It is estimated that approximately 30% of individuals worldwide are affected by mental health problems during their lifetime. Currently, Major Depressive Disorder (MDD) is one of the most prevalent psychiatric disorders and a leading cause of non-lethal disability worldwide. However, despite exposure to known risk factors for MDD, human responses to it vary widely. Whilst some individuals develop MDD, others develop only mild and transient symptoms or no depressive symptomology at all. This ability to 'bounce back' from or 'escape' the development of psychiatric illness is referred to as psychological resilience (Chapter 1). Scientific and clinical interest in resilience has grown exponentially over recent decades, but wide discrepancies are still found in both its definition and measurement. As such, resilience is rarely measured directly, but inferred from the measurement of two specific points of convergence; adversity (its antecedents) and positive adaptation (its consequences). Whilst the study of adversity and positive adaptation has informed our knowledge of resilience it often fails to consider other putative risk factors for MDD (such as genetics), or potential protective factors that may foster resilience despite risk. More recently, examining protective factors have become a focus of research in relation to resilience. This research suggests that numerous protective factors coalesce to contribute to resilient outcomes which give rise to a dynamic resilience process that varies contextually and temporally. Although investigating resilience may be expected to reveal similar findings to studying MDD itself, it does represent a new facet to scientific and clinical research. Specifically, resilience focuses on intervention long before the development of MDD when effects on subsequent suffering may be ameliorated. For this reason, it is imperative to address the concept of resilience, concentrating on the core components of adversity, positive adaptation and protective factors, to move beyond description towards an understanding of individual differences in resilience (Chapter 2). In this thesis, three studies will be presented which aim to examine psychological resilience from multiple perspectives to further delineate the concept. In Chapter 3, the associations and interactions between neuroticism and general intelligence (g) on MDD, and psychological distress were examined in GS:SFHS (Generation Scotland: Scottish Family Health Study) to investigate whether g mitigates the detrimental effects of neuroticism on mental health, as such an association has previously been identified for physical health and mortality. A larger replication was also performed in UK Biobank using a self-reported measure of depression. Across two large samples it was found that intelligence provides protection against psychological distress and self-reported depression in individuals high in neuroticism, but intelligence confers no such protection against clinical MDD in those high in neuroticism. In Chapter 4, a new dataset is presented which was designed to investigate psychological resilience and mental health. Specifically, the STRADL (Stratifying Resilience and Depression Longitudinally) dataset aimed to re-contact existing GS:SFHS participants to obtain repeat measures of MDD and psychological distress in addition to obtaining data on resilience, coping style and adverse life experiences. This dataset has the potential to identify mechanisms and pathways to resilience but also elucidate causal mechanisms and pathways of depression sub-types. Chapter 5 investigated whether neuroticism and resilience are downstream mediators of genetic risk for depression, and whether they contribute independently to such risk. Specifically, the moderating and mediating relationships between polygenic risk scores (PRS) for depression, neuroticism, resilience, and both clinical and self-reported MDD were examined in STRADL. Regression analyses indicated that neuroticism and PRS for depression independently associated with increased risk for both clinical and self-reported MDD, whereas resilience associated with reduced risk. Structural equation modelling suggested that polygenic risk for depression associates with vulnerability for both clinical and self-reported MDD through two partially independent mediating mechanisms in which neuroticism increases vulnerability and resilience reduces it. In Chapter 6, the proportion of phenotypic variance that is attributable to genetic and shared-familial environment was estimated for resilience and three main coping styles; task-, emotion-, and avoidance-oriented coping. Bivariate analyses were conducted to estimate the genetic correlations between these traits and neuroticism. Our results indicate that common genetics affect both resilience and coping style. However, in addition, early shared-environmental effects from the nuclear family influence resilience whereas recent shared-environment effects from a spouse influence coping style. Furthermore, strong genetic overlap between resilience, emotion-oriented coping, and neuroticism suggests a relationship whereby genetic factors that increase negative emotionality lead to decreased resilience. These studies highlight the necessity for complementary multivariate techniques in resilience research to elucidate tractable methodologies to potentially identify mechanisms and modifiable risk factors to protect against psychiatric illness (Chapter 7).

