Technically, SPD and Asperger's cannot exist together because they are defined as mutually exclusive; if an Aspie fits criteria for SPD, he cannot be diagnosed with it because the SPD symptoms occur "during the [life-long] course" of a PDD. However, many Aspies do have schizoid traits. An asexual Aspie who does not find pleasure in socializing could also be diagnosed with SPD if the "during the course" criterion weren't present.
So you have to pick one or the other.
(I think it's possible to be an autistic person with schizoid traits. But as the descriptions stand currently, an Aspie who rejects socialization is only Aspie, not Aspie/Schizoid. Autism takes precedence in diagnosis.)
--SPD is a personality disorder; and while personality has a large heritable component, it doesn't have a neurological aspect like Asperger's does. You're likely to see no real difference from NT in cognition, sensory processing, learning, or coordination. In fact, SPD has its onset by adulthood, as the personality becomes more set for a maturing individual. Asperger's is usually obvious by the toddler years--often, in infancy.
--AS has extra traits that SPD doesn't have--stims, special interests, odd speech, atypical nonverbal communication. If you have somebody who looks schizoid and has those traits, you can be pretty sure that he's autistic.
--Schizoid personality involves not wanting to socialize. Asperger's involves low social skill, regardless of desire. (Many SPD cases do involve low social skills due to lack of practice, but with practice, someone with SPD could get up to NT standard. The issue is that he sees no point in it. But low levels of skill also often lead to low levels of interest--part of why I think more Aspies have schizoid traits than average.)
--Schizoid personality has one trait that is almost never seen in a non-depressed Aspie, by definition: "Takes pleasure in few, if any, activities." That's because Aspie special interests and many stims are sources of great pleasure. I've described my special interests as "almost like falling in love".
--Not all Aspies are asexual. Most people with SPD are.
--Aspies can be extremely sensitive to praise or criticism; an SPD personality makes you rather indifferent to it.
--A significant number of Aspies are extroverted. SPD is extreme introversion.
Myth: Schizoid personality disorder is a kind of schizophrenia.
Truth: No. It's a personality disorder characterized by extreme introversion. While it is associated with an elevated risk of schizophrenia in some studies, SPD does not involve delusions or any other sort of psychosis, nor does it affect cognition.Myth: Schizoid personality disorder means a "dangerous loner" with no morality. It's a recipe for a serial killer.
SPD is probably one of the least troublesome psychological conditions it's possible to have, both for the person diagnosed with it and for society. It doesn't cause psychosis; nor does it impair one's judgment or morality. It may disrupt the typical "mirroring" sort of empathy, in that people with SPD would not immediately feel bad on seeing another person suffer; but it does not appear to affect the chances that the person with SPD would help the suffering individual--they may, in fact, be better at it because they are not emotionally involved in the situation and keep a cool head. (Emotional detachment, an SPD characteristic, is required of doctors, paramedics, police officers, etc.) SPD is defined partly by flat affect, but that is not the absence of feeling; it's the absence of expressed feeling. Additionally: Not all cases of SPD involve flat affect. And some cases of Asperger's do.
There is indeed much speculation that SPD is not actually a mental illness. Unlike the other personality disorders, it seems to cause no direct distress. The only obstacle with SPD seems to be that schizoid individuals may be expected to connect more than they desire to do; and people around them--especially family--may expect more of them than they can give. Finding socialization boring and a waste of time can seem offensive to others who do not understand that this dislike is not personal.
All in all, I think SPD is really on the borderline when it comes to what is "mental illness" and what is not. It doesn't cause distress. It doesn't cause dysfunction. And while it is "deviant" (statistically off the norm), so is genius or saintliness. Introversion is simply a culturally disliked way of life; and mental illness shouldn't be defined by culture. It should be defined by asking questions like, "Does it hurt you? Does it hurt anyone else? Does it lower your ability to function?" And if the answer to those questions is "no", then it shouldn't be a mental illness.
Note: Comments disabled on this post to stop spam. PM me to comment. Thanks.