Attention-deficit/hyperactivity disorder (ADHD) and autism spectrum disorder (ASD) are two of the most common neurodevelopmental disorders typically emerging in early childhood, with global prevalence rates estimated at 5% to 8% and 1% to 4%, respectively [1,2]. In the South Korean population, these rates have been reported to be 8.5% and 2.6%, respectively [3,4]. ADHD and ASD are significant public health concerns, as they are lifelong conditions that often persist into adulthood. These disorders exert long-term impacts not only on the individuals’ functional outcomes but also on the quality of life and well-being of their families, requiring continuous support and comprehensive management across the lifespan [5,6]. ADHD is primarily characterized by inattention, hyperactivity, and impulsivity [7], symptoms that frequently contribute to academic underachievement, difficulty in school adjustment, and life-long economic and social problems [8-10]. In contrast, ASD is characterized by persistent difficulties in social communication and interaction, along with restricted and repetitive behaviors [7], often resulting in social withdrawal, loneliness, and limited participation in group activities [11]. Consequently, children with ADHD and/or ASD are at a heightened risk for bullying and emotional difficulties, including anxiety, depression, and low self-esteem [8,11,12].

Although ADHD and ASD are classified as distinct diagnostic entities, they frequently co-occur. Studies have shown that 50%-70% of children with ASD also meet the diagnostic criteria for ADHD [13], and 21%-30% of children with ADHD exhibit significant autistic traits [14]. Children with comorbid ADHD and ASD often demonstrate additive difficulties across behavioral, cognitive, and social domains, leading to more complex clinical presentations and more affected functional outcomes than those with either condition alone [14,15].

Both disorders are associated with cognitive dysfunction, although their profiles differ. ADHD is strongly linked to relatively weaker executive functions, with differences in attention regulation, working memory, cognitive flexibility, and planning [16]. In contrast, ASD is more commonly associated with cognitive rigidity, difficulty in set-shifting, and a detail-focused rather than holistic cognitive style [17,18]. Nonetheless, overlapping difficulties—particularly in processing speed, working memory, and response inhibition—have been observed in both groups [19,20].

Intelligence quotient (IQ) tests are widely used to assess general intellectual functioning and to delineate cognitive strengths and weaknesses [21]. In individuals with ASD, IQ has consistently emerged as one of the strongest predictors of long-term functional outcomes, often surpassing core symptom severity in its association with adaptive behavior [22]. For children with ADHD, a detailed IQ profile is valuable as it helps identify specific weaknesses and strengths that require targeted educational support [23,24]. The IQ profile can thus serve as a diagnostic aid for ASD and a guide for individualized ADHD interventions.

Several studies have examined the cognitive profiles of children with ADHD and ASD using various intelligence test batteries. A common finding is that children with ASD and ADHD exhibit lower functioning on the Working Memory Index (WMI) and Processing Speed Index (PSI) of intelligence tests [25,26]. However, further distinctions emerge between the two disorders in their IQ profiles. Children with ASD often present with a “spiky” profile on the Wechsler scales, typically showing relative strengths in the Block design subtest alongside weaknesses in the Comprehension subtest [17,27]. In contrast, children with ADHD generally demonstrate weaknesses more specifically in subtests related to working memory and processing speed [23,25,28].

Given these shared and divergent cognitive profiles associated with ASD and ADHD, it is helpful to understand how the distinct symptom patterns of ASD and ADHD are associated with performance across IQ profiles. Previous research has established specific links between ASD or ADHD symptomatology and cognitive abilities. Previous studies on ASD suggested that lower verbal abilities relative to nonverbal abilities are associated with greater communication and social difficulties [29,30]. Similarly, studies conducted on children with ADHD also demonstrated an association between ADHD symptoms and weaknesses in both working memory and processing speed [29,30]. However, those studies did not consider the impact of comorbid ADHD and ASD. Therefore, research is needed to examine the relationship between ASD or ADHD symptoms and IQ profiles while accounting for the effects of comorbid ADHD and ASD.

Most existing research regarding ADHD and ASD has focused on school-aged children or adults, with limited data available on preschool-aged children, a population for whom early identification of cognitive weaknesses is especially important to improving cognitive and academic outcomes [31]. This critical period is characterized by high neuroplasticity, meaning early identification allows for immediate, targeted intervention, potentially maximizing long-term outcomes [32]. Despite the critical need for early and accurate assessment in this population, little is known about the cognitive profiles of children with comorbid ADHD and ASD. One contributing factor to this disparity is the challenge of accurately assessing cognitive function in young children, where factors like test-taking cooperation, short attention spans, and the inherent instability of cognitive scores may complicate data collection and interpretation [33].

