Dahlgren and Whitehead's model of the social determinants of health (Dahlgren and Whitehead, 1991; Dahlgren, 2007) provides a framework from which we can begin to understand the complex interaction of genetic factors and prenatal, perinatal and postnatal events in influencing the development of the individual from childhood, through adolescence and into adult life. The early life stages lay the foundation for adult life and successful ageing. The biological processes of natural selection operate at each of these stages, starting before birth, the outcomes of which are significantly influenced by external factors such as the supply of adequate nutrition, a secure environment and nurturing by parents and others supporting the individual on their journey through life (see Parts Three, Four and Five, this volume). The maturation of the human nervous system, musculature and metabolic processes are essential for independent living; however, the social environment provides the external drivers that shape behaviour, cognitive functioning and decision-making. The primary determinants important in social development include levels of sensory stimulation and prosocial learning, which support communication and the shaping of appropriate behavioural responses, such as positive (eg human bonding) and negative (eg aggressive) relationships, which are prerequisites for healthy relationships within the family, the workplace and the community.

The well-being of children and the safeguarding of vulnerable people generally involves protecting them from abuse, exploitation and physical and psychological damage. Sexual abuse and exploitation, sadly, are not uncommon in modern society. These adverse early life experiences not only affect an individual's relationships with others and mental health in later life, but also impact directly and indirectly on physiological status via the immune system (Baumeister et al, 2016), leading to increased vulnerability to communicable and non-communicable diseases, including childhood cancers (Anon, 2016a). This vulnerability is exacerbated by psychological factors that undermine self-esteem and self-image (see Chapter Sixteen). An overview of the positive aspects of the psychological environment within the wider health context is provided in Chapter Three.

Early childhood experiences and the development of social networks have a major impact on later-life health and well-being, which are mediated by the development of prosocial behaviour, resilience, coping skills and optimal cognitive functioning. These complex issues require a healthy mind and body and the motivation to benefit and learn from a supportive environment.