We tested the hypothesis that conceptual similarity promotes generalization of conditioned fear. Using a sensory preconditioning procedure, three groups of subjects learned an association between two cues that were conceptually similar, unrelated, or mismatched. Next, one of the cues was paired with a shock. The other cue was then reintroduced to test for fear generalization, as measured by the skin conductance response. Results showed enhanced fear generalization that correlated with trait anxiety levels in the group that learned an association between conceptually similar stimuli. These findings suggest that conceptual representations of conditional stimuli influence human fear learning processes.The idea that many fears originate from an association between neutral and aversive stimuli has continued to serve as the basis for influential accounts of human anxiety disorders (Bouton 2002;Mineka and Zinbarg 2006). However, the nexus of many fears defy a straightforward explanation, since following an aversive experience other stimuli that are indirectly related to the experience oftentimes become feared as well. In some cases, generalizing an acquired fear following an emotional learning episode is predicated on an association between stimuli formed before the emotional experience occurred. This form of stimulus generalization is known as sensory preconditioning (SPC) (Brogden 1939;Kimmel 1977;Gewirtz and Davis 2000), and is an important process whereby the representation of a known stimulus is modified following a salient experience with a related stimulus.A typical SPC procedure occurs in three phases: first, subjects learn to associate two neutral cues-for example, a bell and a light. Next, one of the cues, say the light, is associated with a biologically significant unconditioned stimulus (US), which leads to the development of a conditioned response (CR). The bell is called the preconditioned stimulus (PS) and the light is called the conditioned stimulus (CS). Finally, the PS is presented alone. Despite the fact that it has never predicted the US directly, the PS will often elicit a CR, suggesting that an association was formed during preconditioning between the PS and CS. The majority of SPC studies have been conducted in nonhuman animals (Brogden 1939;Rizley and Rescorla 1972). SPC effects have also been reported in a small number of human fear-conditioning experiments (White and Davey 1989;Vansteenwegen et al. 2000), but human research on this topic is very limited relative to experiments using first-order Pavlovian conditioning procedures. Consequently, the features that promote fear generalization between previously associated stimuli are unknown.A well-known factor important for classical conditioning (Rescorla and Furrow 1977) and stimulus generalization overall is similarity to the CS (Pavlov 1927;McLaren and Mackintosh 2002). Classic experiments of stimulus generalization in animals (reviewed in Honig and Urcuioli 1981) have revealed orderly generalization gradients of conditioned responses that tra...