This study therefore aims to: 1) compare the IQ profiles of preschool children diagnosed with ADHD, ASD, comorbid ADHD and ASD, and TD peers, using the Korean Wechsler Preschool and Primary Scale of Intelligence-Fourth Edition (K-WPPSI-IV)—a widely used tool for assessing cognitive abilities in preschool-aged children in Korea [34]; and 2) examine the associations between IQ profiles and clinical measures commonly used to assess ADHD and ASD symptoms.

Based on previous research, it is hypothesized that children with ADHD and those with ASD will share weaknesses in the PSI. Additionally, it is hypothesized that children with ASD will show further weakness in the Comprehension subtest, while those with ADHD will show specific weaknesses in the working memory subtests. Furthermore, it is hypothesized that ASD symptoms will correlate negatively with the Verbal Comprehension Index (VCI), and that ADHD symptoms will relate negatively with the WMI and PSI.

Our findings revealed that children with ADHD and ASD exhibited shared difficulties in working memory and processing speed, yet they differed in verbal abilities, as expected. Specifically, differences were noted on the Vocabulary and Comprehension subtests of the K-WPPSI-IV, with children with ASD scoring significantly lower than those with ADHD alone on the Comprehension subtest. Furthermore, CARS scores showed a negative correlation with WMI, while ATA scores were negatively correlated with PSI. Crucially, while these patterns are consistent with those documented in previous studies examining school-aged children [26,27,46], the novelty of our work is the demonstration of these specific profile differences within the understudied preschool-aged population. This finding provides critical new data for early differential diagnosis.

Working memory is a foundational cognitive mechanism that supports the ability to process stimuli, retrieve relevant information based on past experiences, and maintain attentional focus on ongoing tasks [47]. It also underlies core executive functions, including attention regulation, inhibition, planning, and temporal sequencing [48]. In ADHD, reduced working memory is a well-documented core neuropsychological weakness, strongly associated with behavioral challenges [23,49-51], and typically assessed using the WMI on Wechsler intelligence tests. Numerous studies have confirmed lower WMI scores in ADHD, often attributed to diminished attention [25,28]. Children with ASD also present with a distinctive profile of executive dysfunction, including difficulties with cognitive flexibility, working memory, and selective attention [52,53], which contributes to lower WMI scores [18,26]. While some research reports overlapping working memory weaknesses associated with attentional difficulties in both ADHD and ASD [54,55], other evidence suggests more pronounced spatial working memory difficulties in ADHD, potentially stemming from greater weaknesses in inhibitory control [56]. In contrast, our study found that both the ADHD and ASD groups exhibited weaknesses in working memory, while the WMI scores did not significantly differ between the ADHD, ASD, and ADHD+ASD groups. This finding was further corroborated by the significant correlations observed between the WMI and its subtests and the ATA and CARS scores, suggesting a direct link between working memory weaknesses and core symptom severity. While working memory differences are commonly observed across various neurodevelopmental disorders, they are most consistently reported as a core cognitive characteristic of ADHD [57]. Therefore, low performance in working memory may represent a shared cognitive vulnerability among preschool-aged children with ADHD and ASD. The common weaknesses in working memory across both conditions highlight that interventions focusing on executive function, including working memory, should be integrated into treatment plans for both ASD and ADHD.

As hypothesized, processing speed also emerged as a shared area of weakness in children with ADHD and ASD. Processing speed reflects the efficiency with which individuals perceive, process, and respond to stimuli [58] and is a key determinant of performance across a range of cognitive tasks [59]. Previous research has shown that children with ADHD often display reduced processing speed, particularly when cognitively underaroused, leading to slower responses and impaired output generation [60,61]. Similarly, ASD has been conceptualized as a disorder of general information processing, with relative weaknesses observed in both simple and complex cognitive tasks [62-65]. The PSI in the Wechsler scales assesses visual scanning, sustained attention, and interference control—cognitive functions that are frequently influenced in both ADHD and ASD.66 Consistent with prior studies [18,28,46,67], our results demonstrated uniformly low PSI scores across the ADHD, ASD, and ADHD+ASD groups. Furthermore, PSI scores and their subtests showed negative correlations with visual indices on the ATA, suggesting that reduced processing speed is associated with diminished visual attention capabilities. However, interpreting these shared weaknesses requires accounting for the measurement bias inherent in the K-WPPSI-IV. The substantial verbal demands of this standardized tool can confound performance in the working memory and processing speed domains for children with ASD, often resulting in lower scores that may not fully reflect their innate capacity [68,69]. Additionally, it is important to consider that reduced PSI scores have been observed in various other neurodevelopmental conditions [70], mood disorders [71], and even in states of fatigue [72]. Therefore, while reduced processing speed is common in both ADHD and ASD, they should be interpreted as a non-specific neuropsychological marker rather than a condition-specific weakness.

