Modern evolutionary psychologists, building on Bowlby's important work but incorporating Darwinian principles of sexual selection, conceptualize the attachment system as evolving for two related but distinct adaptive reasons: survival and reproduction. Certainly, adult attachment styles serve a different function than childhood attachment styles. In children, attachment styles help elicit care from parents in order to survive whereas in adults, attachment styles serve to maintain long-term pair-bonds that can increase reproductive success.

Still, there is an emerging consensus among neurobiologists and social-personality psychologists that both parent-infant bonds and long-term couple relationships draw on the same attachment motivational system (Del Giudice & Belsky, 2010). The behavioral and psychological displays of adult bond formation, separation, and loss show striking similarities with the same displays in children (Feeney, 1999) and neurobiological studies also show substantial overlap in the neurochemical and neuroanatomical substrates involved in both types of relationships (e.g., Pedersen et al., 2005).

While other drives surely come into play in adult relations (e.g., sexual attraction), the evidence suggests that the motivational system that underlies parent-infant bonds may have been at least partially co-opted during the course of human evolution to promote long-term bonding in an adult context. The attachment system is closely linked to the stress response system and helps regulate a child's feelings of distress, pain, fear, and loneliness.

Adopting an evolutionary approach, Belsky et al. (1991) and Chisholm (1993) argue that children in the first years of their lives assess their levels of attachment security and use those cues to assess how risky and uncertain their environments are (see Part II, Developing a Life History Strategy). This assessment has a drastic effect on the formation of a person's reproductive strategies and whether he or she will live a fast or slower life. 

Children raised in harsh and unpredictable environments who are also biologically prepared to live the fast life will tend to develop an insecure attachment style, whereas those growing up in safe and secure environments biologically prepared to live a slower life will tend to develop secure attachment styles. 

Insecure children are likely to follow a fast life trajectory, involving early reproduction and physical maturation, short-term, uncommitted relationships with partners, low parental investment, and increased opportunism and risk-taking. Insecurely attached adults do report shorter estimates of their own life expectancy (Chisholm, 1999), so there does appear to be a link between adult attachment security and perceptions of the harshness of the environment. Secure children, on the other hand, are predicted to follow reproductive strategies involving later reproduction, higher parental investment, longer-term couple relationships, and more trusting, mutually beneficial close personal relationships in all aspects of their lives. 

These models also predict the specific aspects of the family environment that act as cues of security to the child. These aspects involve family stress, harmonious parent-child relations, father absence, and marital conflict. Belsky et al. (1991) proposed that rejected or insensitive parenting conveys information about the lack and unpredictability of resources, the low trust and cooperation of people in the environment, and the instability and low commitment in couple relationships (see Part II, Developing a Life History Strategy).

A key assumption in developmental psychology is that humans go through various stages in their development, each stage associated with milestones. What are the important stages in the development of attachment?

Stages of Attachment

Marco Del Giudice, a young, rising superstar in evolutionary psychology, in only the past few years has published a flurry of papers (see References section) relating to the development of the adult attachment system. Building on the work of Belsky et al. (1991) and Chisholm (1999), Del Giudice has shed much light on the developmental time course leading from infant attachment styles to mature, sexually differentiated strategies.

Del Giudice proposes that life history strategies develop in a flexible, multi-stage fashion. According to this view, life history strategies remain open to continual modification depending on life stage and context (although some people may be more flexibly adaptive than others; see Part II, Developing a Life History Strategy). Del Giudice also proposes that throughout human development there are developmental switch points where an individual's genes are calibrated with information from the environment and this integration then shapes an individual's choice of life history strategy (also see West-Eberhard, 2003).

What are these switch points?

While strategy-setting may occur even before birth (exposure to maternal stress hormones), Del Giudice and his colleagues argue that  the passage from early to middle childhood is the first crucial switch point (Del Giudice, Angeleri, & Manera, 2009). During this hormonally mediated turning point, environmental and genetic factors are integrated to redirect the individual’s reproductive strategy in a sexually differentiated way and this neurobiological "switching" mechanism is strongly tied to adrenarche.

Adrenarche is an endocrine maturational event also known as "adrenal puberty" (e.g., Auchus & Rainey, 2004). At about 5-8 years of age, the adrenal glands of both sexes begin increasing output of adrenal androgens. These andogens don't have much effect on physical development but can directly influence psychological functioning directly through neuromodulatory effects, and can indirectly influence psychological functioning in the brain through hormones such as testosterone and/or estrogen in the brain (e.g., Adkins-Regan, 2005; Del Giudice & Belsky, 2010). Research does show that bad parenting on both the mother and the father's part and higher marital discord during early childhood predicts earlier emergence of adrenarche in boys and girls in first grade, and the beginning stages of puberty in girls in fifth grade (ages 10-11; see Ellis & Essex, 2007).