In the last decennia, the reluctance and ambivalence about assessing borderline personality disorder (BPD) in young people has shifted to an increasing focus on adolescence and early adulthood as the developmental phases when BPD commonly has its onset . BPD is defined by high comorbidity and poor outcomes . BPD has been associated, early in the course of the disorder, with high levels of social impairment , such as poorer general psychosocial functioning, poorer peer relationships and problems with family relationships , and impairments in theory of mind and mentalizing . Moreover, research has shown that BPD in young people has a unique predictive value for poor psychosocial functioning, above and beyond mental state disorders and other personality disorder diagnoses [3, 4]. Even at a subthreshold level, the criteria of BPD are associated with poorer social and occupational functioning in adolescence and young adulthood , as well as in adulthood [7, 8].
The diagnosis BPD is not a fixed diagnosis, and the symptoms wax and wane during development. In adolescents, individual changes in psychosocial functioning appear to be related to changes in BPD symptoms, as an increase of BPD symptoms was related to an increase in psychosocial dysfunction, while a decline of BPD symptoms was related to improvement of psychosocial functioning . This is an important finding, given that adolescence is considered a key developmental period concerning the onset of BPD. During adolescence and young adulthood, full threshold and subthreshold BPD may interfere with the process of gradually assuming more adult roles and responsibilities, which are necessary for adequate interpersonal functioning. Problems in interpersonal functioning are considered a central problem in BPD as well as in personality pathology in general , and in contrast to the relatively unstable nature of the diagnosis BPD, both in adolescents and in adults, problems in social functioning are relatively stable and may have long-lasting consequences for the individual’s functioning . Social and interpersonal functioning develop in the context of social relationships. Therefore, social relationships can be seen as the key element of understanding the course of BPD . This suggests that a better concept of relationships in relation to BPD in adolescence and young adulthood is needed in order to grasp the context of development of possible impairments in social functioning better.
Social relationships in young people with BPD
During adolescence and young adulthood the development of social autonomy, establishing intimate relationships, and finding a new balance in the relationship with parents are important developmental tasks . The relationships of adolescents and young adults with both threshold and subthreshold BPD can be challenged. Relatives, partners and friends of adults with BPD report considerable elevated objective and subjective burden and mental health problems, including depression and anxiety . Similarly, families and friends of young people with BPD features report elevated levels of distress, negative caregiving experiences, and family environments high in expressed emotion, such as criticism and emotional overinvolvement, also when compared to adults in the general population or families and friends of young people with other serious illnesses . In addition, reciprocal relations between BPD symptoms and parenting factors in adolescent girls in the community have been found, indicating that parenting may affect subsequent BPD symptoms and vice versa .
Considering the current social relationships with parents, it is found that adolescents who are developing personality disorders are more likely to experience conflicts with family members throughout the transition to adulthood . In turn, persistent conflict with family members may have an adverse impact on psychosocial development throughout this important transitional period. Different explanations have been proposed for the findings that personality disorder traits were associated with both elevated contact and elevated conflict with family members . One of the hypotheses the authors stated was that due to social skills deficits and interpersonal conflict adolescents with personality disorders may find it difficult to maintain satisfying relationships with others outside the family circle. In addition, they may tend to maintain frequent contact with family members during the transition to adulthood because they need sustained support from the family. This raises the question whether BPD, in addition to challenges in relationships with parents, is also associated with challenges in the relationships with friends during adolescence and young adulthood.
As the importance of parental physical presence decreases, relationships with peers are increasingly taking over functions of the relationship with parents, including intimacy, advice on behaviours and feelings, and social influence [18, 19]. Friendships are particularly important for socialization towards more mature roles in late adolescence and early adulthood, and play an important part in adaptive social development. Indeed, in a review Brechwald and Prinstein  have shown that in comparison to risky peer influence, healthy peer socialization processes can provide potential protection from maladaptive outcomes. Especially in adolescence, beyond friendships, the closeness elicited in best friends provides the context in which intimacy, trust, and emotional support are established and tested .
