Having noted the high incidence of co-occurrence of personality disorders with other disorders, the question arises as to why this might be so. To date, there are no universally agreed upon explanations. It may ultimately be the case that each pairing of disorders will end up having its own unique set of reasons for co-occurrence. It is also possible that the diagnostic system is not yet accurate enough. We may eventually discover that in cases where there appear to be two disorders, there is really only one. While we cannot provide definitive explanations for the high rate of co-occurrence, we will describe few of the current hypotheses and discuss some of the possibilities that researchers are currently considering:
1.) It may be that the presence of co-occurring disorders is exactly as it appears: there are simply two distinctly different disorders occurring within a person at the same time. In other words, people may literally have more than one unrelated condition at a time. For instance, someone with a cancer diagnosis can also get the Flu. We would be able to notice some symptom overlap such as fatigue and perhaps nausea, but there are two separate and distinct disorders present. Similarly, it could be possible that Borderline Personality Disorder and Eating Disorders share some symptom over-lap, but there are still two distinct disorders present. While the simplicity of this explanation is appealing, it does not explain why such disorders should be observed to co-occur more frequently than can reasonably attributed to chance.
2.) In a related manner, some disorder co-occurrence may be due to overlapping diagnostic criteria. For instance, the diagnostic criteria for Social Phobia and Avoidant Personality Disorder are so similar that it is not surprising that there is a high degree of co-occurrence between these two conditions. Another example would be the co-occurrence among Cluster B personality disorders which share impulsivity as a common diagnostic criterion. Impulsive behaviors are a common feature of the Cluster B personality disorders, so it makes sense that those personality disorders often occur together. Impulsivity can also be found in people with ADHD and Bipolar Disorder so we would expect these two disorders to co-occur with each other, and with Cluster B personality disorders. This explanation provides a technical basis for understanding why co-occurrences might be observed, but it begs the question of whether different disorders sharing common criteria are actually different disorders in the first place.
3.) It is possible that one co-occurring disorder might be a milder or more limited version of the other; similar to the way a cold might be considered a milder version of the flu. This hypothesis has been entertained by some experts to help explain the relationship between Borderline Personality Disorder and Bipolar Disorder, and Bipolar or the relationship between ADHD with Clusters B and C personality disorders.
4.) It may be that one co-occurring disorder might be a precursor of the other. Recall the research we reviewed provided evidence for a link between childhood ADHD and heightened risk for developing a personality disorder in adulthood. It is possible that childhood ADHD is an earlier expression of what will later develop into a personality disorder. However, this explanation does not explain why only some people with ADHD later develop a personality disorder, while others do not.
5.) A final hypothesis many experts entertain is that a common, yet unidentified, underlying factor may explain the existence of two or more co-occurring disorders. In such a case, the co-occurring conditions would appear to be different expressions of the same underlying, yet-to-be-identified cause. To illustrate this concept we might easily (but erroneously) conclude that ice cream causes accidental drowning. Of course, the hidden, underlying factor is higher temperatures during summer months. The higher temperatures cause an increase in both ice cream sales and drowning. This unidentified, underlying factor hypothesis was investigated by Ted Reichborn-Kjennerud and colleagues (2007) who examined the relationship between people with Avoidant Personality Disorder and people with Social Phobia. They concluded there were identical genetic risk factors (yet unidentified) in both groups. Different environmental factors appear to exert an influence that determines which disorder eventually emerges.
In summary, the distinction between similar disorders is often difficult to make. It is clear that some disorders co-occur quite frequently. However, our current state of knowledge has not yet uncovered the exact reasons for this phenomenon. Whether this represents a technical problem with the present-day diagnostic system, or whether there is some deeper, underlying influence at work, we do not yet know. As research continues, the answers will eventually emerge. In the meanwhile, the definitive causes, course, and biological underpinnings of many of the mental illnesses remains unknown.