"What can we learn about emotion by studying psychopathy?", Marsh 2013:

Psychopathy affects both children and adults. Markers of psychopathy may emerge early in childhood (Glenn et al., 2007; Wang et al., 2012), are moderately reliable predictors of adult psychopathy (Lynam et al., 2008), and the core affective features of psychopathy appear to be highly heritable (Larsson et al., 2006). The heritability coefficient of the core callous and unemotional features has been estimated to be at least 0.43 (Larsson et al., 2006) and as high as 0.71 (Viding et al., 2005, 2008). An individual's risk for engaging in antisocial behavior during childhood or adulthood can be increased by any number of life history events, including trauma exposure, low socioeconomic status, or delinquent peer groups (Lynam et al., 2008), but these factors do not seem to precipitate the emergence of psychopathic traits in children (often termed callous-unemotional traits). In fact, callous-unemotional traits may paradoxically serve as a protective factor against parental maltreatment: among children with callous-unemotional traits, there is little correspondence between the quality of parenting that children receive and the severity of their antisocial behavior problems (Wootton et al., 1997). Instead it appears that life stressors that result in heightened stress responding represent a distinct etiological route toward antisocial behavior (Blair, 2001). Among children without high levels of callous-unemotional traits, parental maltreatment is associated with increased antisocial behavior (Wootton et al., 1997). In addition, antisocial behavior in the absence of callous-unemotional traits does not appear to be highly heritable, supporting the role of environmental stressors in leading to antisocial behavior in the absence of callous-unemotional traits (Viding et al., 2005, 2008).

...Although Cleckley's descriptions of psychopathy reflect a psychodynamic orientation, his observations are consistent with more recent experimental data assessing fear responding in psychopathy. A focus on fear responding emerged from the observation that psychopathic offenders are particularly likely to re-offend, suggesting that the threat of future punishments is not sufficiently motivating for them (Corrado et al., 2004; Hare, 2006). Fear is, in essence, the state that accompanies the anticipation of an aversive outcome (i.e., punishment) and promotes avoidance and escape behaviors (Stein and Jewett, 1986; Panksepp, 1998; LeDoux, 2000). Fear being the emotion that promotes avoidance of behaviors that result in punishment (LeDoux, 2003), it is ostensibly is the mechanism by which punishing criminal behavior serves to deter it. Early hypotheses proposed that dysfunctional fear responding renders psychopaths less likely to avoid engaging in criminal behaviors that result in punishments like imprisonment, and were supported by laboratory findings that psychopaths are less likely to modulate their behavior in response to anticipated punishments ranging from electrical shock to loss of points in a computer game (Lykken, 1957; Hare, 1966; Newman and Kosson, 1986; Blair et al., 2004).

Abundant psychophysiological research supports the notion that psychopaths' responses to the threat of an aversive outcome are muted. Psychopathy impairs anticipatory skin-conductance responses (Lykken, 1957; Aniskiewicz, 1979; Herpertz et al., 2001; Birbaumer et al., 2005; Rothemund et al., 2012), fear-potentiated startle responses (Patrick et al., 1993; Levenston et al., 2000; Herpertz et al., 2001; Rothemund et al., 2012), and contraction of the corrugator muscle underlying the brows (Herpertz et al., 2001; Rothemund et al., 2012) during threat anticipation. Psychopathy also impairs aversive classical conditioning (Flor et al., 2002) as well as other fear-relevant responses such as the recognition of fear from the face, body, and voice (Marsh and Blair, 2008; Dawel et al., 2012). These differences are particularly evident for psychopathic offenders characterized as “primary” psychopaths who exhibit the core callous and unemotional personality features of the disorder (Lykken, 1957; Aniskiewicz, 1979; Dawel et al., 2012). This is in contrast to “secondary” psychopaths, in whom antisocial behavior may primarily reflect social disadvantage or maltreatment and who may present with increased anxiety (Newman et al., 2005; Kimonis et al., 2012). Finally, both anecdotal reports and empirical evidence indicate that subjective experiences of fear are reduced in psychopathy
...There is very little evidence available that describes other types of emotional reactions in psychopathy, although what evidence exists suggests that disgust responding remains intact, and there is little evidence for consistent impairments in happiness or surprise (Marsh and Blair, 2008; Marsh et al., 2011; Dawel et al., 2012). One emotion for which the present literature is genuinely ambiguous is sadness, with meta-analytic findings generally showing some deficits in recognizing sadness expressions in psychopathy, albeit less consistently and with generally smaller effect sizes than for fear. Very little literature explores sadness responses in psychopathy in other contexts, and results from these studies are equivocal (e.g., Blair et al., 1995; Brook and Kosson, 2013) In general, the neurobiological basis of sadness is not as well understood as that of fear, and further development of the neurocognitive basis of sadness may be required to develop targeted tasks assessing it in psychopaths.

...From a societal perspective, understanding empathic deficits for others' fear may be the most important question of all that the study of psychopathy helps to answer. Although amygdala lesion cases can illuminate the amygdala's role in fear, because these lesions usually occur in late adolescence or adulthood, their effects on the development of other brain regions and behavior is more limited. This may be why amygdala lesions in adulthood are not associated with heightened aggression, whereas the case of psychopathy suggests a strong relationship between developmental deficits in fear and aggression. Fear plays an important role in preventing or ending aggression during social encounters (Blair, 1995, 2005b), and fearful emotional facial expressions elicit empathic concern and the desire to help from people who perceive them, even subliminally (Marsh and Ambady, 2007). The rationale for much research on psychopathy is that individuals with this disorder are responsible for a disproportionate amount of suffering, as they engage in a variety of antisocial, criminal, and violent behaviors that cause others distress and fear (Hare, 1993; Rutter, 2012). There is limited evidence that failure to exhibit empathic responses to others' pain is related to lower self-reported empathic concern or aggressive or antisocial behavior (Singer et al., 2004, 2006). In contrast, the evidence linking the failure to exhibit empathic responses to others' fear, both on a neural and a behavior level, is abundant. Psychopaths, in whom the failure to recognize others' fear or to generate empathic activation in the amygdala and autonomic nervous system is a hallmark feature, exhibit profound impairments in empathic concern for others and notoriously commit antisocial acts. Thus, as important as the study of psychopathy is for answering fundamental psychological and neuroscientific questions about the nature of emotion and empathy, an improved understanding of emotion and empathy as they pertain to psychopathy may be critical to developing improved means of ameliorating psychopaths' harmful effects on others.