Maartje Boer

CHAPTER 5 144 increased impulsivity at T3 ( β = 0.51, p < 0.001). The strength of this relation was equal to the relation between SMU problems at T2 and attention deficit at T3 ( χ ²(1) = 0.41, p = 0.522). However, increased SMU problems at T1 did not increase impulsivity at T2. We also did not find that increased SMU problems increased hyperactivity over time. Considering these results, Hypothesis 4 is partially confirmed. Additional Findings Although adolescents who reported high SMU intensity also reported more SMU problems at the between-person level (Table 5.4), the results in Table 5.5 show that on the within-person level, adolescents whose SMU intensity increased did not report increased SMU problems one year later. Neither did adolescents whose SMU problems increased report increased SMU intensity one year later. In addition, adolescents whose SMU problems increased also reported increased SMU problems one year later, across all waves with relatively large effect sizes (from T1 to T2 β = 0.79, p < 0.001; from T2 to T3 β = 0.99, p < 0.001). This suggests that increased SMU problems were persistent over time. Such a pattern was not observed regarding SMU intensity. Also, adolescents whose attention deficits increased at T1 reported increased attention deficits at T2 ( β = 0.42, p = 0.001). Increased hyperactivity at T1 was associated with increased hyperactivity at T2 ( β = 0.53, p < 0.001). Increased impulsivity at T2 was associated with increased impulsivity at T3 ( β = 0.38, p = 0.008). In addition, all measures at the within-person level were positively correlated within T1 (Table 5.6). This means that during this wave, increases in SMU intensity, SMU problems, attention deficits, impulsivity, and hyperactivity occurred simultaneously. These associations were also found in T2, with the exception that increases in SMU intensity were not correlated with increases in attention deficits or impulsivity. During T3, increases in SMU intensity were not associated with increases in ADHD-symptoms in the same wave, but SMU problems increased simultaneously with impulsivity. Discussion This study investigated the direction of the relation between ADHD- symptoms and both SMU intensity and SMU problems among adolescents,