Recently I had someone ask me if you can have both Schizotypal Personality and an autism spectrum condition. I've gotten permission to repost my reply here. According to the person who asked me the question, my reply was quite helpful.

I think it probably is possible to be both schizotypal and AS, although it would be tricky to determine because some of the symptoms overlap. Both conditions have odd mannerisms, poor social skills and probably executive dysfunction (don't know if EFD has been observed in schizotypal, but it's definitely present in schizophrenia, which is a related but more severe condition). I suspect that sensory sensitivities are common in schizotypal as well. The way to tell if someone has both would be to look for the symptoms that aren't shared between the two conditions. 

Look at the DSM-IV criteria for schizotypal and for autism: 

Schizotypal Personality: 

A. A pervasive pattern of social and interpersonal deficits marked by acute discomfort with, and reduced capacity for, close relationships as well as by cognitive or perceptual distortions and eccentricities of behavior, beginning by early adulthood and present in a variety of contexts, as indicated by five (or more) of the following: 

(1) ideas of reference (excluding delusions of reference) 

(2) odd beliefs or magical thinking that influences behavior and is inconsistent with subcultural norms (e.g., superstitiousness, belief in clairvoyance, telepathy, or "sixth sense"; in children and adolescents, bizarre fantasies or preoccupations) 

(3) unusual perceptual experiences, including bodily illusions 

(4) odd thinking and speech (e.g., vague, circumstantial, metaphorical, overelaborate, or stereotyped) 

(5) suspiciousness or paranoid ideation 

(6) inappropriate or constricted affect 

(7) behavior or appearance that is odd, eccentric, or peculiar 

(8) lack of close friends or confidants other than first-degree relatives 

(9) excessive social anxiety that does not diminish with familiarity and tends to be associated with paranoid fears rather than negative judgments about self 

Autism (DSM-IV Asperger Syndrome requires only A and C): 

(I) A total of six (or more) items from (A), (B), and (C), with at least two from (A), and one each from (B) and (C) 

(A) qualitative impairment in social interaction, as manifested by at least two of the following: 

1. marked impairments in the use of multiple nonverbal behaviors such as eye-to-eye gaze, facial expression, body posture, and gestures to regulate social interaction 

2. failure to develop peer relationships appropriate to developmental level 

3. a lack of spontaneous seeking to share enjoyment, interests, or achievements with other people, (e.g., by a lack of showing, bringing, or pointing out objects of interest to other people) 

4. lack of social or emotional reciprocity ( note: in the description, it gives the following as examples: not actively participating in simple social play or games, preferring solitary activities, or involving others in activities only as tools or "mechanical" aids ) 

(B) qualitative impairments in communication as manifested by at least one of the following: 

1. delay in, or total lack of, the development of spoken language (not accompanied by an attempt to compensate through alternative modes of communication such as gesture or mime) 

2. in individuals with adequate speech, marked impairment in the ability to initiate or sustain a conversation with others 

3. stereotyped and repetitive use of language or idiosyncratic language 

4. lack of varied, spontaneous make-believe play or social imitative play appropriate to developmental level 

(C) restricted repetitive and stereotyped patterns of behavior, interests and activities, as manifested by at least two of the following: 

1. encompassing preoccupation with one or more stereotyped and restricted patterns of interest that is abnormal either in intensity or focus 

2. apparently inflexible adherence to specific, nonfunctional routines or rituals 

3. stereotyped and repetitive motor mannerisms (e.g hand or finger flapping or twisting, or complex whole-body movements) 

4. persistent preoccupation with parts of objects 

So, if you look at those two, the following overlaps are present: 

* schizotypal criteria 3 overlaps with autism B3 and B2 (verbal communication issues and odd language use) 

* schizotypal criteria 6 is the same as autism A1 (impairment in expressive use of nonverbal cues) 

* schizotypal criteria 7 can result from autism A1 and C3 

* schizotypal criteria 8 is a more severe form of autism A2 (you can meet A2 and have friends if your friendships are atypical or you have a lot of trouble making friends, but if you don't have any friends you meet both 8 and A2) 

In addition, I've often seen schizotypal 9 and possibly 5 in AS people as a result of bullying. A lot of AS people are too trusting, but if they get hurt too much by others, they can switch to not trusting enough. 

Now, schizotypal is not supposed to be diagnosed if it occurs 'during the course of a pervasive developmental disorder' (which is outdated language given that we now know autism spectrum conditions are not episodic, like schizophrenia or bipolar disorder are). That probably means that you can't diagnose a person as schizotypal if an autism spectrum condition could explain all their schizotypal traits. 

So, that would probably mean that you don't count 6 or 8, and only count 3 and 7 if they are not due to autistic traits (eg if the language problems are more typical of thought disorder than autism, and the eccentric appearance is not due to stimming and/or odd nonverbal communication). Criteria 1, 2 and 3 could be counted, because those have nothing to do with autism, and criteria 5 and 9 could be counted as long as they're in more extreme than would be expected based on the person's history of negative social experiences. That could add up to the required five, but it would be harder to meet criteria than if you didn't have AS.

Also keep in mind that each clinician kind of interprets the rules differently. The existence of AS as a diagnosis is proof of this, because AS is supposed to be diagnosed only if autism criteria aren't met (ie, they meet A and C but not B), and some of the criteria in A and B are actually different ways of describing the exact same thing (eg A3 or A4 will cause B2, because you can't have that kind of social impairment and not have it affect your conversations with others). So research has shown almost everyone who meets criteria A actually meets B as well, but if the person had no language delay or fits the clinician's stereotype of AS in some way, they will ignore the B criteria and diagnose AS instead of autism. (Autism does require delay in social, communication or pretend play, but anyone who meets criteria A has a delay in social skills by definition.) So it's possible that if you were seen by a different clinician, you could have gotten a different diagnosis. Maybe another clinician would diagnose you as Schizotypal and AS, or decide Schizotypal fits you better than AS does. There's a certain amount of subjectivity to it.