Cette thèse avait pour objectif d’examiner les liens longitudinaux entre les relations d’amitié et l’évolution des comportements d’agressivité physique en début de scolarisation. Guidé par les principes énoncés par les théoriciens de l’apprentissage social, de l’attachement, du développement de la personnalité et de la théorie du jugement moral, le rôle principal et modérateur de certaines dimensions spécifiques à la qualité de la relation d’amitié, ainsi que des attributs comportementaux des amis et des caractéristiques personnelles de l’enfant a été évalué. Des données provenant de l’Étude Longitudinale du Développement des Enfants du Québec (ELDEQ), de l’Étude des Jumeaux nouveau-nés du Québec (EJNQ) et de l’évaluation des effets d’un programme d’intervention dyadique ont été analysées. Les mesures utilisées dans cette thèse ont été collectées entre la maternelle et la 2e année du primaire, soit de 5 à 8 ans, directement auprès des enfants, de leurs amis, leurs pairs, leurs parents et leurs enseignants par le biais de questionnaires, d’entrevues sociométriques et de mises en situation hypothétiques. En lien avec la perspective de l’apprentissage social, les résultats ont montré que l’association à des amis agressifs en maternelle est liée à une augmentation des comportements d’agressivité physique chez l’enfant. Cependant, en lien avec les théories du développement de la personnalité et la perspective de l’attachement, le fait d’établir une relation d’amitié de bonne qualité est reliée à une diminution des comportements agressifs à travers le temps. De plus, une interaction entre la qualité de la relation et les attributs comportementaux des amis a indiqué que le risque lié à l’association à des amis agressifs est atténué dans le contexte d’une relation d’amitié de bonne qualité. Les résultats indiquent également que chez les garçons, la présence de conflits entre amis à la maternelle est associée de façon linéaire à de plus hauts niveaux de comportements agressifs, indépendamment du risque génétique de l’enfant face à cette problématique. Une interaction triple a par ailleurs révélé que le conflit n’était pas lié à une augmentation de l’agressivité physique dans le contexte d’une relation d’amitié caractérisée par l’affect positif et une bonne capacité à régler les conflits. Enfin, les résultats ont montré un effet indirect d’une intervention dyadique sur la diminution des comportements d’agressivité physique, qui opère à travers l’amélioration de la capacité des amis à régler leurs conflits. Ces résultats appuient le rôle bénéfique de la qualité de la relation d’amitié sur l’évolution des manifestations de comportements d’agressivité physique et suggèrent que cet aspect relationnel soit pris en compte dans les programmes de prévention des conduites agressives. En somme, la mise en évidence d’associations et d’interactions significatives entre la qualité des relations d’amitié, les attributs comportementaux des amis et les manifestations de comportements d’agressivité physique en début de scolarisation suggère que certains aspects et dimensions relationnelles positives peuvent être bénéfiques au développement des enfants agressifs. La prévention du maintien et de l’aggravation des conduites agressives par l’entremise de l’amélioration de la qualité des relations d’amitié représente une avenue prometteuse. / The aim of the present dissertation was to investigate the prospective links between friendship relationships and physical aggression development during the early school years. Within a theoretical framework based on social learning, personality, attachment and socioconstructivist theories, we examined the unique and combined role of various dimensions of friendship quality, friends’ behavioral characteristics and child’s pre-existing vulnerabilities in regard to the persistence of young children’s physical aggression. Data from the Quebec Longitudinal Study of Child Development (QLSCD), from the Quebec Newborn Twin Study (QNTS) and from an efficacity test through a dyadic intervention program were used. Measures in this dissertation were collected from kindergarten to grade 2, when the children were aged from 5 to 8 years old, with the children themselves, their friends, their classmates, their parents, as well as their teachers, through various questionnaires, a peer evaluation procedure and hypothetical scenarios. In line with the social learning perspective, friends’ aggression was linked to a significant increase in children’s physical aggression. However, in line with the social bonding perspective, a good friendship quality played both a compensatory and a protective role, by respectively reducing children’s initial level of physical aggression and by mitigating the associations between friends’ and children’s physical aggression. Results also show that friendship conflict was associated to a linear increase with boys’ but not with girls’ physical aggression over time. Shared positive affect and conflict resolution skills were found to mitigate the prospective association between friendship conflict and children’s physical aggression. These results were independent of children’s sex and genetic risk for physical aggression. No interaction effects were found between the friendship dimensions and genetic risk for aggression, suggesting that children are equally affected independently from their genetic liability. Results showed an indirect effect of the dyadic intervention on decreasing levels of physical aggression through the improvement of one specific feature of friendship quality: conflict resolution. These results support the causal role of friendship quality on the developmental course of physical aggression and point to the inclusion of this relational aspect in prevention programs targeting young aggressive children. Overall, the main and moderating effect found between friendship quality, friend’s aggression and child personal characteristics bring both theoretical and practical implications. Indeed, these results suggest that fostering a positive relationship between friends in the early school years may decrease physical aggression even if the friends are aggressive. Besides, high friendship quality may also buffer against the risk associated with experiencing conflict. These findings underscored the importance of taking into account the relational characteristics, such as conflict resolution and positive affect, in order to better understand the impact of friendship relationships on children’s physical aggression development.

