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Figure 2. Independent pathway multivariate model. Observed variables are depicted in squares and latent variables in circles. Ac:
Additive genetic factors common to the phenotypes; Cc: shared environmental factors common to the phenotypes; Ec: unique
environmental factors common to the phenotypes. A: additive genetic factors specific to the phenotypes; C: shared environmental
factors specific to the phenotypes; E: unique environmental factors specific to the phenotypes.

Figure 3. Common pathway model. Observed variables are depicted in squares and latent variables in circles. Ac: Additive genetic
factors common to the phenotypes; Cc: shared environmental factors common to the phenotypes; Ec: unique environmental factors
common to the phenotypes; A: additive genetic factors specific to the phenotypes; C: shared environmental factors specific to the
phenotypes; E: unique environmental factors specific to the phenotypes; L: common latent susceptibility factor.

limitations of categorical classifications are not negligible, as they have often been the central reason for
revisions and reeditions of the manuals. Caspi & Moffitt emphasized how the existence of such a high
[26]
number of diagnostic categories directly leads to high rates of comorbidity among the disorders themselves:
the presence of transdiagnostic symptoms and the tendency to categorize psychopathology into specific
labels mean that a significant portion of individuals diagnosed with one disorder may meet the criteria for
the diagnosis of other conditions. The authors' proposal would be to adopt a more parsimonious approach
to diagnosis, considering the distinction between INT-EXT disorders, which could further promote the
development of dimensional psychodiagnostic approaches .
[26]
Adopting a dimensional perspective: the three factors model
As mentioned, one of the most discussed implications of the categorical approach regards the high
comorbidity rates that can be found among different disorders. Research has shown that most people
diagnosed with a psychiatric disorder would often match the criteria for at least another
psychopathology . These comorbidity rates make it necessary to evaluate the hypothesis that psychiatric
[26]
disorders might be analyzed using a more parsimonious model able to aggregate the disorders into macro-
categories, rather than identifying them as independent entities . One of the first studies to introduce a
[26]
[27]
new perspective on the existing taxonomic system was conducted by Krueger in 1999 . The research in
question involved a confirmatory factor analysis, through which the comorbidity (and hence correlations)
of the ten most common mental disorders (major depressive episode, panic disorder, dysthymia, social