Cultural evolution with sparse testimony: when does the cultural ratchet slip?

Cultural Evolution with Sparse Testimony: when does the Cultural Ratchet Slip? Andrew Whalen (aczw@st-andrews.ac.uk) Department of Biology, University of St Andrews, St Andrews, Fife KY16 9TH UK Luke Maurits (luke.maurits@berkeley.edu) Michael Pacer (mpacer@berkeley.edu) Thomas L. Griffiths (tom griffiths@berkeley.edu) Department of Psychology, University of California, Berkeley, Berkeley, CA 94720 USA Abstract Humans have accumulated a wealth of knowledge over the course of many generations, implementing a kind of “cultural ratchet”. Past work has used models and experiments in the iterated learning paradigm to understand how knowledge is ac- quired and changed over generations. However, this work has assumed that learners receive extremely rich testimony from their teacher: the teacher’s entire posterior distribution over possible states of the world. We relax this assumption and show that much sparser testimony may still be sufficient for learners to improve over time, although with limits on the con- cepts that can be learned. We experimentally demonstrate this result by running an iterated learning experiment based on a classic category learning task. Introduction The sciences are impressive; humanity can be proud. How- ever the work of science can hardly be conceived of, let alone realized, in a single generation. Data needs to accrue over time to shine light on different theories; with new in- formation, the landscape of theories changes, making some plausible while rendering others unthinkable. In addition to science, humanity has accumulated a vast body of practical knowledge and technology which has permitted our adapta- tion to nearly every environment on Earth (Boyd & Richer- son, 1988). Our species is distinguished by this accumulation of knowledge, but our understanding of the process underly- ing this “cultural ratchet” (Tomasello, 1994, 1999) is in its early stages. One aspect of the cultural ratchet’s operation that may have significant consequences for cultural evolution is the amount and kind of information that is passed between generations. Beppu and Griffiths (2009) investigated this aspect of the cul- tural ratchet in the context of an iterated function learning task. When participants in this task provided testimony to fu- ture participants which consisted only of demonstrated data (assumed to be undifferentiated from data observed “in the wild”), groups performed no better than single learners who received only data from the world — no ratcheting effect oc- curred. However, when participants provided their entire the- ory about how the world works — in Bayesian learning terms, their complete posterior beliefs — then the groups eventually learned the correct function. We know that when copious and rich information is passed from generation to generation, the cultural ratchet works flawlessly; given a steady stream of data from the world, sci- ence marches forward. We also know that under conditions of much poorer information passing, the cultural ratchet “slips”; a constant stream of data does not guarantee progress. Be- tween these two extremes are a range of possibilities, which may lie closer to actual human information passing than ei- ther extreme. What forms of testimony passing are needed for the cultural ratchet to catch more often than slip? We take a first step toward addressing this question in this paper. We construct a simple model of iterated learning with “sparse” testimony and consider three forms of evaluating social testimony. We apply this model to a category learn- ing task and find that limited social testimony may lead to iterative improvements across generations; however we also find that these improvements may not allow learners to find the correct hypothesis. We find that in hard category learning tasks, with limited personal data, learners may not perfectly learn the category, although they perform better than the ini- tial learners in the chain. These predictions are confirmed by an iterated learning experiment using a similar category learning task. In the experiment, we find that participant ac- curacy improved across generations, however in most of the conditions the amount of improvement is limited depending on the difficulty of the task and the amount of private data re- ceived. These results suggest that while passing limited testi- mony can still be sufficient to improve the accuracy of groups compared to receiving no testimony, it may not be enough to learn particularly challenging tasks with limited data. Iterated Learning and Cultural Evolution Iterated learning is a widely used computational and exper- imental paradigm for understanding how inductive biases might shape linguistic preferences over the course of multiple generations and influence how languages develop and change (Kirby, 2000, 2001; Perfors & Navarro, 2011). It has since been generalized beyond this setting. Griffiths and Kalish (2007) showed that if learners receive only social testimony then the long term distribution of beliefs of the population will be the same as the prior beliefs of each learner. Much of human learning does not take place purely on the basis of socially transmitted information. Human learn- ers also receive data directly from the world: be they scien- tists measuring the behavior of particles in a laboratory or hunter-gatherers testing new tools in an unfamiliar environ- ment. When learners receive outside data as well as testi- mony, the convergence to the prior shown by Griffiths and Kalish does not hold, and learners’ long-term behavior de-