## 4 Cognitive Disengagement Syndrome (CDS) and Twin Studies  
### CDS: Symptoms, Impairments, and Differentiation with ADHD  
Cognitive Disengagement Syndrome (CDS) can be defined as a cluster of symptoms impacting cognitive function and its underlying processes [1]. The core characteristics of CDS include excessive daydreaming, difficulties in sustaining and directing attention, and deficits in information processing, particularly in terms of accuracy and speed [2]. Furthermore, CDS is associated with motor dysfunction, manifesting as hypoactivity, prolonged sedentary behavior, and slowed, reduced, or delayed motor responses [3,4]. These impairments contribute to significant deficits in academic performance, social interactions, and overall adaptive functioning across both youth and adulthood [1,4]. This condition was formerly named Sluggish Cognitive Tempo (SCT), a designation that emphasized cognitive sluggishness, mental foginess, and slowed cognitive and motor processes [4]. However, this terminology shift from SCT to CDS reflects both conceptual and terminological advancements. The former label primarily underscored cognitive slowness while failing to encompass the broader symptomatology, which includes inattention, excessive daydreaming, hypoactivity, and psychomotor slowing. Additionally, the term \"sluggish\" carried a stigmatizing connotation, whereas \"disengagement\" offers a more neutral and clinically precise characterization. The revised nomenclature more accurately captures the multifaceted nature of the disorder and aligns with contemporary research emphasizing its complex clinical presentation [4,47,48]. The impact of CDS extends beyond cognitive and motor deficits, influencing broader functional domains such as academic performance and social interactions [4]. Research has increasingly focused on distinguishing CDS from related conditions, particularly ADHD, by examining their distinct neurocognitive profiles. Empirical findings indicate that ADHD and CDS exhibit notable differences in neurocognitive functioning. [PERSON] et al. [49] identified significant differences in EF deficits among adults with comorbid ADHD and CDS compared to those diagnosed solely with ADHD. Specifically, CDS appears to be primarily associated with deficits in processing speed, which manifest as delayed responses and difficulties in information processing under increased cognitive demands. These impairments hinder the effective implementation of strategic approaches in working memory tasks. The distinct cognitive profile of CDS is characterized by slower response times and specific patterns of cognitive strategy use, differentiating it from ADHD. Consequently, the presence of processing speed deficits suggests that CDS imposes an additional cognitive burden on individuals with both ADHD and CDS compared to those with ADHD alone. Given these findings, EF performance emerges as a critical dimension for distinguishing CDS from ADHD. [PERSON] et al. [49] propose that, as CDS is associated with more pronounced EF deficits than ADHD, EF impairments could serve as a key discriminative factor between the two conditions. A comprehensive distinction between CDS and ADHD is widely acknowledged, not only due to differences in neurocognitive profiles but also in relation to symptom manifestation, potential pharmacological interventions, and longitudinal trajectories. Morespecifically, the inattention observed in CDS seems to differ qualitatively from that seen in ADHD. In CDS, the difficulty in maintaining attentional focus is predominantly triggered by internal stimuli. Conversely, in ADHD, inattention is more strongly influenced by external stimuli from the environment [50,51]. This distinction is further supported by evidence suggesting that the presence of CDS as the primary subtype of ADHD is associated with poor adherence to and limited efficacy of methylphenidate treatment.