Attachment is an important developmental domain and is known to have an impact on other domains throughout life as well, like mental and physical health, academic achievement and social functioning. Attachment theory (Bowlby, 1969) points at the importance of effective care during stress to develop a secure attachment and build trust. This means that whenever a child is distressed, it can count on its caregiver to provide help and support. Bowlby suggested that children develop trust and secure attachment representations when they have multiple experiences of receiving help and support when needed. The trust and secure attachment in their turn stimulate care seeking behavior. However, when care is lacking or inconsistent, the child more likely develops an insecure attachment. As a consequence, they fail to adequately turn to their caregivers for help during distress. According to Bowlby, these experiences become internalized in Internal Working Models. These are cognitive representational models in which experiences of interaction become organized as memories. These Internal Working Models shape expectancies for future interactions and guide the individual’s behavior. In learning theory, the probability of the co-occurrence of two events, here distress together with care by the attachment figure, is called contingency. Repeated experiences of receiving care from a caregiver during distress can become internalized as an experienced probability that the caregiver provides care when feeling distressed. Although there seems to be stability in the Internal Working Models over time, they still can change or become more complex based on additional experiences between early childhood and adolescence. At that period in one’s life, biological processes enhance brain capacity to organize experiences during social interactions in a cognitive script-like fashion. Therefore, middle childhood is a suitable period to study the development of attachment. When investigating the development of attachment, research has focused mainly on social influences such as parenting. However, little research has been done on the underlying endocrinological processes linked with attachment. Focusing on just one of the possible domains that might influence attachment could be limiting. Therefore, we want to look at the interplay between social influences and endocrinological processes. More precisely, our purpose is to investigate in what way endocrinological processes contribute to the internalization of experiences related to care into Internal Working Models. Specifically, we expect that increases in oxytocin and vasopressin and decreases in cortisol during care play a role in the development of attachment over time. We will be conducting a longitudinal study with 3 waves within 1 year. The study will consist of a combination of an experimental design in which we will measure oxytocin, vasopressin and cortisol, and an Event Sampling Methodology design using smartphones for 2 weeks. The purpose of the current study is to investigate whether and how attachment measures and contingency of effective care relate to biophysiological changes in children in middle childhood.