This thesis examined the correlates of delinquency and substance use by focusing on the psychosocial environmental influences that lead to sibling similarities and differences in these areas. It was shaped by data from studies strongly suggesting that environmental influences played the most important role in development and adjustment, but that these influences were not shared by siblings in the same family. The evidence for the importance of nonshared environment has led to the conclusion that family variables, such as parenting style and family events, which are shared between siblings, are less important. Adopting this conclusion is highly relevant to the study of the environmental influences that place an adolescent at-risk for problem behaviours, because family variables are important in most sociological treatments of criminal and delinquent behaviour. Therefore, a key issue of this thesis was to investigate the extent of sibling similarity for delinquency and substance use, as well as the significance of a range of variables of theoretical interest in the study of adolescent problem behaviour. A second issue of interest was to determine whether there were associations between differences in sibling experiences and differences in their delinquency and substance use. These issues were examined in 361 primarily opposite-sex adolescent sibling pairs from Wave 1 of the “Sibling Study”, a longitudinal study designed to identify those factors that contribute to adolescent engagement in illegal behaviours and those that inhibit such engagement. The design of this study provided several advantages over previous investigations of within-family differences. First, the sample used in the study consisted of siblings at the extremes of the dimensions of problem behaviour rather than based solely on a sample of students, making it possible to investigate the relative influence of shared and nonshared environment at the extremes. Second, a broad set of factors across the domains of school, family, peer, and individual were assessed rather than solely focussing on parental treatment, which is where much research is concentrated. Third, the relationship between shared family context effects and differential experience was considered. Finally, the associations between the correlates of differences for delinquency and substance use were investigated, whereas the relatively small body of previous research on sibling differences has largely ignored substance use. The results indicated substantial within-family differences in experiences of the environment and problem behaviour. Although all siblings differed, sibling pairs where one sibling was identified as an offender or at-risk for offending were significantly more different from one another than siblings identified as normative. Sibling constellation variables were less important in accounting for differential experiences than were the shared family context characteristics. Almost 60% of the variance in sibling delinquency difference scores and 38% of the variance in sibling substance use difference scores could be accounted for by differences in sibling experiences. In general, extra-familial experiences, such as peer influence, school involvement, and stressful events, and individual characteristics, such as self-control, definitions, and attitudes to authority were more important sources of systematic nonshared environment than parental treatment and sibling interaction items. In addition, different patterns were found for low and high risk families, and for delinquency and substance use. These results support the importance of nonshared environmental factors although future research is needed to confirm that these sibling differential experiences are the most important for explaining problem behaviour. The empirical, theoretical, and practical implications of this thesis for the development of problem behaviour were discussed.

Children's relationships with their parents are considered to be a major influence in their development. Parents' use of discipline with their children, in particular, constitutes a major component of the parent-child relationship and is linked to children's later social skills and behavioral and emotional functioning. Prior studies have shown that parents treat their children differently. The purpose of this dissertation was to examine whether parents use differing levels of discipline with their children and to delineate parent, child, and contextual factors associated with differential parental discipline. This study utilized a behavior genetics twin study design to control for extraneous influences, such age and life changes, which may significantly impact a parents' use of discipline. Participants in this study consisted of 38 families (36 twin pairs, 2 sets of triplets) who had previously participated in the Southern Illinois Twins and Siblings Study (SITSS; DiLalla, 2002) and were now between the ages of 5 ½ to 10 ½. Children and their parents were each asked to report on various parenting practices and discipline techniques used within their home. Parents completed a variety of questionnaires assessing socioeconomic status, interparental conflict, parent personality, child temperament, and child emotional and behavioral functioning and mothers tracked their discipline for one week through a diary. Results from this study showed that children living in the same family experience different levels and types of discipline. Child temperament and emotional/behavioral functioning were related to differential discipline practices. The twin in the family rated as exhibiting greater levels of extraversion, effortful control, and prosocial skills received more sensitive parental discipline. In contrast, the twin showing more conduct problems, peer problems, emotional symptoms, and inattention/ hyperactivity experienced harsher parental discipline. Importantly, parents and their children did not agree on the amount of differential discipline used in their homes. Parents reported using similar levels of discipline with both of their children and children reported large differences in the type and amount of discipline shown to them and their co-twins. Findings from this study could add to the effectiveness of many parent training programs by helping parents to understand the interactions between various parent, child, and contextual characteristics in the initiation of parental disciplinary strategies. Through this understanding, parents may begin to match their disciplinary styles to best meet their children's phenotypic characteristics and needs and environmental demands. Lastly, the search for various genetic and environmental factors associated with harsh discipline practices will greatly enhance the success of prevention programs. By being able to determine familial and contextual characteristics associated with harsh or aversive punishment practices, we may be able to predict those parents most at risk for harsh discipline and to intervene before abusive discipline practices are used.