Psychopathy is a broad spectrum of behavioral disorders, one of which is boldness or fearlessness.
Low fear including stress-tolerance, toleration of unfamiliarity and danger, and high self-confidence and social assertiveness.
Not all psychopaths have this trait, but those who do experience less fear and have a different physiological reaction to frightening situations. They can understand that people who literally stare down the barrel of a gun fear for their life, but they cannot recreate this feeling in the same situation.
The study Adolescents with psychopathic traits report reductions in physiological responses to fear reports:
Results: As predicted, psychopathy was associated with reductions in the subjective experience of fear relative to other emotions. Children and adolescents with psychopathic traits reported fewer symptoms associated with sympathetic nervous system arousal during fear‐evoking experiences.
Conclusions: Rather than being related to uniformly impoverished emotional experience, psychopathic traits appear to be associated with greater deficits in subjective experiences of fear. This pattern of responding supports and extends previous observations that psychopathy engenders deficits in fear learning, physiological responses to threats, and the recognition of fear in others.
And another study examined measurable physiological reactions to fear in psychopaths:
Inmates with high scores on both PCL-R factors and inmates with high scores on only the social deviance simension (Factor 2) display smaller increases than nonpsychopathic inmates in both heart rate and skin conductance when asked to imagine fear-provoking versus neutral situations. Thus, some of the reduced responsiveness to fear stimuli may be a function of ASPD, not psychopathy.
Patrick and his colleagues have also reported that psychopaths display an unusual attenuation of startle responses to intense stimuli presented during the viewing of negative effective slides. Whereas nonpsychopaths display larger eyeblinks if startled while viewing slides of negative valence, psychopaths display smaller blinks under such conditions. [...]
Such group differences are interpreted as reflecting reduced activity in a fedence/withdrawal system in psychopathic individuals. (Source: page 117)
And, lastly, the study Psychopathy, Fear Arousal and Anticipated Pain summarizes:
The MMPI Pd scale was used to separate Ss [subjects] into High and Low Pd [psychopathic disorder] groups of 10 Ss each. Skin resistance was recorded while the numbers 1 to 12 were consecutively presented on a memory drum, with Ss having been previously informed that an electric shock would be received when 8 appeared. The results indicated that as shock became imminent, the increase in conductance was greater, more rapid, and began earlier for the Low Pd Ss than for the High Pd ones. The results were related to the clinical observation that the psychopathic person is relatively unaffected by the threat of punishment.
The predisposition to develop psychopathy is inheritable:.
In this respect, psychopathic personality traits [...], specifically callous-unemotional (CU) traits, have been proposed as the most important specifiers within antisocial populations16 and have been included as such in the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-V). This is not only because of their clinical relevance [...] but also for their associated genetic risk and neurobiological deficits, including aberrant reactivity in limbic and prefrontal brain regions (Marsh et al.18 and Finger et al.,19 for a review see Viding and McCrory20). (Source)
The study Evidence for substantial genetic risk for psychopathy in 7‐year‐olds concludes:
Results: DeFries–Fulker extremes analysis indicated that exhibiting high levels of CU [callous-unemotional] is under strong genetic influence. Furthermore, separating children with AB [antisocial behaviour] into those with high and low levels of CU showed striking results: AB in children with high levels of CU is under extremely strong genetic influence and no influence of shared environment, whereas AB in children with low levels of CU shows moderate genetic and shared environmental influence.
Conclusions: The remarkably high heritability for CU, and for AB children with CU, suggests that molecular genetic research on antisocial behaviour should focus on the CU core of psychopathy. Our findings also raise questions for public policy on interventions for antisocial behaviour.