Sex, stress, and fear: individual differences in conditioned learning.

It has long been recognized that humans vary in their conditionability, yet the factors that contribute to individual variation in emotional learning remain to be delineated. The goal of the present study was to investigate the relationship among sex, stress hormones, and fear conditioning in humans. Forty-five healthy adults (22 females) underwent differential delay conditioning, using fear-relevant conditioned stimuli and a shock unconditioned stimulus. Salivary cortisol samples were taken at baseline and after acquisition training and a 24-h-delayed retention test. The results showed that acquisition of conditioning significantly correlated with postacquisition cortisol levels in males, but not in females. This sex-specific relationship was found despite similar overall levels of conditioning, unconditioned responding, and cortisol. There was no effect of postacquisition cortisol on consolidation of fear learning in either sex. These findings have implications for the understanding of individual differences in fear acquisition and risk factors for the development of affective disorders.