Behavioural maturation correlates with brain-based development. Maturation is not a linear story, there is an ebb and flow that has to be precisely timed for normal development – too fast can be just as damaging as too slow. In fact, a slower development of the prefrontal cortex can result in a cortical thickness that has been associated with a higher intelligence quotient (IQ), while a rapid development of the brain is associated with autism. Progressive increases in the thickness of pyramidal cells and synaptic density peak and ebb over childhood. Brain weight and head circumference increase and plateau at various ages.