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Page 103 De Francesco et al. J Transl Genet Genom 2024;8:102-18 https://dx.doi.org/10.20517/jtgg.2023.51

procedures as well as setting up preventive interventions for CDS.

Keywords: Cognitive disengagement syndrome, internalizing disorders, externalizing disorders, twin studies,
comorbidity

INTRODUCTION
For several decades now, the importance of behavioral genetics in the field of psychopathology research has
been widely recognized, as it constitutes a unique investigative method that allows the analysis of genetic
and environmental etiological factors underlying psychological disorders. Quantitative genetic studies,
especially twin studies, are particularly applied in research on developmental psychopathology and have
significantly contributed to enriching our current understanding of Internalizing and Externalizing
symptoms. The terms Internalizing and Externalizing are used to refer to two groupings of behavioral,
[1]
social, and emotional problems . Internalizing (INT) symptoms are usually related to the self and
underestimated by external raters as not effortlessly recognizable, while externalizing (EXT) symptoms
[2]
generally occur in the interaction with the social environment .

Behavioral genetics, besides shedding light on the etiological factors of individual disorders belonging to
each of these domains, has greatly enhanced our knowledge regarding the existence of a common
susceptibility factor underlying the frequent comorbidity between these symptom’s clusters. However, in the
last two decades, interest has grown in a set of dysfunctional behaviors that cannot be categorized within the
aforementioned domains. Since the late 80s, factor analysis studies have shown that some of these
behaviors could be enclosed within a distinct condition originally termed Sluggish Cognitive Tempo
[3]
(SCT) . This condition refers to a set of developmentally inappropriate caregiver-reported behaviors and
symptoms involving slowed-down cognitive processing speed, excessive daydreaming and mind-wandering,
mental confusion/fogginess, task-unrelated thoughts, difficulty initiating and sustaining effort, low
motivation, drowsiness, and marked hypoactivity that can impair a child’s daily functioning in several
[5]
[4]
domains . The term SCT has recently been replaced with Cognitive Disengagement Syndrome (CDS ),
which more accurately represents the nature of the syndrome and aligns more precisely with the current
scientific understanding and terminology preferences. Given the nature of the features that characterize it,
[6,7]
originally, CDS was considered a subgroup of Attention-Deficit/Hyperactivity Disorder (ADHD) . ADHD
is amongst the most pervasive psychological disorders affecting children in their schooling years. It is
characterized by symptoms such as hyperactive behavior, impulsivity, and inability to sustain attention.
Depending on which of these problems best characterizes the individual’s symptomatic profile, they can be
categorized into three different presentations: ADHD-I (predominantly inattentive type), ADHD-H
(predominantly hyperactive-impulsive type), or ADHD-C (combined type) .
[8]

The recognition of CDS as a distinct nosological entity has gained significant scientific validity following a
meta-analysis conducted by Hartman et al. in 2004, whose results indicated a clear differentiation between
[9]
CDS and the inattentive subtype of ADHD . Being a relatively young syndrome, twin research on CDS is
still in its early stages but has already provided important insights, both confirming its differentiation from
ADHD-I and identifying some common etiological factors with specific INT symptoms. However, despite
the high rates of comorbidity between CDS and INT-EXT symptoms that have already been found in a
growing body of literature, twin research on this topic still needs to be further deepened.

In light of the above, this review aims to discuss the potential significance of twin studies in highlighting the
genetic and environmental basis of the co-occurrence between INT-EXT and CDS manifestations. After an