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The results of this work thus highlighted that although CDS and anxiety-related problems represent distinct
phenomena at the clinical level, they possess many common etiological factors, consisting of both life
[73]
experiences and biological predisposition .

Twin studies on CDS and Externalizing symptoms
To date, twin studies have mainly focused on the comorbidity between CDS and ADHD, whereas the
genetic patterns underlying the co-occurrence between CDS and other EXT have not yet been analyzed.
Moruzzi et al. have investigated the relationship between CDS, Inattentive Problems (INP), and
[74]
Hyperactivity-Impulsivity Problems (HIP) in a sample of 398 Italian twins aged 8 to 17 years, highlighting
that these domains are correlated both genetically and environmentally, thus remaining distinct . More
[74]
specifically, in this case, the best-fitting twin model was a Cholesky model, which underlined that the
covariation between CDS, INP, and HIP was influenced by additive genetic factors and non-shared
[74]
environmental factors . In particular, stronger correlations have been highlighted between CDS and INP
compared to those between CDS and HIP. Given that genetic correlations between these phenotypes also
appear to be significant, the co-occurrence of CDS and ADHD-INP may be attributable to genetic
components . Moreover, within the examined sample, symptomatology related to CDS exhibits the lowest
[74]
heritability, suggesting the plausible influence of environmental factors in accounting for variances in CDS
scores among twins, compared to those related to HIP. This underscores the presence of partially distinct
etiological patterns between CDS and ADHD . The CDS traits, HIP and INP, still appear to be correlated
[74]
at both genetic and environmental levels, suggesting that they may share, to some extent, genetic
underpinnings and environmental risk factors . This result sustains the hypothesis that CDS might
[74]
develop from partially different patterns of genetic influence and might be considered a distinct
[74]
phenomenon, despite sharing some etiological factors with the main ADHD presentations . Another twin
study that has examined the relationship between CDS and ADHD is the one conducted by Leopold
et al. . The authors highlighted a trend of hyperactivity-related symptoms decreasing over time through a
[75]
longitudinal twin study where phenotypes were measured over a span of 10 years. In contrast, CDS levels
tended to increase in opposition to the inattentive symptoms’ characteristic of ADHD, which exhibited
greater stability. In the analyzed sample, the correlations between CDS and ADHD dimensions were high
and stable but always below r = 1.0, further confirming the hypothesis that these two disorders would be
[75]
distinguishable from each other, despite their frequent comorbidity . These findings are consistent with
the previously discussed literature about the genetic contribution to the comorbidity of CDS and INT and
with the body of literature that emphasizes the need to differentiate between CDS and other
psychopathological domains. However, more research is needed to further examine the genetic
contributions to the comorbidity between CDS and other EXT domains.

CONCLUSIONS
There is a substantial need for further investigation into the etiology of CDS and its potential classification
as a distinct disorder in forthcoming diagnostic manuals. The comorbidity between CDS and INT has been
profusely investigated [54,62,64] , to the extent that the identification of CDS symptoms as transdiagnostic
dimensions of anxiety and depressive disorders has been hypothesized . However, genetic evidence
[51]
[40]
regarding the co-occurrence of CDS symptoms, INT and EXT disorders is limited . Given the evidence of
the significant contribution of twin studies in highlighting the genetic influence on the comorbidity between
INT-EXT disorders , it is important to implement this investigation considering CDS as a distinct
[40]
nosological entity. Despite the emergence of some evidence in this area [73,74] , results provided so far by the
existing literature are far from being fully conclusive. Up to now, research on this topic has highlighted that,
although having some overlapping genetic pathways with both INT symptomatology and ADHD, CDS
might be considered a unique syndrome with significant internal validity [69,70] . Furthermore, as stated above,
more research is needed to address the comorbidity between CDS and other EXT disorders.