He characterizes their way of thinking as being vague and void of thoughts and as sometimes having a "defective perceptual scanning". Because they often do not perceive cues that trigger affective responses, they experience fewer emotional reactions.For Millon, SzPD is distinguished from other personality disorders in that it is "the personality disorder that lacks a personality." He criticizes that this may be due to the current diagnostic criteria: They describe SzPD only by an absence of certain traits, which results in a "deficit syndrome" or "vacuum". Instead of delineating the presence of something, they mention solely what is lacking. Therefore, it is hard to describe and research such a concept.He identified four subtypes of SzPD. Any individual schizoid may exhibit none or one of the following:

Differential diagnosis
While SzPD shares several symptoms with other mental disorders, there are some important differentiating features:

Comorbidity
SzPD is often found to be comorbid with at least one of several disorders or pathologies. Sometimes, a person with SzPD may meet criteria for an additional personality disorder; when this happens, it is most often avoidant, schizotypal or paranoid PD. Alexithymia (the inability to identify and describe emotions) is often present in SzPD. Sharon Ekleberry suggests that some people with schizoid personality features may occasionally experience instances of brief reactive psychosis when under stress. Substance use disorder
Very little data exists for rates of substance use disorder among people with SzPD, but existing studies suggest they are less likely to have substance abuse problems than the general population.
The study found that social interaction impairments, stereotyped behaviours and specific interests were more severe in the individuals with Asperger syndrome also fulfilling SzPD criteria, against the notion that social interaction skills are unimpaired in SzPD. The authors believe that substantial subgroup of people with autism spectrum disorder or PDD have clear "schizoid traits" and correspond largely to the "loners" in Lorna Wings classification The autism spectrum (Lancet 1997), described by Sula Wolff. The authors of the 2019 study hypothesised that it is extremely likely that historic cohorts of adults diagnosed with SzPD either also had childhood-onset autistic syndromes or were misdiagnosed. They stressed that further research to clarify overlap and distinctions between these two syndromes was strongly warranted, especially given that high-functioning autism spectrum disorders are now recognised in around 1% of the population. Low weight
A study which looked at the body mass index (BMI) of a sample of both male adolescents diagnosed with SzPD and those diagnosed with Asperger syndrome found that the BMI of all patients was significantly below normal. Clinical records indicated abnormal eating behaviour by some patients. Some patients would only eat when alone and refused to eat out. Restrictive diets and fears of disease were also found. It was suggested that the anhedonia of SzPD may also cover eating, leading schizoid individuals to not enjoy it. Alternatively, it was suggested that schizoid individuals may not feel hunger as strongly as others or not respond to it, a certain withdrawal "from themselves".