3.2 Human Development

The brain does not arrive fully formed at birth. It develops in a sequence, with simpler functions coming online before complex ones. This principle is called brain maturation, and it shapes cognition, emotion, and behaviour from infancy through to early adulthood.

The clearest example of this sequence is the prefrontal cortex, the region responsible for decision-making, planning, and impulse control. It is the last part of the brain to mature, continuing its development well into a person's mid-20s. This fact alone explains a great deal about adolescent behaviour.

At the cellular level, two types of cells drive development:

• Neurons carry information. Each has a soma (cell body), dendrites that receive incoming signals, and an axon that transmits signals outward as electrical impulses called action potentials.
• Glial cells support neurons: they maintain homeostasis, generate myelin, assist in signal transmission, and supply neurons with nutrients.

Myelination is the process by which axons become coated in a fatty myelin sheath, dramatically increasing the speed of neural transmission. It proceeds from the back of the brain to the front, which is why children develop basic motor skills well before they can manage higher-order reasoning.

Brain maturation involves several overlapping processes:

• Neurogenesis: stem cells divide into progenitor cells, which become neurons or glial cells.
• Cell migration: new cells travel to their correct location in the brain.
• Cell differentiation: stem cells specialise into their final cell type.
• Synaptogenesis: synapses form between neurons.
• Apoptosis: excess neurons and synapses are eliminated in a process of programmed cell death.

Johnson (2001), in a literature review of brain development research, concluded that although most neurons are present at birth, synaptogenesis is especially active during the first year of life, and full visual processing does not develop until around age 2. Johnson also identified three perspectives on how brain development drives cognition: the maturational perspective (cognition follows brain maturation), the specialisation approach (skills emerge from interactions between cortical areas), and the skill-learning hypothesis (different brain regions support a skill while it is being learned, then hand off to other areas once it is mastered).

The concept of critical periods implies that timing matters, not just experience. Language acquisition is the most studied example: children who receive little or no linguistic input before puberty typically fail to develop full grammatical competence later, even with intensive instruction.

Bowlby's attachment theory applies the same logic to emotional development. John Bowlby argued in the 1950s that the critical period for forming secure attachments runs from birth to approximately two to three years. During this window, consistent and responsive caregiving builds an internal working model that shapes how a person relates to others throughout life. Inconsistent care during this period is associated with less stable relationships in adulthood.

Simons et al. (2014) drew on data from an existing longitudinal study to examine whether early attachment predicted adult romantic relationships. They found that supportive parenting and secure childhood attachment were correlated with secure attachment styles in adult romantic relationships. Poor early attachment was also associated with higher rates of aggression towards partners in adulthood. The longitudinal design is a strength here, as it tracks the same individuals over time, but the study cannot establish causation, and later relationship experiences may also reshape attachment patterns.