We have examined personality traits to provide further information about the validity of diagnosing ADHD in adults when the onset is late or the numbers of symptoms reported from childhood are subthreshold. As hypothesized, we found that full ADHD and late-onset ADHD showed similar personality profiles with significant deviations on all TCI scales except reward dependence and self-transcendence. By contrast, subthreshold cases only showed deviations on novelty seeking and self-directiveness. Our findings could not be accounted for by differences in psychiatric co-morbidity among subgroups. These data further the notion of the similarities between late-onset ADHD and full ADHD, further calling into question the stringent age at onset criteria for adults when making retrospective diagnoses of ADHD.
Our results regarding age at onset are consistent not only with our prior work described in the introduction (Faraone et al. 2006b
) but also with work from other groups. One study comparing teenagers with onset before or after age 13 found no link between age at onset and severity of symptoms, types of adjustment difficulties or the persistence of ADHD (Schaughency et al. 1994
). Rohde et al. (2000)
compared clinical features between adolescents meeting full criteria for ADHD and those meeting all criteria except age at onset. Because these two groups had similar profiles of clinical features, the authors concluded that the DSM-IV age at onset criterion should be revised. In an epidemiologically ascertained sample of adolescents, Willoughby et al. (2000)
found that adolescents meeting full criteria for combined type ADHD had worse clinical outcomes than those failing to meet the age at onset criterion but found no differences attributable to age at onset for the inattentive subtype of ADHD. In DSM-IV field trials, requiring an age at onset of 7 reduced the accuracy of identifying currently impaired cases of ADHD and reduced agreement with clinician judgments (Applegate et al. 1997
We were particularly interested in the subthreshold group because our prior work suggested that this group might be heterogeneous, with some cases having an atypical manifestation of ADHD and others not. If, when compared with full ADHD, this group had a qualitatively different set of personality traits, we should have seen a marked difference in their personality profiles. We did not. However, as shows, the deviations of the subthreshold group from the non-ADHD group, albeit smaller, are in the same direction as the deviations we observed for late-onset and full ADHD. Our findings for the subthreshold group are consistent with prior papers of ours in which, compared with full ADHD, we found lower rates of familial ADHD (Faraone et al. 2006c
), psychiatric co-morbidity (Faraone et al. 2006c
), substance use disorders (Faraone et al. 2007b
) and neuropsychological dysfunction (Faraone et al. 2006b
) for subthreshold subjects. However, in each of these domains, the subthreshold subjects were more impaired than non-ADHD controls. Such data support the idea that ADHD, like blood pressure, is a dimensional trait.
Several other lines of evidence suggest that a dimensional perspective on ADHD is valid. Many studies have found an excellent correspondence between quantitative measures of ADHD (e.g. scales derived from the Child Behavior Checklist, the Conners Scales and the ADHD Rating Scale IV) and the categorical diagnosis (Edelbrock, 1986
; Bird et al. 1987
; Biederman et al. 1993
; Chen et al. 1994
; Boyle et al. 1997
; Hudziak, 1997
). These studies suggest that children with ADHD are at one extreme of a quantitative dimension and that, on this quantitative dimension, there is no obvious bimodality that separates children with ADHD from other children.
Quantitative measures of ADHD are highly heritable, about 70–90%, which is similar to the heritability of the ADHD diagnosis (Edelbrock et al. 1995
; Thapar et al. 1995
; Silberg et al. 1996
; Judy et al. 1997
; Sherman et al. 1997
). In fact, twin studies of ADHD have generally applied dimensional rating scales, with clinical cut-offs applied when diagnostic categories were required. These studies all show high heritabilities regardless of where these cut-offs have been made and regardless of whether diagnostic or continuous criteria have been applied (Goodman & Stevenson, 1989a
; Stevenson, 1992
). Twin studies have used mathematical modeling techniques to directly test the hypothesis that the clinical diagnosis of ADHD is the extreme of a quantitative trait. Gjone et al. (1996)
applied a mathematical model to determine if the heritability of attention problems increased with their severity. This model is useful because cases at the severe end of the dimension might be expected to have a categorical disorder such as ADHD. If ADHD accounted for the heritability of attention problems we would see increasing heritability with increasing severity. However, heritability did not change with severity, so the authors concluded that there was, in the population, a continuously distributed dimension of genetic liability to attention problems. A similar approach was applied by Levy et al. (1997)
, who concluded that ADHD was best viewed as the extreme of a behavior that varies genetically throughout the entire population rather than as a categorical disorder. Similar findings were reported by Willcutt et al. (2000)
. Thus, available data support the idea that ADHD can be viewed as the extreme expression of a trait that varies quantitatively in the population. This, in turn, suggests that personality deviations seen in ADHD can also be viewed from a dimensional perspective.
