In this longitudinal study, we identified three linear trajectories of mother-rated Attention Problems in boys and girls from 6 to 12 years: stable low, low-increasing and high-decreasing symptom levels. Most of the children followed the stable low trajectory, which is what we hypothesized. Further, we expected two other stable trajectories with a possible decrease late in childhood. Instead of these stable trajectories, we found a low-increasing trajectory, and a high-decreasing trajectory.
Our findings are in agreement with the study from Malone et al. [27
] which identified three trajectories that, when considering children from middle childhood until early adolescence, included increasing and decreasing classes. Van Lier et al. [43
] also identified a high-increasing trajectory in children from the Zuid-Holland Study using the DSM-oriented ADHD scale of the CBCL. Two earlier studies that reported a stable high trajectory included children with already elevated risk (i.e., parental alcoholism, and low-socioeconomic position) [17
]. It is possible that a stable high trajectory only presents in high risk populations, or populations characterized by low use of pediatric health care. In the US low-socioeconomic groups have a lower use of pediatric care, while there is no association between SES and help seeking in the Netherlands, where there are no major financial constraints to receiving professional help [51
]. It could also be that a stable high trajectory appears only in children with both attention deficits and hyperactivity problems. In a general population study, it may be more likely that a decreasing trajectory appears, in accordance with theory that attention deficits diminish as self-regulation increases, and in response to adequate treatment. These reasons may explain why a stable high trajectory was not identified in our study.
It might be argued that a fourth class should be included, both for the boys and the girls. However, an additional class did not provide additional information, and appeared to split the high-decreasing trajectory into two ordered classes. This seems to be an example of a so-called ‘indirect interpretation of mixture models’ where classes do not represent different types of subjects, but rather approximate different parts of the joint distribution of observed data. Not including the fourth class does not change the conceptual interpretation of the modeling results.
Mean parent-rated AP scores larger than 9 (up to age 11) or 10 (age 12) are in the subclinical range, and scores larger than 12 (up to age 11) or 13 (age 12) are considered clinical [1
]. None of the trajectories exceeded these levels at any age. A post hoc analysis among the boys showed that 2.3% of the twins and 4.5% of the singletons had mean AP scores of 9 or higher on at least two assessments. About two-third of these children were assigned to the high-decreasing trajectory. Since DSM diagnoses of ADHD were not available, we could not investigate whether children with specific ADHD-subtypes would tend to be either in the low-increasing or high-decreasing class. However, it was found that children with a low AP-score obtained a negative ADHD diagnosis in 96% of the cases [10
]. Furthermore, children with a high AP-score obtained a positive diagnosis in 59 (boys) and 36% (girls) of the cases. Since hyperactivity tends to decrease over time [18
], we hypothesize that children with the hyperactive-impulsive or the combined type of ADHD would be overrepresented in the high-decreasing trajectory. Children on the low-increasing trajectory seem at risk for having high levels of ADHD symptoms later in childhood. As this risk may arise from a combination of several genetic, biological and environmental factors [18
], further research is needed to identify specific predictors of the trajectories.
Linear growth provided the best description of the development of Attention Problems for the observed time. Attention deficits may increase during childhood as academic demands, such as demands on impulse control and response inhibition, increase. Linearity does however not mean that the regression line will go up indefinitely, but that linear models best describe the observed time (6–12 years). With longer follow-up of these children a quadratic model could provide support for declining levels of Attention Problems in adolescence.
The second aim of this study was to investigate if similar trajectories could be identified in singletons. For both boys and girls, singletons followed three trajectories identical to twins, with similar class proportions. The mean intercepts and slopes of the trajectories did not differ between twins and singletons. Therefore, we conclude that twins and singletons are comparable with respect to the development of ADHD symptoms in childhood. The findings from the teacher ratings support this conclusion, as we observed no consistent differences in the mean AP scores between twins and singletons. This conclusion confirms the generalizability of twin studies to singleton populations with regard to ADHD symptoms in middle and late childhood. Our findings are in agreement with a cross-sectional twin-singleton comparison, in which twins were compared to their non-twin siblings, that reported no consistent differences with respect to the prevalence of ADHD [12
This is the first study that investigates trajectories of Attention Problems in middle childhood in the general population. Strengths of the study are the use of prospective data over a 6-year period, the representativeness of the samples, large sample sizes, and the use of advanced person-centered statistical analyses. Nevertheless, several limitations of this study must be considered. First, there was a modest association of non-response with SES, which may have led to underestimating the proportion of children in the high-decreasing and the low-increasing trajectories, especially in the twin sample. Also, the twin and singleton samples differed with respect to SES, with a higher proportion of twins from higher SES backgrounds. The singleton sample consists of families who were randomly selected from municipal registers, after which participation was strongly pursued, e.g., by means of home-visits, making participants (especially those from low SES) more likely to participate. In contrast, the twin sample depends on voluntary participation and families are encouraged to remain on the register, even when they do not take part in each survey for which they are approached. Secondly, the twin and singleton samples were comprised of different cohorts. For twins, birth cohort did not predict mean AP scores at ages 7, 10 and 12 [9
]. For singletons, an earlier study did not find evidence for secular changes in parent-rated AP over a 10-year period (1983–1993), but small secular changes were reported for teacher-rated AP [48
]. Similarly, small increases in Dutch children’s parent- and teacher-rated AP scores were found over a 20-year period (1983–2003) [41
]. As these differences were very small (Cohen’s d
< 0.2), it is unlikely that cohort effects confound our findings. Thirdly, the twin and singleton samples were recruited from different regions of the country (data collection is nation-wide for twins, whereas for singletons a specific part of the country is included). However, an earlier study showed there were no significant differences in CBCL scale scores between children living in Zuid-Holland and children living elsewhere in The Netherlands [41