Evidence is accumulating that abundance in cerebral connectivity is the neural basis of human behavioural variability, i.e., the ability to select adaptive solutions from a large repertoire of behavioural options. Recently it was demonstrated that variability in motor behaviour- the hallmark of typical development--emerges coincident with the onset of synaptic activity in the embryonic cortex. This inspired the hypothesis that variability results from cortical activity and that its expression depends on integrity of cortical connectivity. Recent findings in preterm infants with cerebral white matter injury and in children with autism spectrum disorder (ASD) allow the elaboration of this hypothesis: diffuse damage of the cerebral white matter is associated with an overall reduction in variability, i.e., in a reduction of movement complexity and variation, whereas reduction in long-distance cortical connectivity, as described in children with ASD, is associated with reduced variability, in which movement complexity may be conserved.