I shouldn't have used the word automatic; I meant reflexive or involuntary.
These are some quasi-clincal definitions that seem to agree with what I learned back when I studied psych:
Direct identification with, understanding of, and vicarious experience of another person's situation, feelings, and motives.
emotional awareness and understanding of another person's thoughts, feelings, and behavior, even those that are distressing and disturbing. Empathy emphasizes understanding; sympathy emphasizes sharing of another person's feelings and experiences.
Both of the above are from the medical section here. Note that the one you cited and the first of these both use the word vicarious, which seems (to me, anyway) to imply an emotional, rather than cognitive response. But perhaps you see it differently.
Clinically speaking, sociopathy is related to ASPD (antisocial personality disorder), and is defined in both the DSM-IV and the ICD-10, the latter of which lists the first criterion as:
Callous unconcern for the feelings of others and lack of the capacity for empathy.
Sympathy is not included in either manuals' definition.
Lack of empathy is also specifically associated with Psychopathy and Narcissistic personality disorder.
I might have empathy for a serial killer, in that I may wince if he were to get stuck with a pin (a reflexive response), but I may not have sympathy for him, because I cognitively determine that he deserves it.
More importantly, who remembers the awesome Star Trek (tos) episode, The Empath ? I think that clears it up perfectly.