* Almost always chooses solitary activities;
* Has little, if any, interest in having sexual experiences with another person;
* Takes pleasure in few, if any, activities;
* Lacks close friends or confidants other than first degree relatives;
* Appears indifferent to the praise or criticism of others;
* Shows emotional coldness, detachment, or flattened affectivity.
B. Does not occur exclusively during the course of schizophrenia, a mood disorder with psychotic features, another psychotic disorder, or a pervasive developmental disorder and is not due to the direct physiological effects of a general medical condition.
Or, as the Howard H. Goldman (Ed.) in the "Review of General Psychiatry" [4th Edition. London, Prentice Hall International, 1995] puts it:
"The person with Schizoid Personality Disorder sustains a fragile emotional equilibrium by avoiding intimate personal contact and thereby minimising conflict that is poorly tolerated."
Intuitively, a connection between SPD and NPD seems plausible. After all, NPDs are people who self-sufficiently withdraw from others. They love themselves in lieu of loving others. Lacking empathy, they regard others as mere instruments, objectified "Sources" of Narcissistic Supply. With the exception of criterion 6 above - the classic narcissist would tend to fit all the others.