Attention-deficit/hyperactivity disorder (ADHD) is a neurodevelopmental disorder characterized by hyperactivity, impulsivity, and inattention.1 The worldwide prevalence of ADHD is estimated to be approximately 5% [1,2], with a male sex bias estimated to be between 2:1 and 3:1 [1,3]. ADHD is usually diagnosed at school age and symptomatically improves as children grow [4,5]; notably, hyperactivity and impulsivity improve earlier than does inattention, which might drive the change in the prevalence of diagnostic subtypes of ADHD observed with aging [6,7]. However, recent studies have reported that symptoms in approximately half of the patients diagnosed with childhood ADHD persist into adulthood and cause some degree of functional impairment [8-10].

The ADHD Rating Scale (ARS) is a norm-referenced checklist that was developed to assess the severity of ADHD symptoms. It was originally developed with 14 items based on the diagnostic criteria of the Diagnostic and Statistical Manual of Mental Disorders, Third Edition (DSM-III), and was revised to include 18 items in the DSM-IV [11,12]. The ARS has been validated in multiple studies and translated and validated in a number of countries, including Iceland and Denmark [13-15]. The Korean version of the ARS (K-ARS) was validated by So et al. [16] and has been used in multiple studies with Korean children [17,18].

Normative data of K-ARS scores among Korean elementary school children have been previously reported [19,20], but these studies were limited by small sample sizes. With a large enough sample size, collecting normative data from a population provides important indications of symptom distribution in the general population and allows for culture-specific optimization of screening thresholds. Hypothetically, the score distribution of a scale such as the K-ARS may differ from one country to another because of cultural differences that might alter the expression of symptoms. The present study thus aimed to determine the normative data of K-ARS scores in a large population of Korean school-age children. Furthermore, we compared these data to those from previous Korean studies, as well as to those obtained in the United States to explore the expression of ADHD symptoms possibly unique to Korean populations.

The present study investigated the normative data collected using the K-ARS in a large community-based sample of elementary school children aged 6 to 12 years. Total K-ARS scores and scores on both subscales were significantly higher in boys than in girls, which is consistent with previous studies of normative ARS data from Korean and American general populations [20,21]. In the analysis of K-ARS scores by age group, the scores of 8-year-olds were both the highest of all age groups and significantly higher than the lowest scores (those of the 12-year-olds) in both sexes. Although K-ARS scores slightly declined with age, it the trend was marginal. This finding is inconsistent with reports documenting a significant decline in ADHD symptom severity with age [22]. However, it should be noted that these data were collected from the general population in which most children do not have ADHD. In fact, the findings of a previous study on normative data of the K-ARS were consistent with those of our study [19]. Jang et al. [19] also reported a slight decline in K-ARS scores from a peak at 8 years of age among Korean elementary school children.

Neurobiological changes concomitant with aging may account for the declining severity of ADHD symptom after 8 years : ‘concrete operational period,’ formerly conceptualized by Piaget [23], begins at 7-8 years of age. From this point onward, inhibitory control increasingly develops throughout the childhood and adolescent period [24-26]. Imaging studies on brain structures also indicate that gray-matter formation follows a u-shaped trajectory, where the apex is at 8 years of age [27]. These findings suggest that the development of this period is different from that of the preschool period, which is characterized by expansive mental growth and blossoming nature [28].

The mean total K-ARS scores among boys and girls ranged from 9.19 to 10.48 and 6.34 to 7.15, respectively. This is consistent with other studies that have analyzed normative K-ARS data in Korean populations; one such study found scores for the same age group to range from 9.08 to 11.03 (boys) and 5.38 to 8.37 (girls) [19]. On the other hand, the investigation of the US population that used the original version of the ARS demonstrated some significant differences from our study. While the inattention subscale scores did not differ between the Korean and US populations for either sex, the hyperactivity-impulsivity subscale scores of both sexes and the total scores of boys were significantly higher in the US population than in our study. The scores for the same (80^th, 90^th, 93^rd, and 98^th) percentile cut-offs as those used by DuPaul et al. [21] were also higher than those found in the present study. As aforementioned, the US population featured total score cut-offs of at least 3-4 points higher than those of the population considered by the present study; further, the 98th percentile of the former scored 11 points higher than the latter. Notably, a previous study that compared ARS scores between Japanese and US populations [29] reported that the two populations exhibited a similar difference: hyperactivity-impulsivity subscale scores among both sexes and all age groups were higher in the US population than the Japanese population.

There are some plausible explanations for these differences. DuPaul et al. [21] reported scores differed according to ethnicity. The disparity in scores might reflect real behavioral differences between populations that can be affected by various genetic, environmental, or cultural factors. For instance, the prevalence of the dopamine D4 receptor 7-repeat allele, which is correlated with an increased expression of ADHD, is reported to be very low in Asian populations [30,31]. In addition, differences in dietary habits or other daily routines might also influence the behavior of children. For example, Howard et al. [32] reported an association between a Western dietary pattern and ADHD. Finally, behavior is also affected by cultural factors. Compared to Western cultures, Asian cultures are more collectivistic and emphasize group awareness over individual interest; this difference has been hypothesized to influence the expression of hyperactivity [29,33]. Asian children are also reportedly more introverted than their American counterparts; such introversion could partially mask hyperactivity [29,34].

The differences in parental recognition of ADHD and its symptoms could explain-at least in part-the score differences between the Korean and US populations. Parental recognition may be related to ethnicity and socioeconomic status. For example, Bussing et al. [35] reported that Caucasian parents were more likely than African-American parents to apply medical labels, expect a lifelong course of treatment, and include school interventions in the overall treatment plan. Cultural differences may also affect the scoring style of parents; i.e., on Likert scales, raters from collectivistic cultures tend to moderate their scores (i.e., score middle values), while those from individualistic cultures are more likely to use extreme values [36].

Interestingly, despite these cultural differences, the prevalence of ADHD did not differ significantly between cultures. The prevalence of the condition in Korean populations is reportedly between 5.90% to 6.50%, similar to the worldwide prevalence [17,37,38]. In a meta-analysis of the worldwide prevalence of ADHD, Polanczyk et al. [2,39] also reported that geographic location plays a limited role in its large variability. Currently, there is no straightforward interpretation for these differences and similarities in ADHD symptom scores in the general population. Future cross-cultural studies will be required to identify the pathophysiology and ethnic differences underlying ADHD.

This study provides data that advance our understanding of the similarities and differences in expression of ADHD across cultures and confirm the results of previous Korean studies with smaller sample sizes. Furthermore, our study will help clinicians to evaluate their patients using the K-ARS and determine therapeutic plan by referencing the normative data.