Why an iconic gesture takes its particular form is a largely open question, given the variations one finds across both situations and speakers. We present results of an empirical study that analyzes correlations between contextual factors (referent features, discourse) and gesture features, and tests whether they are systematic (shared among speakers) or idiosyncratic (inter-individually different). Based on this, a computational model of gesture formation is presented that combines data-based, probabilistic and model-based decision making.