As proposed by retrospective studies of adults 
and studies of children with OCD exhibiting hoarding symptoms 
our study suggests that hoarding symptoms are relatively prevalent (approximately 2%) among adolescents although somewhat less prevalent than in similar adult studies 
. Mean scores on HRS-SR among young hoarders (mean
12–40) in the current study were also similar to scores reported in adults (mean
. However, a notable difference between hoarding symptoms in adolescence and adulthood is that they seem to affect females to a larger extent in the younger age group, a finding that also has been reported earlier 
. An additional difference emerged when examining the frequency of excessive acquisition amongst individuals classed as having clinically significant hoarding difficulties. Approximately 30–40% of these individuals also reported at least moderate problems with acquisition, indicating that excessive acquisition may be less prominent in adolescents than it is in adults (about 85%; 
). This lends further support to the inclusion of excessive acquisition as a diagnostic specifier rather than a core criterion in DSM−5. It is possible that excessive acquisition difficulties develop later in life when individuals with hoarding tendencies become financially independent and also have more control over their acquisitive behavior and space.
The obtained prevalence estimate of hoarding symptoms also indicates that these are at least as common as OCD during adolescence 
, a finding that contradicts the notion that hoarding is merely is a subtype of OCD.
Given the lack of published data on the hoarding phenotype in adolescence and the potential of familial effects on clutter we also estimated a prevalence rate of 3.7% when excluding the requirement to have significant clutter in the participant’s room. It is still unknown whether the phenomenology of hoarding changes across the lifespan and whether the proposed diagnostic criteria for HD can be applied to adolescents without adaptation. Further exploration of the phenomenology of hoarding during adolescence involving face-to-face interviews and home visits will be needed in order to determine which features of hoarding are most prominent and problematic during this age period and whether the diagnostic criteria require adapting for their use in adolescents. Nevertheless, our estimates provide an important first step towards estimating the possible prevalence of hoarding difficulties in this age group.
Our findings also showed that OCD, to which HD has been linked traditionally, co-occurred in only 2.9% of adolescents who fulfilled hoarding criteria and at a similar rate among those who did not. This finding from a non-clinical sample suggests that the link between the two disorders might be especially tenuous during adolescence. Previous findings of high comorbidity between HD and ADHD in clinically ascertained samples of adults 
and youth 
were not confirmed. We also found comparable rates of ASD in the hoarding and non-hoarding groups. Similarly, research in adults suggests that symptoms of autism are not more prevalent in subjects with HD compared to psychiatric controls 
. Taken together, our findings suggest that in the majority of cases at the population level, hoarding symptoms are frequently present in the absence of other neurodevelopmental disorders, lending further support to the notion that HD might be a distinct nosological entity.
Finally, the twin analyses indicated that hoarding symptoms were moderately heritable in adolescence, although the magnitude of the genetic influence was much stronger in boys than in girls. This somewhat unexpected finding – hoarding symptoms are clearly heritable in adult men and women 
– was related to considerable shared environmental influences on hoarding symptoms amongst girls. However, similar findings of sex differences during adolescence have been reported previously regarding genetic and environmental influences for OCD, 
, and pubertal development 
. Furthermore, dynamic developmental genetic effects from childhood to young adulthood, expressed in genetic attenuation and innovation (i.e. the decline in impact of certain genes and the activation of genes which previously had no effect) in symptoms of anxiety and depression are well documented 
. It seems plausible that hoarding may become progressively more heritable over time as the influence of shared familial environment decreases when young people leave their parents’ home and have stronger control over their own living space. Longitudinal studies of the heritability of hoarding symptoms in young adulthood and beyond are needed to elucidate developmental trajectories of hoarding in both sexes.
The results should be interpreted considering several limitations. First, we based our classification of clinically significant hoarding symptoms on a measure not specifically validated in an adolescent population. Thus, the possibility that it did not capture hoarding symptoms equally well as in adults cannot be ruled out. Hopefully, the modification of the clutter item increased its relevance for this age group. Second, prevalence estimates should be seen as indicative rather than definitive because we could not conclusively rule out other medical or psychiatric conditions that are known to lead to hoarding behavior 
. Third, although ADHD, ASD and OCD are relatively stable prevalence-wise in the age span from 9/12 to 15 years, more precise estimates of their co-occurrence with hoarding symptoms would have been obtained had they been assessed at age 15 years and not at age 9/12; hence, we cannot totally exclude an under- or possibly overestimation of ADHD, ASD and OCD comorbidity. Fourth, albeit modeled closely after DSM-IV-TR-criteria 
, comorbid OCD was not determined using a validated measure, and was based solely on parental report. Thus, the OCD comorbidity rate might have been underestimated by parents and should therefore be interpreted cautiously. Fifth, and finally, since ASD and ADHD were significantly more common among non-responders, we cannot rule out that the true comorbidity of hoarding symptoms and neurodevelopmental disorders might be higher.
This study is the first to investigate the occurrence of hoarding symptoms in a large population based sample of adolescents. Hoarding symptoms were prevalent among adolescents and usually appeared without co-occurring ADHD, ASD and OCD. Furthermore, the same etiological factors seemed to influence hoarding symptoms in both sexes although the genetic effect was much stronger in boys. Longitudinal studies are required to elucidate the developmental trajectories of hoarding symptoms and their heritability from adolescence onto adulthood.