These are learned ways of behaviour in relationships that come from imitating the over-reactive, under-reactive, or just right (Goldilocks) nervous systems of our caregivers.
Attachment style stems from research done on attachment theory carried out by psychologist John Bowlby, who worked as a psychiatrist in London, treating emotionally disturbed children. His work led him to consider the impact that a child’s relationship with his or her mother had on their social, emotional, and cognitive development. He found that the impact was significant, and that separation from the mother caused great distress in babies.
Bowlby defined attachment as a ‘lasting psychological connectedness between human beings.’1 He also suggested that attachment can be considered in an evolutionary context, in that the caregiver provides safety and security to the infant.2
1 Schaffer, H. R., & Emerson, P. E. (1964). The development of social attachments in infancy. Monographs of the Society for Research in Child Development, 1-77.
2 McLeod, S. A. (2017, Feb 05). Attachment theory. Simply psychology: https://www.simplypsychology.org/attachment.html