BPD features are consistently associated with problematic functioning across facets of peer functioning, such as friendship quality, peer victimization and bullying, and peer aggression . Friendships during adolescence can be characterized by intense intimacy, and conflict is likely to occur faster and have a bigger impact when contact in relationship is very frequent . Close friendships appear to place special burdens during early adolescence, and the expectations for exclusivity of best friendship in particular may be a marker of BPD . Although different findings in associations between parental and best friend relationship quality and BPD were reported [11, 22, 25], research is needed to consider the dynamics of exclusive “best friend” roles in relation to BPD .
Given the major (psycho) social changes key to adolescence and young adulthood in general, and the development of psychopathology (including BPD) in some young people specifically, it is important to increase our understanding of BPD in the context of social functioning within adolescence and young adulthood. Many types of psychopathology, including personality pathology and BPD specifically, have their onset in (early) adolescence. Based on the Children in the Community Study (CIC) , a developmental trajectory for personality pathology was suggested describing an onset in early adolescence. In addition, a peak in prevalence rates of BPD was observed around middle to late adolescence followed by a decline into adulthood [27, 28]. At the same time, normative changes in relational functioning occur in adolescence and young adulthood. Bowlby’s theory of attachment  states that in infancy and childhood, children are primarily attached to their parents because they can offer them security and protection. As children enter adolescence, the importance of physical protection and security decreases, and adolescents become more independent and start to explore their environment on their own . During the transition to young adulthood (18–25 years) the scope of change and more independent exploration of life’s possibilities increases further, gradually leading to more enduring choices in love, work, and worldviews . Throughout adolescence and young adulthood, internal representations of relationships which shape mental concepts of interpersonal behaviour in close relationships across the life span, are considered to become resistant to change and generalized to other close relationships [29, 32].
The literature paints a developmental picture on how friendship relationships may be seen as an extension of the attachment relationship with the parents developed during childhood. However, the developmental path from childhood until adulthood is not a straightforward pathway, and there are several gaps in the current literature. Firstly, research on BPD features and aspects of the relationships with mothers focuses mostly on maladaptive parenting, such as maternal abuse or neglect [20, 21], or affective behaviours during a discussion task [19, 22]. While findings from these studies provide general support for the importance of the parent-child relationship, it remains unclear which specific features of this mother-child relationship are related to BPD in young people. Moreover, although previous research has suggested that the experience of the parent-child relationship can differ substantially between parent and child , to the best of our knowledge associations between quality of relationships and BPD have not been studied yet from the perspectives of both the young person and the mother. Secondly, while the increasing importance of relationships with peers have been reported across the literature , few studies have integrated the findings on quality of parental and best friend relationships, leaving a gap in how the quality of both parental and best friend relationships might be associated with BPD. Third, although studies have focused on parental and best friend relationships in young people, it is unclear whether these associations are related to age during the developmental phases of adolescence and young adulthood.
The current study
Taken together, gaining insight into the associations between BPD features and social relationships with mothers and a best friend in adolescents and young adults would provide a window into the context for the development of psychosocial functioning during a crucial phase in the onset of BPD. The current study aims to add to the literature by examining relations between the quality of relationships with both mothers and a best friend and BPD in young people, investigating the role of age in these relationships, and comparing the results on the quality of relationships with the mother as reported both by the young person and the mother.
We expect that less supportive interactions and more negative interactions with mothers are related to more BPD symptoms. Similarly, we expect that less supportive interactions and more negative interactions with a best friend are related to more BPD symptoms [20, 34]. With regards to the role of age, it is hypothesized that the link between maternal supportive and negative interactions and BPD becomes weaker when adolescents grow older. Thus, associations between the maternal factors (maternal supportive and maternal negative interactions) and BPD might be smaller for older individuals and larger for younger individuals (i.e., moderation). For relationships with best friends, the opposite pattern is hypothesized; that is, we expect that associations between the best friend factors (best friend supportive and negative interactions) and BPD might be larger for older individuals and smaller for younger individuals.
Finally, in addition to studying the quality of parental relationships from a young person’s perspective, a parent’s perspective on this relationship can add to a more informative view by understanding individual differences between youths’ and parents’ reports of family relationships. Therefore, we will test the robustness of our findings by comparing the results based on self-report with results from a subsample in which supportive and negative interactions with mothers were rated by the mother.