Connections between neurons get shaped and strengthened by experience. The brain evolves rapidly over early childhood and continues to evolve over the first two decades
of life. In the first three years neurons migrate, differentiate, and build up synaptic strength.
After age three, the brain is constantly sculpted (a critical process known as cortical pruning) and the strength of the connections in a child’s brain improved (becoming
increasingly myelinated) . The different brain systems thus become more fully evolved towards being ‘adult-like’.
Across childhood and adolescence there are peaks in brain development – at age 3, 8, 11 through to 15 and even later at 19 . Such ‘peaks’ are – like iceberg tips – only a small
indication of the complexity of the underlying changes happening in brain systems and their related cognitive and emotional functions.
The frontal system begins to assume control over socio-emotional and purposeful behaviour from 3-4 years. A four year old child may understand how more ‘smiley faces’ on a chart could be linked to a trip to a play park and an ice cream and decide (or
not) that the trade-off is worth sharing a toy for.
This capacity to link behaviour and consequence accelerates rapidly in development from around 7 to 11, with language skills allowing logical deduction and more abstract
thought . See Figure 2. The consequences of such cognitive changes, in terms of control of behavior, can be seen in the ability to resist distraction being relatively
matured by 6 years or so and impulse control becoming established by age ten, and these abilities continue to evolve over early adolescence, with planning and dual
attention improving with age .
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(Fox  p. 28)