Adults are more efficient in creating and transmitting novel signalling systems than children

Iterated language learning experiments have shown that meaningful and structured signalling systems emerge when there is pressure for signals to be both learnable and expressive. Yet such experiments have mainly been conducted with adults using language-like signals. Here we explore whether structured signalling systems can also emerge when signalling domains are unfamiliar and when the learners are children with their well-attested cognitive and pragmatic limitations. In Experiment 1, we compared iterated learning of binary auditory sequences denoting small sets of meanings in chains of adults and 5-7-year old children. Signalling systems became more learnable even though iconicity and structure did not emerge despite applying a homonymy filter designed to keep the systems expressive. When the same types of signals were used in referential communication by adult and child dyads in Experiment 2, only the adults, but not the children, were able to negotiate shared iconic and structured signals. Referential communication using their native language by 4-5-year old children in Experiment 3 showed that only interaction with adults, but not with peers resulted in informative expressions. These findings suggest that emergence and transmission of communication systems is unlikely to be driven by children, and point to the importance of cognitive maturity and pragmatic expertise of learners as well as feedback-based scaffolding of communicative effectiveness by experts during language evolution. Language transmission in adults and children 3 Introduction Languages are shaped by two sets of constraints: the need to be learnable so they can be transmitted to the next generation, and the need to be expressive to ensure successful communication (Tamariz, 2017). Empirical evidence for this insight comes from experimental semiotics studies of novel signalling systems (Garrod & Galantucci, 2011) which comprise iterated language learning experiments, where the outcome of learning a mini-language by one participant serves as input for the next participant in a chain (Cornish, Smith & Kirby, 2013; Cornish, Dale, Kirby & Christiansen, 2017; Kirby, Cornish & Smith, 2008; Kirby, Tamariz, Cornish & Smith, 2015; Silvey, Kirby, and Smith, 2014; Verhoef, Kirby & Padden, 2011; Verhoef, 2012; Verhoef, Kirby & de Boer, 2014; 2016; Verhoef, Roberts & Dingemanse, 2015), as well as referential communication tasks and signalling games, where multiple participants negotiate meanings of novel signals over several rounds of communicative interaction (Dingemanse, Blasi, Lupyan, Christiansen & Monaghan, 2015; Fay, Ellison & Garrod, 2014; Garrod, Fay, Lee, Oberlander & MacLeod, 2007; Roberts et al., 2015, Selten & Warglien, 2007). These studies have shown that unstructured stimuli become increasingly easier to learn and to use because innovations are shaped by learners’ implicit biases towards simpler, more transparent (Jones, Vinson, Clostre, Zhu, Santiago & Vigliocco, 2014; Dingemanse et al., 2015; Roberts et al., 2015) and more compressible (Kirby, Griffiths & Smith, 2014; Tamariz & Kirby, 2015; 2016 Xu & Griffiths, 2010) languages. To date, these types of laboratory experiments have been conducted mainly with adults. It is conceivable that adults, especially when presented with language-like signals, albeit artificial ones, will invoke their considerable meta-linguistic knowledge about what language ought to be like. Yet language is primarily acquired by children who lack this metalinguistic knowledge and are subject to a range of cognitive constraints that differ from those operating in adults. To gain a better understanding of the generalisability of findings from experimental semiotics, and to explore the role of the cognitive and pragmatic constraints imposed by child learners, this study compares transmission and creation of unfamiliar signalling systems between adults and children. To predict in what ways children may alter the way consistent and communicatively efficient signalling systems emerge we first need to consider what research in experimental semiotics tells us about how such systems emerge in adults. Language transmission in adults and children 4 The findings can be summarised with respect to three crucial features of language: iconicity, combinatorial structure and compositional structure. Iconicity emerges when adult learners are faced with novel signal-meaning mappings, and attempt to exploit transparent links between physical properties of the signals and dimensions of the associated meanings (Dingemanse et al. 2015; Roberts et al., 2015), capitalising either on abundant neonatal cross-modal connections or acquired knowledge about statistical regularities or cross-modal co-occurrences (Spence, 2011). Emergence of iconicity has been demonstrated not just when learners negotiate novel signalling systems during communication but also in simple iterated learning experiments