.
For some researchers, CC describes a process of a gradual increase in the efficiency and/or complexity of a cultural trait through repeated innovation and transmission events (often over generations; Dean et al., 2014; Mcguigan et al., 2017; Mesoudi and Thornton, 2018; Schofield et al., 2017). Within this framework, the actual level of efficiency/complexity of the final product resulting from such a process is not relevant for determining whether the process is deemed cumulative. Therefore, we will refer to this definition as process-focused (Fig. 1a). For example, Schofield et al. (2017) suggested that food-washing behaviours in Japanese macaques has increased in efficiency over a period of 60 years and might thus represent a case of cumulative cultural evolution in non-human primates. The question of whether the most efficient observed technique of food-washing—digging a separate pool of water for rinsing potatoes—could have been invented from scratch without dependency on the older, less efficient techniques—was therefore not decisive for the authors’ conclusion for CC. Another example is the increase in flight distance of paper planes that has been observed in transmission chain experiments with human adults (Caldwell and Millen, 2008, 2009): while the authors acknowledge that the flight distances achieved at the end of the transmission chains could also have been reached by a few individuals without the opportunity of social learning within the experiment, they argue that the observed increase in flight distance, via cycles of learning and innovation, validate the experiment as a laboratory model of cumulative cultural evolution. Such process-focused definitions of CC correspond to the recently defined “core criteria” of CC by Mesoudi and Thornton (2018).
In contrast, other researchers, in addition to describing CC as a process of gradual increase in the efficiency/complexity of a cultural trait, require the efficiency/complexity of the trait (i.e., the product of this gradual increase) to go beyond the limits of what any individual of the species could re-innovate from scratch (i.e., on their own; Aplin, 2019; Boyd and Richerson, 1995; Boyd et al., 2011; Charbonneau, 2015; Henrich and Tennie, 2017; Miton and Charbonneau, 2018; Reindl et al., 2017; Richerson and Boyd, 2005; Tennie et al., 2018, 2009; Tomasello et al., 1993; Vale et al., 2017; Fig. 1b). We refer to this definition as product-focused. While for product-focused researchers the criterion of the trait being highly likely to be impossible to innovate by a single individual is a necessary part of the definition of CC, process-focused researchers would see this as an additional feature that only some cumulative cultural products possess (Mesoudi and Thornton, 2018). Note that while we claim that scholars generally fall into one of these categories of defining CC (and some may use both, see e.g., Fay et al., 2019, 2018), we do not suggest that either of these definitions is currently commonly accepted or that one is followed by the majority of researchers in the field.
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).
