Individual differences in fear: isolating fear reactivity and fear recovery phenotypes.

Although different people respond differently to threatening events, animal research on the neural basis of fear tends to focus on typical responses. Yet there are substantial individual differences between animals exposed to identical behavioral procedures. In an effort to begin to understand the nature and causes of fear variability and resilience, we separated outbred Sprague-Dawley rats into high and low reactivity, and fast and slow recovery phenotypes, based on freezing levels during fear conditioning and extinction, respectively. Subsequent tests revealed stable differences in both measures, indicating that fear responses reflect trait-like phenotypes in outbred animals. Because clinical disorders may reflect extreme phenotypes, identification of the biological basis for these differences could provide insights into human individual differences in fear.