without communication (Jones et al., 2014). Iconic signal-meaning mappings are subsequently aligned and refined during communicative interaction, resulting in conventionalised signals that become increasingly arbitrary (Garrod & Galantucci, 2011; Lister & Fay, 2017). Emergence of combinatorial structure can be demonstrated in iterated learning experiments with novel stimuli that are not linked to referents, e.g. whistle sounds (Verhoef, 2012; Verhoef et al., 2014; 2015), colour sequences (Cornish, Smith & Kirby, 2013), doodles (del Giudice, 2012; Tamariz & Kirby, 2015) or random dot patterns (Kempe, Gauvrit & Forsyth, 2015). As a result of iterations through consecutive cycles of learning, such meaningless stimuli become more systematic and structured as sub-components like pitch contour segments or small colour sequences are recombined to generate potentially unlimited sets, in the same way as phonemes are combined to form morphemes and words in natural languages. When such unfamiliar stimuli are linked to meaning, combinatorial structure can also arise from the pressure to minimise confusion between signals as an increase in the number of signals renders them increasingly difficult to discriminate (Nowak, Krakauer & Dress, 1999; Zuidema & deBoer, 2009), but also from intrinsic signal features such as rapid fading (Roberts & Galantucci, 2012), or limited iconic affordances of the signalling domain (Roberts, Lewandwoski & Galantucci, 2015). Compositional structure has been shown to emerge when the signals are not only subjected to iterated learning but also used to communicate meaning (Kirby et al., 2015), when meaning spaces undergo expansion (Selten & Warglien, 2007), when communication involves multiple interlocutors in social networks (Raviv, Meyer & Lev-Ari, 2019) or when context-based predictability of referents is low (Winters, Kirby & Smith, 2018). In these situations, sub-components of the signals become Language transmission in adults and children 5 systematically associated with dimensions of the meanings, akin to morpho-syntactic rules in natural languages (del Giudice, 2012; Kirby, Cornish & Smith, 2008; Kirby, Tamariz, Cornish & Smith, 2015; Silvey et al., 2014; Verhoef, Roberts & Dingemanse, 2015). In the present study we ask if and how these basic results would change when novel signals are learned and used by children. Children differ from adults with respect to cognitive capacities, pragmatic abilities, pre-existing real-world knowledge and prior linguistic experience. It is therefore important to investigate more directly how children create and transmit novel signalling systems in order to gain a better understanding of the underlying constraints operating in this process and the role that children may play in language change, especially in light of claims that diversity of linguistic structure is linked to the proportion of child vs. adult learners of a language and the differences in learning constraints this may impose on the process of language transmission (Lupyan & Dale, 2010; Dale & Lupyan, 2012). Predictions about what constraints children impose on the emergence of communicatively efficient signalling systems and in what ways these constraints differ from those imposed by adults should address both emergence of iconicity and emergence of structure. Findings from child language development research suggests that such predictions will not necessarily be straightforward: With respect to the emergence of iconicity, the Iconic Bootstrapping Hypothesis (Imai & Kita, 2014) proposes that children benefit from iconic signal-meaning mappings because such mappings are transparent and hence easier to comprehend thereby alleviating the burden of learning. Consequently, children should be more predisposed than adults to capitalise on transparent cross-modal associations between signal features and meaning dimensions. However, the developmental origins of transparent cross-modal association are not clear. A recent meta-analysis of the emergence of the kiki-boubaeffect in infancy and early childhood (Ford, Lammertink, Peperkamp, GuevaraRukoz, Fikkert & Tsuji, 2018) suggested that some cross-model correspondences (e.g. the bouba-effect which refers to the association of round shapes with back vowels and voiced consonants) are present early on while others (e.g. the kiki-effect which refers to the association of spiky shapes with front vowels and voiceless consonants) tend to emerge over time. This would lead to fairly complex predictions according to which some iconic mappings may be preferred by children while others should more easily accessible to adults based on their greater experience with statistical regularities in the Language transmission in adults and children 6 environment yet the literature at present does not allow us to make predictions with regards to specific age-dependent cross-modal preferences. Predictions are also inconsistent with respect to the emergence of structure. On the one hand, in accordance with the Less-Is-More-h