Intra-individual variability in behavior and functioning is ubiquitous among children with attention-deficit/hyperactivity disorder (ADHD), but it has not been systematically examined or integrated within causal models. This article seeks to provide a conceptual, methodologic, and analytic framework as a foundation for future research. We first identify five key research questions and methodologic issues. For illustration, we examine the periodic structure of Eriksen Flanker task reaction time (RT) data obtained from 24 boys with ADHD and 18 age-matched comparison boys. Reaction time variability in ADHD differed quantitatively from control subjects, particularly at a modal frequency around .05 Hz (cycle length approximately 20 sec). These oscillations in RT were unaffected by double-blind placebo and were suppressed by double-blind methylphenidate. Together with converging lines of basic and clinical evidence, these secondary data analyses support the speculative hypothesis that the increased power of multisecond oscillations in ADHD RT data, and by inference, in attentional performance, represents a catecholaminergic deficit in the ability to appropriately modulate such oscillations in neuronal activity. These results highlight the importance of retaining time-series data and quantitatively examining intra-subject measures of variability as a putative endophenotype for ADHD.

Fluctuations in low-frequency RT variability predict the diagnosis of ADHD beyond the effect associated with global differences in variability. Future studies will examine whether such spectrally specific fluctuations in behavioral responses are linked to intrinsic regional cerebral hemodynamic oscillations that occur at similar frequencies.

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In this study the stability over the first 13 months of life of measures of infant cardiac activity (heart period and heart-period variability), their relations with each other, and their relations with a continuous-variable index of infant-mother attachment were investigated. The indexes of cardiac activity changed in an orderly way with development (increasing heart-rate variability, decreasing heart rate). There were moderate to high intercorrelations among the cardiac measures, particularly those indexing heart-rate variability (i.e., vagal tone, heart-period variance, and heart-period range). Regression analyses showed that the measures of heart-rate variability at 3,6, and 9 months were significant predictors of the continuous-variable index of security. The higher the infants' heart-rate variability, the higher were their attachment insecurity scores. Analyses of whether the conventional secure/insecure classification was related to the early infant cardiac measures indicated that measures of heart-rate variability were significantly higher in the insecure children.Recently, researchers have investigated the relation between attachment classification and measures of temperament (Belsky & Rovine, 1987; Bretherton, CConnell, &Tracey, 1980;Miyake, Chen, & Campos, 1985). This research has demonstrated a relation between concurrent measures of temperament and attachment and suggests that temperament, assessed during early infancy, may be a reliable predictor of attachment classification. For example, Miyake et al. (1985) reported that infant irritability measured at 3 months of age was predictive of patterns of resistant behavior during reunion in the strange situation at 1 year. These findings suggest that temperament may contribute significantly to the prediction of the quality of attachment.Neurophysiological processes may underlie the construct of temperament and mediate aspects of the relation between temperament and attachment. Contemporary theoretical perspectives have conceptualized temperament as a psychobiological construct (e.g., Goldsmith & Campos, 1982;Rothbart & Derryberry, 1981;Thomas & Chess, 1977). For example, Rothbart and Derryberry defined temperament in terms of constitutional differences in physiological reactivity and self-regulation of reactivity. Thus, children's approach and avoidant behaviors, observed in the strange-situation procedure (Ainsworth, Ble-

Study goals were to explore whether children clustered into groups based on reactions to witnessing bullying and to examine whether these reactions predicted bullying intervention. Seventy-nine children (M = 10.80 years) watched bullying videos in the laboratory while their heart rate (HR) was measured, and they self-reported on negative emotion after each video. Bullying intervention was assessed by school peers. Two groups emerged based on reactions to the bullying videos: The Emotional group (43% of children) displayed HR acceleration and reported high negative emotion, whereas the Unemotional group (57% of children) showed HR deceleration and reported low negative emotion. Group membership predicted bullying intervention, with peers reporting that Emotional children were more likely to stop a bully than Unemotional children.