Address correspondence to:

Miller (1965) identified his famous limit of 7+-2 items based in part on absolute identification – the ability to recognize stimuli which differ on a single dimension, such as lines of different length. An important aspect of this limit is its independence from perceptual effects and its application across all stimulus types. Recent research however, has identified several exceptions. We investigate an explanation for these results which can reconcile them with Miller's work. We find support for the hypothesis that the exceptional stimulus types have more complex psychological representations, which can therefore support better identification. Our investigation uses data sets with thousands of observations for each participant, which allows the application of a new technique for identifying psychological representations: the structural forms algorithm of Kemp and Tenenbaum (2008). This algorithm supports inferences not possible with previous techniques, such as multi-dimensional scaling. Absolute identification (AI) is the fundamental task of identifying stimuli that vary only on one physical dimension. For example, tone frequency (e.g. Hartman, 1954; Pollack, 1952), tone loudness (e.g. Garner, 1953) or line length (e.g. Lacouture, 1997). In a typical AI task, stimuli are first presented to the participant one at a time, each with a unique label. In the test phase, th