Cumulative cultural evolution in the laboratory: an experimental approach to the origins of structure in human language

Abstract

We introduce an experimental paradigm for studying the cumulative cultural evolution of language. In doing so we provide the first experimental validation for the idea that cultural transmission can lead to the appearance of design without a designer. Our experiments involve the iterated learning of artificial languages by human participants. We show that languages transmitted culturally evolve in such a way as to maximize their own transmissibility: over time, the languages in our experiments become easier to learn and increasingly structured. Furthermore, this structure emerges purely as a consequence of the transmission of language over generations, without any intentional design on the part of individual language learners. Previous computational and mathematical models suggest that iterated learning provides an explanation for the structure of human language and link particular aspects of linguistic structure with particular constraints acting on language during its transmission. The experimental work presented here shows that the predictions of these models, and models of cultural evolution more generally, can be tested in the laboratory.

Fig. 1.

An example string–picture pair.

Fig. 2.

Transmission error and a measure of structure by generation in 4 chains. a shows the increase in learnability (decrease in error) of languages over time. b shows structure in the languages increasing. The dotted line in b gives the 95% confidence interval so that any result above this line demonstrates that there is a nonrandom alignment of signals and meanings. In other words, structure in the set of signals reflects structure in the set of meanings. In 2 cases, this measure is not defined and therefore is not plotted (see Methods). The language discussed in the paper is circled.

Fig. 3.

An example evolved language in the first experiment. This language exhibits systematic underspecification, enabling learners to reproduce the whole language from a fragment.

Fig. 4.

Transmission error and structure by generation in the experiment in which ambiguous data were removed from the training set at each generation. a gives error for the whole language; b gives structure. These results show that, despite the blocking of underspecification, structure still evolves that enables the languages to become increasingly learnable. The language discussed in the paper is circled.

Fig. 5.

An example evolved language in the second experiment. The language is structured: the string associated with a picture consists of substrings expressing color, shape, and motion, respectively. The hyphens represent 1 way of analyzing the substructure of these strings and are added purely for clarity; participants in the experiment always produced strings of characters without spaces or any other means of indicating substructure.