Cette thèse avait pour objectif d’examiner les liens longitudinaux entre les relations d’amitié et l’évolution des comportements d’agressivité physique en début de scolarisation. Guidé par les principes énoncés par les théoriciens de l’apprentissage social, de l’attachement, du développement de la personnalité et de la théorie du jugement moral, le rôle principal et modérateur de certaines dimensions spécifiques à la qualité de la relation d’amitié, ainsi que des attributs comportementaux des amis et des caractéristiques personnelles de l’enfant a été évalué. Des données provenant de l’Étude Longitudinale du Développement des Enfants du Québec (ELDEQ), de l’Étude des Jumeaux nouveau-nés du Québec (EJNQ) et de l’évaluation des effets d’un programme d’intervention dyadique ont été analysées. Les mesures utilisées dans cette thèse ont été collectées entre la maternelle et la 2e année du primaire, soit de 5 à 8 ans, directement auprès des enfants, de leurs amis, leurs pairs, leurs parents et leurs enseignants par le biais de questionnaires, d’entrevues sociométriques et de mises en situation hypothétiques. En lien avec la perspective de l’apprentissage social, les résultats ont montré que l’association à des amis agressifs en maternelle est liée à une augmentation des comportements d’agressivité physique chez l’enfant. Cependant, en lien avec les théories du développement de la personnalité et la perspective de l’attachement, le fait d’établir une relation d’amitié de bonne qualité est reliée à une diminution des comportements agressifs à travers le temps. De plus, une interaction entre la qualité de la relation et les attributs comportementaux des amis a indiqué que le risque lié à l’association à des amis agressifs est atténué dans le contexte d’une relation d’amitié de bonne qualité. Les résultats indiquent également que chez les garçons, la présence de conflits entre amis à la maternelle est associée de façon linéaire à de plus hauts niveaux de comportements agressifs, indépendamment du risque génétique de l’enfant face à cette problématique. Une interaction triple a par ailleurs révélé que le conflit n’était pas lié à une augmentation de l’agressivité physique dans le contexte d’une relation d’amitié caractérisée par l’affect positif et une bonne capacité à régler les conflits. Enfin, les résultats ont montré un effet indirect d’une intervention dyadique sur la diminution des comportements d’agressivité physique, qui opère à travers l’amélioration de la capacité des amis à régler leurs conflits. Ces résultats appuient le rôle bénéfique de la qualité de la relation d’amitié sur l’évolution des manifestations de comportements d’agressivité physique et suggèrent que cet aspect relationnel soit pris en compte dans les programmes de prévention des conduites agressives. En somme, la mise en évidence d’associations et d’interactions significatives entre la qualité des relations d’amitié, les attributs comportementaux des amis et les manifestations de comportements d’agressivité physique en début de scolarisation suggère que certains aspects et dimensions relationnelles positives peuvent être bénéfiques au développement des enfants agressifs. La prévention du maintien et de l’aggravation des conduites agressives par l’entremise de l’amélioration de la qualité des relations d’amitié représente une avenue prometteuse. / The aim of the present dissertation was to investigate the prospective links between friendship relationships and physical aggression development during the early school years. Within a theoretical framework based on social learning, personality, attachment and socioconstructivist theories, we examined the unique and combined role of various dimensions of friendship quality, friends’ behavioral characteristics and child’s pre-existing vulnerabilities in regard to the persistence of young children’s physical aggression. Data from the Quebec Longitudinal Study of Child Development (QLSCD), from the Quebec Newborn Twin Study (QNTS) and from an efficacity test through a dyadic intervention program were used. Measures in this dissertation were collected from kindergarten to grade 2, when the children were aged from 5 to 8 years old, with the children themselves, their friends, their classmates, their parents, as well as their teachers, through various questionnaires, a peer evaluation procedure and hypothetical scenarios. In line with the social learning perspective, friends’ aggression was linked to a significant increase in children’s physical aggression. However, in line with the social bonding perspective, a good friendship quality played both a compensatory and a protective role, by respectively reducing children’s initial level of physical aggression and by mitigating the associations between friends’ and children’s physical aggression. Results also show that friendship conflict was associated to a linear increase with boys’ but not with girls’ physical aggression over time. Shared positive affect and conflict resolution skills were found to mitigate the prospective association between friendship conflict and children’s physical aggression. These results were independent of children’s sex and genetic risk for physical aggression. No interaction effects were found between the friendship dimensions and genetic risk for aggression, suggesting that children are equally affected independently from their genetic liability. Results showed an indirect effect of the dyadic intervention on decreasing levels of physical aggression through the improvement of one specific feature of friendship quality: conflict resolution. These results support the causal role of friendship quality on the developmental course of physical aggression and point to the inclusion of this relational aspect in prevention programs targeting young aggressive children. Overall, the main and moderating effect found between friendship quality, friend’s aggression and child personal characteristics bring both theoretical and practical implications. Indeed, these results suggest that fostering a positive relationship between friends in the early school years may decrease physical aggression even if the friends are aggressive. Besides, high friendship quality may also buffer against the risk associated with experiencing conflict. These findings underscored the importance of taking into account the relational characteristics, such as conflict resolution and positive affect, in order to better understand the impact of friendship relationships on children’s physical aggression development.