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Lack of empathy is usually very obvious...if you suspect it's an issue, it probably is. A person may have a tiny bit of it somewhere or in some situations, but for the most part a person either has it or doesn't.
Diagnosis of antisocial personality is accomplished more with observed or reported actions. Given a couple situations where they didn't hold up social norms and let others suffer for their own reward----and especially to the criminal extent, and you've got someone with anti-social traits.
If this question is for a class....PPD doesn't have to have any issues with empathy, so it wouldn't really be used in differential diagnosis. High order autism or aspergers will demonstrate lack of empathy, Aspergers will usually train themselves on the rules of empathy and go forward acting like they have the trait because they understand it's place in relationships. Antisocials can do this during the seduction phase, but it wears thin within hours or days.
Here is a rule of thumb: If they have a brittle presentation when stressed or challenged, are smart, aloof, OCD-ish and keep forgetting to include the human factor in relationships, you're pointing toward Aspergers.
If they go through the world feeling entitled and above the laws and norms to the point that they scam and lie to get their way without ever looking back, it's antisocial. They don't learn from their mistakes.
If they're consumed with 'what people are taking from me' in a wield way, it's usually Paranoid PD.
Empathy just isn't the deciding factor in this.
My best to you.
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