Psychopathy is defined as a mental (antisocial) disorder in which an individual manifests amoral and antisocial behavior, shows a lack of ability to love or establish meaningful personal relationships, expresses extreme egocentricity, and demonstrates a failure to learn from experience and other behaviors associated with the condition.
Henry R. Hermann
Persons with psychopathy show inhibition of the startle response to both emotionally pleasant and unpleasant stimuli. Thus, psychopathic persons appear to have a specific insensitivity to unpleasant or aversive stimuli, and thus respond equivalently to pleasant and unpleasant stimuli. Psychopathy is highly refractory to psychotherapy, perhaps due to a lack of emotional sensitivity or awareness required for empathy.
O’Donnell, Hetrick
Neuroimaging research indicates that the atypical empathic outcomes observed in psychopathic individuals may be associated with reduced activity in regions associated with emotional processing, such as the AI and amygdala.
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Reduced gray matter volume in the AI and amygdala has similarly been shown to correlate with instances of aggressive behavior and levels of empathy in adolescents with conduct disorder.
Thompson et al.
The consent of most modern studies show that psychopathy has nothing to do with "only being cold" or "being rational". There is a clear divide between the medical standpoint and how the word is often used out of context in daily life.
That said, like most mental disorders, the clear definition is used on the core cases while we see a wide spectrum of "lighter cases" with only some or light symptoms, which might or might not fit multiple different diagnoses. You don't have a heart attack just because your chest hurts a bit. Same with the "little boxes" we use for mental problems.
"Pure psychopaths" are missing the mental capacity to experience certain things. Something that could also be seen for certain kinds of brain damages that could lead to similar inabilities. A psychopath doesn't struggle with his own desires or acts, because they don't realize that what they do might hurt others or overstep community rules. They are like children who haven't learned the rules. There is no "wrong" in their act because to them, the act itself is correct.
Psychopaths will display emotions, but studies have shown, that at least part of that is an act "to fit in" or "reach a goal". The actual brain activity to emotional stimuli is lower or none. Though there is also a difference between "empathy based emotions" and direct stimuli. They will still become angry for themselves, but rarely will they become angry for someone else based on that person's feeling. So a psychopath could still get angry if you go against his wife, but it might be based on the disrespect you show him through that rather than the emotions his wife feels.
It's also shown that a majority of psychopaths are good actors, who will use adisplay of emotions to further their cause. With these cases, it's also really hard to say which reactions are genuine and which are just a display. You can't check everyone's brain to make sure... let alone check which action was the actual trigger of a genuine feeling. You can't go around and divorce a man ten times and see under which conditions he reacts in what way.
That said, if you want to write a struggling character, compulsive acts might be a better way to go. They are also linked to mental pressure which makes it easier "to trigger" at the right moments in the story. People with such problems also often realize their "different behavior" and struggle with it. Those also much more closely fits the "image of the psychopath" in modern media. Something else to look at would be traumata that can fuck up a person.
The "calculative emotionless husk" is the core idea of a psychopath and can be explained through brain topology and activity. The struggling kind of psychopath is more often than not a different kind of mental disorder, all of them combined under the umbrella of the word "psychopath" as we use it in our daily life. Though the majority of those cases probably aren't psychopaths by the definition. We just can go and test everyone, and use really expensive monitoring and long tests on every single person.
It's a bit of how we call the majority of "feeling bad"-cases a depression nowadays. A lot of these cases aren't depressions under the medical definition, but people use that word because it describes a set of symptoms. And the treatment for a lot of stuff will be similar, no matter the "actual cause" of the symptoms.
That said, if you want to look into something to write about it, it's probably better to use the correct definitions and look for a cause that will bring similar symptoms as you want the character to have. The differences between antisocial-behavior, split personalities, compulsive acts, and psychopaths might also be a good place to find more ideas and creativity for your character...
