The emergence of focus in an experimental communication game

We report an experiment investigating the emergence of focus, the prosodic or morphosyntactic marking of critical elements (Schmitz, 2008) in a sentence. Stevens (2016) argued for a theory of focus based in information theory (Shannon & Weaver, 1949; Schmitz, 2008; Bergen & Goodman, 2015). Language users must deal with noise – the random deletion or alteration of parts of a signal. A solution is to compensate by adding redundancy (e.g., greater prosodic or morphosyntactic emphasis). However, redundancy costs both effort and time, so we should expect speakers to restrict redundancy to critical elements, particularly when effort and time pressures are high. (Redundancy on critical elements will be referred to here as critical redundancy, as compared with non-critical redundancy on other elements.) These factors should be expected to operate over multiple timescales. In a single interaction, speakers respond dynamically to perceived noise, time and effort pressures (Krauss & Weinheimer, 1964; Clark, 1996; Brennan & Clark, 1996). Developmentally, language learners acquire strategies for adding redundancy (Romaine, 1984). Over generations, such strategies can be expected to become grammaticalized as focus systems (Tamariz & Kirby, 2016). We thus expect focus-like behavior to emerge and evolve in any communication system that involves sending messages under similar constraints and make the following predictions: (1) Overall message length should vary according to time and effort costs; (2) longer messages should differ from shorter messages not only with respect to length – shorter messages should also have lower proportions of non-critical redundancy; (3) critical redundancy should be higher when noise is higher, both in an absolute sense and in a relative sense (more critical than noncritical redundancy); (4) unless noise and time pressures actually prevent accurate communication, communicative accuracy should remain relatively constant, because focus is designed to help maintain accuracy under different conditions. We tested these predictions experimentally by having participants play a simple communication game. Players sat separately and took turns to be “Sender” or “Receiver”. The Sender would see three grids, two with line figures (Figure 1; in 422