Iterated learning and the cultural ratchet

Iterated Learning and the Cultural Ratchet Aaron Beppu (abeppu@berkeley.edu) Thomas L. Griffiths (tom griffiths@berkeley.edu) Department of Psychology and Cognitive Science Program University of California Berkeley, Berkeley, CA 94720 USA Abstract How does the behavior of individuals in a society influence whether knowledge accumulates over generations? We explore this question using a simple model of cultural evolution as a process of “iterated learning,” where each agent in a sequence learns and passes on a piece of information. Using both math- ematical analyses involving rational Bayesian agents and lab- oratory experiments with human participants, we vary whether agents observe data from the environment and what kind of information they receive from the previous agent. Our mathe- matical and empirical results both suggest that merely observ- ing the behavior of other learners is not sufficient to produce cumulative cultural evolution, but that knowledge can be accu- mulated over generations when agents are able to communicate the plausibility of different hypotheses. Introduction What we as humans know about the world is largely informed by what we can learn from others. Our beliefs and behaviors are built up not only by our own interactions with the environ- ment around us, but by observations of the behavior of oth- ers, and by receiving explicit statements of belief. Through this ability to build off the knowledge and behaviors of those around us, human societies are able to accumulate informa- tion, the so-called “ratchet effect” (Tomasello, Kruger, & Rat- ner, 1993). Large, lasting changes in our beliefs develop by accruing small, incremental changes over many generations. In this paper, we use a simple model to characterize the con- ditions necessary for this accrual, and compare the predic- tions of this model to experimental results. In particular, we suggest that these changes will accumulate if learners have access to the actual beliefs of their teachers, as opposed to merely observations of behavior informed by those beliefs. Much of the previous discussion about cumulative cultural evolution has focused on the relationship between the teacher and the learner. For example, Tomasello (1993) largely claims that the ratchet effect is a product of sophisticated imi- tative learning, emphasizing the advantages of joint attention and a theory of mind. Gergely and Csibra (2005) suggest that “natural pedagogy” plays a role, focusing on how human teachers go out of their way to ease the learning process for children, providing highly tailored information and a greatly enriched learning environment. Both of these mechanisms increase the fidelity of cultural transmission, providing the opportunity for the ratchet effect occur. Rather than focusing on the specific actions or abilities of teachers and learners, we consider the kind of informa- tion passed between generations. We model the process of cultural evolution using an “iterated learning” framework (Kirby, 2001), in which successive generations are modeled as a sequence of agents in which each agent learns from the previous agent. Iterated learning models have recently been used in simulations exploring the emergence of linguis- tic structures (Kirby, 2001) and in experiments with human participants revealing inductive biases in specific domains (Kalish, Griffiths, & Lewandowsky, 2007). We explore vari- ants of this framework where each agent learns not only from the behavior of the previous generation, but also from ob- serving the world and from theories summarizing the infer- ences of the previous generation. The consequences of each variant for the accumulation of knowledge are determined through a combination of mathematical analyses of sequences of Bayesian agents and an experiment in which cultural evo- lution is simulated in the laboratory with human participants. Our mathematical and empirical analyses produce two re- sults that help to identify the conditions under which cumu- lative cultural evolution can occur. First, we show that if the information passed from teacher to learner is limited to the learner’s observations of the teacher’s behavior, then cumula- tive cultural evolution will not take place. We then go on to show that cumulative cultural evolution can occur if teachers give learners information about which hypotheses they be- lieve are plausible, rather than just behaviors consistent with the most plausible hypothesis. These results suggest that a capacity for communicating such beliefs may be necessary in order for the cultural ratchet to operate. The plan of the paper is as follows. The next section intro- duces our formal framework, and summarizes our mathemati- cal results. We then outline the motivation for our experiment with human learners, and describe the procedure and results of that experiment. The paper concludes with a discussion of the implications of these results for understanding the con- ditions under which the cultural ratchet can operate, and the kinds of cognitive capacities that teachers and learners need to possess in order to satisfy these conditions. Analyzing Cultural Transmission Our first step in exploring the conditions that result in cu- mulative cultural evolution is analyzing variants of iterated learning in which the learners are rational Bayesian agents. The assumption of rationality allows us to examine how ideal learners use the information transmitted between generations, and allows us to be explicit about the biases that influence learning. Specifically, we assume that all agents have a shared set of hypotheses H, and a prior distribution over these hy- potheses p(h) describing the degree to which each h ∈ H is believed to be true in the absence of any evidence. Starting with these prior beliefs, agents observe the world and receive information from other agents, resulting in data,