In addition, a growing group of studies has begun to document effects of the 5-HTTLPR on intermediate behavioral and physiological processes that map onto these alterations in brain structure and function. The S allele is associated with increased acquisition of conditioned fear responses (67), increased auditory startle response (68, 69), and greater sympathetic reactivity when simply observing another person receiving shock (70). Moreover, the 5-HTTLPR S allele has been associated with increased HPA axis reactivity to aversive or threatening stimuli in a number of studies (71–74). The S allele typically has no impact on baseline levels of HPA function in these studies, underscoring its documented effect on threat-related amygdala reactivity. In addition, the S allele has been linked with difficulty disengaging from, or preferential attention toward, threat-related stimuli (75–79), a more negative information-processing bias (80), emotion-induced retrograde amnesia (81), sensitivity to financial loss (70, 82), and even social blushing (83). Although this literature is not without inconsistencies (e.g., some reported associations are sex-specific and others have not replicated), it does suggest that the effects of the 5-HTTLPR S allele on
