Systematicity and Idiosyncrasy in Iconic Gesture Use: Empirical Analysis and Computational Modeling Kirsten Bergmann and Stefan Kopp SFB 673 Alignment in Communication, Bielefeld University Sociable Agents Group, CITEC, Bielefeld University P.O. Box 100 131, D-33615 Bielefeld, Germany {kbergman,skopp}@techfak.uni-bielefeld.de Abstract. Why an iconic gesture takes its particular form is a largely open ques- tion, given the variations one finds across both situations and speakers. We present results of an empirical study that analyzes correlations between contextual fac- tors (referent features, discourse) and gesture features, and tests whether they are systematic (shared among speakers) or idiosyncratic (inter-individually differ- ent). Based on this, a computational model of gesture formation is presented that combines data-based, probabilistic and model-based decision making. Key words: Iconic gesture, meaning-form mapping, systematicity, idiosyncrasy 1 Introduction The use of speech-accompanying iconic gestures is ubiquitous in human-human commu- nication, especially when spatial information is expressed. Current literature on gesture research states that the question “why different gestures take the particular physical form they do is one of the most important yet largely unaddressed questions in gesture research” [2, p. 499]. This holds especially for iconic gestures, for which information is mapped from some mental image into (at least partly) resembling gestural form. Although their physical form, hence, corresponds to object or event features like shape or spatial properties, empirical studies have revealed that similarity with the referent cannot fully account for all occurrences of iconic gestures [19]. Rather, recent findings indicate that a gesture’s form is influenced by specific contextual constraints or the use of more general gestural representation techniques such as shaping, drawing, or placing [17, 4]. In addition to those systematic patterns in gesture use, human speakers are of course unique and inter-subjective differences in gesturing also hold (cf. [12]). For example, while some people rarely make use of their hands while speaking, others gesture almost without interruption. Similarly, individual variations are seen in preferences for partic- ular representation techniques or low-level morphological features such as handshape [4]. Such inter-subjective differences in gesture behaviour are common and reflect an idiosyncrasy of iconic gestures – gestures are created locally by speakers while speaking, without adhering to any conventionalized standards of good form. McNeill & Duncan [25, p. 143] conclude that, by this idiosyncracy, “gestures open a ‘window’ onto thinking that is otherwise curtained”.