As and show, ADHD was associated with deviations for each dimension of temperament. Our finding of elevated novelty seeking in ADHD confirms three prior studies of these personality traits in ADHD (Anckarsater et al. 2006
; Jacob et al. 2007
) and in a sample of smokers with ADHD (Downey et al. 1996
). As discussed by Cloninger (1987)
, individuals high in novelty seeking are quick-tempered, curious, easily bored, impulsive, extravagant, and disorderly. We also found that ADHD was associated with increases in harm avoidance and decreases in reward dependence and persistence. Based on the work of Cloninger and co-workers (Cloninger, 1987
; Cloninger et al. 1991
; Svrakic et al. 1993
), we can interpret these traits as follows. People high in harm avoidance are fearful, socially inhibited, shy, passive, easily tired, and pessimistic even in situations that do not worry other people. Although the higher harm avoidance of ADHD subjects is consistent with ADHD's co-morbidity with anxiety disorders and depression (Biederman et al. 2006
; Kessler et al. 2006
), the group differences we observed were not accounted for by these disorders, which suggests that this personality subscale may tap subclinical traits. The low persistence scores of adults with ADHD suggest that they tend to give up easily when frustrated, that they are less likely to strive for higher goals, and less likely to persevere at tasks. Low persistence is consistent with the executive functioning deficits seen in many patients with ADHD (Biederman et al. 2007a
). The low reward dependence seen in patients with ADHD suggests that they are practical, tough-minded, socially insensitive, and indifferent.
and also show that ADHD was associated with TCI character dimensions. Adults with ADHD had increases in self-transcendence and decreases in self-directedness and cooperativeness. The higher self-transcendence seen for adults with ADHD suggests that they tend to be spiritual, unpretentious, humble, and fulfilled. Low self-directedness suggests that adults with ADHD are less responsible, reliable, resourceful, goal-oriented and self-confident and that they find it difficult to define, set and pursue meaningful internal goals. The results from the cooperativeness scale suggest that adults with ADHD show tendencies to being self-absorbed, intolerant, critical, unhelpful, and opportunistic. Such persons tend to be inconsiderate of other people's rights or feelings. These traits are consistent with ADHD's co-morbidities with ODD and conduct disorder.
Our findings should be viewed in light of some limitations. The diagnoses of ADHD relied entirely on the self-report of adult subjects. Although this method allowed us to evaluate the validity of retrospective self-reports, these findings may not generalize to diagnoses defined using data from informants. As Barkley et al. (2002)
showed in a study of youth followed into adulthood, informant reports can boost the validity of diagnosing ADHD in adulthood. In addition, because we had relatively few subthreshold subjects, our power to detect differences with this group was limited. Because we did not assess Axis II diagnoses, we cannot draw conclusions about personality disorders in our sample or the relationship between dimensional measures of personality and personality disorders. Because we have relied on retrospective diagnoses, our data cannot discriminate true age at onset effects from recall effects. For example, it is possible that cases of late-onset ADHD are simply patients who could not accurately recall the times when symptoms of ADHD first occurred.
Despite these limitations, these data suggest that, as regards the traits assessed by the TCI, adults with late-onset ADHD are very similar to adults with full ADHD, which provides further support for the idea that the DSM-IV age at onset criterion may be too stringent (Applegate et al. 1997
; Barkley & Biederman, 1997
; Faraone, 2000
; Faraone et al. 2000
). In the context of previous reports about psychiatric co-morbidity, family history and neuropsychological functioning and substance use disorders (Faraone et al. 2006b
), our data about personality traits suggest that late-onset ADHD would meet Robins & Guze (1970)
criteria for validity. Future research should consider what age of onset would provide the optimal balance for minimizing Type I and Type II diagnostic errors in ADHD. Although our current project did not address what age would provide the most parsimonious criterion, our data do suggest that age 7 may be too stringent. Subthreshold ADHD seems to be a milder form of the disorder. More work is needed to clarify their diagnostic status. Adults with late-onset or full ADHD should be considered for the diagnosis of ADHD with implications for treatment. Although our results have important implications for diagnostic criteria, because nearly all the TCI dimensions were deviant in ADHD cases, they appear to provide an overall index of poor functioning rather than yielding insights into the nature of personality in ADHD patients.