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This study examines the prevalence, stability, and development of physical aggression, as reported by mothers and fathers, in a sample of children initially recruited at 12, 24, and 36 months (N=2,253) and in a subsample followed up 1 year later (n=271) in a cross-sequential design. Physical aggression occurred in 12-month-olds, but significantly more often in 24- and 36-month-olds. The rates of physically aggressive behaviors increased in the 2nd year of life, and declined from the 3rd birthday onward. Stabilities were moderate for 12-month-olds and high for 24- and 36-month-olds. At the ages of 24 and 36 months, boys were more aggressive than girls. The results confirm and extend R.E. Tremblay's (2004) hypothesis about the early development of physical aggression.

An inverse relation between cortisol (re)activity and externalizing behavior has been hypothesized, but research findings seem equivocal. We tested this hypo(re)activity hypothesis in two meta-analyses, one for basal cortisol (k ¼ 72 studies, N ¼ 5,480) and one for cortisol reactivity to a stressor (k ¼ 29 studies, N ¼ 2,601). No association was found between cortisol reactivity and externalizing behaviors (r ¼ À.04, 95% CI ¼ À.11, .02). However, the relation between basal cortisol and externalizing behavior was significant but small (r ¼ À.05, 95% CI ¼ À.10, À.002). The age of the children significantly moderated this relation: Externalizing behavior was associated with higher basal cortisol (hyperactivity) in preschoolers (r ¼.09, 95% CI ¼ .002, .17), and with lower basal cortisol (hypoactivity) in elementary school-aged children (r ¼ À.14, 95% CI ¼ À.19, À.08). There was no significant relation between cortisol and externalizing behavior in adolescents. ß

In a randomized controlled trial we tested the role of genetic differences in explaining variability in intervention effects on child externalizing behavior. One hundred fifty-seven families with 1- to 3-year-old children screened for their relatively high levels of externalizing behavior participated in a study implementing Video-feedback Intervention to promote Positive Parenting and Sensitive Discipline (VIPP-SD), with six 1.5-hr intervention sessions focusing on maternal sensitivity and discipline. A moderating role of the dopamine D4 receptor (DRD4) variable-number tandem repeat (VNTR) exon III polymorphism was found: VIPP-SD proved to be effective in decreasing externalizing behavior in children with the DRD4 7-repeat allele, a polymorphism that is associated with motivational and reward mechanisms and Attention Deficit Hyperactivity Disorder (ADHD) in children. VIPP-SD effects were largest in children with the DRD4 7-repeat allele whose parents showed the largest increase in the use of positive discipline. The findings of this first experimental test of (measured) gene by (observed) environment interaction in human development indicate that children may be differentially susceptible to intervention effects depending on genetic differences.

The home-based intervention program Video-feedback Intervention to promote Positive Parenting and Sensitive Discipline (VIPP-SD) was tested in a randomized controlled trial with 237 families screened for their 1-to 3-year-old children's relatively high scores on externalizing behavior. VIPP-SD, based on attachment theory and coercion theory, focuses on mirroring and discussing actual parent-child interactions in six 1.5-hr sessions with individual families at home. VIPP-SD proved to be effective in enhancing maternal attitudes toward sensitivity and sensitive discipline and in promoting sensitive discipline interactions in the intervention group as compared with the control group. Moreover, in families with more marital discord and in families with more daily hassles, the intervention resulted in a decrease of overactive problem behaviors in the children. The authors conclude that VIPP-SD should become an important module in attachment-based interventions.

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The present study investigated longitudinal pathways from specific early preschool behavioral problems (ages 2-3 years) to internalizing and externalizing problems in preadolescence (ages 10-11 years), and the role of social problems at school entry (ages 4-5 years) in such pathways. Path analyses were performed using both parent and teacher reports in a sample of 251 to 346 children from the general population, depending on the availability of parent and teacher data at each time of assessment. Structural equation modeling revealed homotypic internalizing and externalizing pathways, predictions from early preschool externalizing problems to later internalizing problems, and negative predictive paths from early internalizing problems to externalizing problems in preadolescence. Cross-informant predictions spanning 8 years were found between parent-reported aggression and overactivity at ages 2-3 years and teacher-reported externalizing problems at ages 10-11 years. Further, results showed that boys' pathways were more complex and showed greater predictive validity than pathways for girls, and that social problems at school entry played a significant role in pathways to internalizing problems, but only for boys. The results are discussed from a developmental psychopathology perspective.

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