According to Margaret S. Stroebe and John Archer:
“By the close of the last century, attachment theory had evolved into what Cassidy and Shaver (1999) described as “one of the broadest, most profound, and most creative lines of research in 20th-century psychology” (Cassidy & Shaver, 1999, Preface; p. x).” (p.28)
“Attachment can be defined simply as an emotional connection to someone, evidenced by proximity seeking, feelings of security in the persons’ presence, and protest on separation from this attachment figure. Early theorists were interested in the infant’s attachment to its mother, the need for such a connection being a fundamental part of human experience (Ainsworth, Blehar, Waters, & Wall, 1978). Affect regulation was understood to be the force impelling proximity seeking during periods of separation (anxiety) from the attachment figure. The clinging of the infant to the mother was regarded as serving the biological purpose of keeping the infant close and thereby increasing its chance of survival (originally in an environment beset by predators). Infants maintain proximity to their caregiver, they stay nearby and safe, and this provides a source of comfort and protection from threatening experiences. As such, attachment provides a secure base (the feeling of safety provided by an attachment figure, see Ainsworth, 1982; Bowlby, 1969/1982, 1988) for the infant to explore outwardly, and the possibility of returning to this safe haven (i.e., the place to seek reassurance when distressed). Thus, the infant is biologically disposed to use the caregiver as a haven of safety while exploring the environment. Infants need to know that the caregiver is dependable, in the sense of being there when needed, providing a sense of secure attachment, a solid base from which to explore the world.
“Secure attachment is reflected in normal distress when the caregiver leaves, and pleasure on her return. However, there are also patterns of insecure attachment, which—importantly—are linked to inconsistent treatment, neglect, and/or rejection by the caregiver. As such, the style of attachment of the caregiver has a major impact on that of the infant/child. There is a certain degree of concordance, cross-generation transmission of patterns of attachment (e.g., George & Solomon, 1999; Rosenstein & Horowitz, 1996). Three insecure forms of attachment have been identified (e.g., Ainsworth et al., 1978; Main & Solomon, 1986). Ambivalent (preoccupied) attachment is characterized by clinging when the caregiver leaves, and rejection and anger on return. Avoidant (dismissing) infants stay calm when the mother leaves, but avoid and reject her on return. Disorganized (disoriented) infants display variable behavior on such occasions, showing contradictory and sometimes apparently disoriented behavior.” (p.29)
“Although there are long-term effects of early attachment insecurity experiences on mental health (e.g., they are associated with psychopathology, [-p.30] such as personality disorders), it is also important to note that insecure attachment styles are considered adaptive in the sense that they enable children to cope in the face of less-than-optimal parental circumstances (Shorey & Snyder, 2006).
Not surprisingly, given such long-term connections, the idea of continuity or stability in patterns of attachment across the life span is fundamental to attachment theory.” (pp.29-30)
Ref: (italics in original; emphases in blue bold, mine) Margaret S. Stroebe and John Archer (2013) Origins of Modern Ideas on Love and Loss: Contrasting Forerunners of Attachment Theory. Review of General Psychology Vol. 17, No. 1, 28–39
“Abstract: In this article we examine some origins of John Bowlby’s attachment theory, a highly influential scientific approach to love and loss in contemporary society. Although some potential influences have been well-documented, others have either received no recognition or have failed to have an impact. We focus specifically on three of Bowlby’s predecessors, exploring how these were differentially influential on his work. The first of these, Charles Darwin, was amply endorsed by Bowlby, both in terms of the adaptive background to his theory and more specifically in relation to Darwin’s study of the emotions associated with grief. The second, Alexander Shand, was recognized as important but is cited little and omitted from the central issue of the resolution of grief. The third, Bertrand Russell, formulated ideas on attachment and separation before Bowlby, and possibly contributed to the intellectual forces that influenced him too. To our knowledge, Russell’s work was not cited by Bowlby, despite the fact that it contained the seeds of many of Bowlby’s ideas on attachment. It remains unclear whether this was because he had not read Russell or through omission; there is no definitive evidence either way. Tracing these historical origins illustrates how theory development involves a process of integration and selection, how even radical theories are rooted in previous scholarship, and how it can take decades for inspiring ideas to develop into full-blown, well-tested, theories.”