In terms of verbal cognitive profiles, our results showed significantly lower scores on the Vocabulary and Comprehension subtests in children with ASD compared with those with ADHD, despite no significant group differences in overall VCI scores. The Comprehension subtest demands complex verbal reasoning and social understanding, while Vocabulary assesses verbal expression [26]. These tasks vary in the degree of linguistic and social-cognitive processing required, which may explain the particular difficulty children with ASD experience with Comprehension [17,73]. Language delays are well-documented in ASD, with approximately 75% of preschool-aged children showing mild-to-severe variations [74]. Although relative language difficulties in ASD are heterogeneous, there is a broad consensus that many children struggle with linking language with real-world referents—a process requiring both semantic understanding (word meaning) and pragmatic skills (contextual language use and discourse), which are closely tied to social reasoning [73]. Prior research has also identified profound weaknesses in speech comprehension and pragmatic-semantic language use in young children with ASD [75,76]. In contrast, children with ADHD did not exhibit similar difficulties on these subtests. These findings of differential subtest performance highlight a distinctive verbal cognitive profile associated with ASD. This finding offers clinical insight for differential diagnosis within the preschool context and underscores the necessity of early intervention programs targeting social-cognitive skills and verbal reasoning, particularly in real-world situation.

Several limitations should be considered when interpreting our findings. First, this study utilized a retrospective chart review design, which inherently limits the consistency and depth of data collection. Given the young age of our sample, this design also precludes the ability to track diagnostic trajectories over time. Some children currently classified with a single diagnosis may later meet the criteria for co-occurring ASD and ADHD, as one condition often precedes the other during development [77]. Second, sample sizes varied across clinical assessments within each group because different assessments were selectively administered based on presenting symptoms. Consequently, caution is warranted when interpreting the correlational analyses. This variability, however, does not compromise the interpretation of the K-WPPSI-IV profiles, which was the primary focus of this study. Third, clinical diagnoses were based on DSM-5 criteria rather than standardized structured diagnostic tools such as the autism diagnostic interview-revised, autism diagnostic observation schedule, or Korean version of the kiddie-schedule for affective disorders and schizophrenia for school-age children-present and lifetime version, which may limit the diagnostic reliability of the sample. However, we sought to mitigate this limitation by utilizing supplementary rating scales, such as the CARS and the ARS, to support diagnostic validity. Fourth, the exclusion of children with FSIQ below 70, as well as children requiring psychotropic medication, resulted in participants primarily composed of high-functioning individuals with relatively less severe symptoms. Thus, our findings may not fully represent the cognitive profiles of children with comorbid intellectual disabilities or those with greater clinical severity. Fifth, the TD group’s unusually low Vocabulary score relative to their average FSIQ suggests a potential sampling bias in verbal ability. This bias may compromise the validity of our verbal comparisons. Last, the relatively small sample size, especially for the TD group (n=29), constitutes a significant limitation. This restricts the statistical power to reliably detect medium- or small-sized effects. Future research should prioritize prospective designs and recruiting a larger, more representative TD cohort to enhance power and strengthen the validity of the findings.

Despite these limitations, our study has several notable strengths. First, we focused exclusively on preschool-aged children (4-6 years), ensuring that children were at similar stages of cognitive and neurodevelopmental maturity. Second, all children were drug-naive and not taking psychotropic medications at the time of assessment. Since stimulant medications are known to influence FSIQ and other cognitive measures on the K-WPPSI-IV, limiting the sample to unmedicated children allowed for more accurate assessment of their baseline cognitive profiles. Third, our sample comprised Asian children—a population underrepresented in previous comparative studies of cognitive functioning in ADHD and ASD. The replication of cognitive profile distinctions largely consistent with Western findings suggests that the fundamental cognitive manifestations of ASD and ADHD may be robust across cultural boundaries, adding to the generalizability of the findings. Fourth, we included four distinct groups: children with ADHD only, children with ASD only, those with comorbid ADHD and ASD, and TD controls. This comprehensive design allowed for a nuanced examination of the unique and overlapping cognitive features associated with each diagnostic category.

Overall, our findings suggest that preschool-aged children with ADHD and those with ASD share common weaknesses in working memory and processing speed. However, children with ASD showed greater difficulties in verbal expression and complex reasoning, indicating meaningful differences in cognitive profiles between the two conditions. These results provide preliminary insights into early cognitive characteristics of ADHD and ASD. Notably, children with comorbid ADHD and ASD demonstrated more variable patterns, underscoring the need for further research to better characterize the cognitive abilities associated with comorbidity in this young population.