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While much of the early work of human behavioral ecologists and evolutionary psychologists focused on adult decision making and social behavior, by the late 1980s some evolutionists (e.g., Blain & Barkow, 1988) began to explore the relationship between selective forces and human development. One of the first was Jay Belsky. In 1991 Belsky, alongside co‐researchers Laurence Steinberg and Patricia Draper, began to develop the argument that, given the type of environment that many children grow up in, rather than being deviant and maladaptive, insecure attachments might actually be adaptive. Informed by an earlier study by Draper and Harpending (1982), which showed that rearing in the presence or absence of a father had a knock‐on effect on the social behavior of boys and girls, Belsky et al. (1991) developed a hypothesis which they called psychosocial acceleration theory.

Psychosocial acceleration theory proposes that children are sensitive to the conditions of their early family environment and they use this to alter their development in adaptive ways. Those growing up in harsh, unsupportive environments (frequently where the father is absent) mature more rapidly and form less‐successful relationships with other children (and later with romantic partners). Boys growing up in the absence of a father tend to exhibit lower levels of social trust and higher levels of competitive aggression, while girls tend to have negative attitudes towards men and begin puberty earlier. According to their model, due to an evolutionary history where the availability and predictability of resources would vary greatly, the rearing conditions of the first 5–7 years of life help to form an enduring view of how trustworthy interpersonal relationships are likely to be. Where resources, including high biparental investment, are relatively plentiful then children adopt a slow LH lifestyle in which they are more trusting and able to show interpersonal warmth to others. In contrast where life is harsh and there are limited resources, there is a tendency to gravitate towards a fast LH (see Figure 4.1). Hence, the rearing context helps shape the specific LH strategies of individuals within our species. Note that in this model, physiological changes (due to differing environmental inputs) accompany psychological ones. In particular, the sympathetic nervous system and adrenal glands are believed to act as mediators between environmental input and the social pathway that children take during development. Moreover, a number of experts today consider that the level of gene expression also plays a role in calibrating physiological and behavioral responses (de Baca & Ellis, 2017).

In 1997, Belsky developed this psychosocial acceleration theory further by proposing adaptationist accounts for each of Ainsworth’s three attachment styles. Belsky focused primarily on adult outcomes but he also considered how childhood behaviors can be related to these forms of attachment. In the case of secure attachments, children come to see the world as one in which relationships are rewarding and enduring. Such children, therefore, make lasting friendships during childhood and tend not to respond to social challenges with aggression and resentment. Where insecure‐avoidant attachments are formed, Belsky suggests resources may be limited and others cannot be trusted. Hence, both boys and girls find it difficult to form trusting relationships. Both sexes are also more likely to be opportunistic in how they treat other children (and later in adult relationships). For those children who experience insecure‐resistant attachments there is a tendency towards clinginess and demands for others’ attention. To Belsky such children are the least likely to go on to have children themselves but instead shift their investment to other relatives such as younger siblings or nephews and nieces. Belsky suggests in the case of insecure‐resistant attachments children develop a “helper‐at‐the‐nest” role and shift to an indirect reproductive role.

According to modern day evolutionists, both forms of insecure attachment can be seen as alternate LH strategies to boost inclusive fitness under challenging conditions (which may have been common in the ancestral past, see for example, Del Giudice [2009a]). Encouragingly, Belsky’s approach to LH theory and psychosocial complexity is broadly compatible with the now widely accepted view, discussed earlier, that human brain size evolved in response to increased group size and social complexity.

In Figure 4.1, the family context (e.g., level of spousal harmony) feeds in directly to childrearing practices, and this has a major impact on psychological and behavioral development (as briefly outlined earlier). This has a knock‐on effect on the activation of the sympathetic nervous system (the part of the autonomic nervous system that is particularly active during times of stress). It also affects gene expression (that is, it has epigenetic effects; see Chapter 2, this volume). This, in turn, shifts behavior in ways that fit in with a fast or slow LH strategy (or, in reality, a continuum between the two). Finally, following this chain of events can lead to predictions about overall health and even life‐expectancy. Note that, in this model, childrearing practices on each “step down” can also feedback and alter the level above in this diagram. Hence, for example, the child’s behavior will also influence the way the parent (or parents) deals with the child. Likewise, the relationship between the sympathetic nervous system and behavior is a reciprocal one.

Figure 4.1 Developmental pathways for fast and slow life history strategies (based on Belsky et al., 1991 and de Baca and Ellis, 2017).

Although Belsky and his co‐worker’s model was largely theoretical when originally introduced, today, as evolutionary developmental psychology has progressed, evidence to support specific aspects of it has accumulated. Exposure to stress or maltreatment early on in life, for example, is now known to be associated both with various forms of gene expression related to stress and with internalizing behaviors (behaviors such as feeling sad, lonely, and withdrawn) between the ages of 3 to 5 (Conradt et al., 2016; de Baca & Ellis, 2017; Romens et al., 2015).