structural:summarization, structural:reformatting, quality:corruption
Changes in the brain

The human brain is not finished developing by the time a person reaches puberty, or even finishes it. The frontal lobe of the brain has been known to shape itself well into one's 30s. Neuroscientists often cannot agree precisely on when this developmental period ends or if there is an exact age for the end of brain development. Below the age of about roughly 30, the human brain has been implicated in human behavior and social immaturity. However, there has been no empirical study indicating a causal relationship with the development of the prefrontal cortex in adolescence and into early adulthood with any irrational behaviors. The brain reaches 90% of its adult size by six years of age. Thus, the brain does not grow in size much during adolescence.

Over the course of adolescence, the amount of white matter in the brain increases linearly, while the amount of grey matter in the brain follows an inverted-U pattern. Through a process called synaptic pruning, unnecessary neuronal connections in the brain are eliminated and the amount of grey matter is pared down. However, this does not mean that the brain loses functionality; rather, it becomes more efficient due to increased myelination (insulation of axons) and the reduction of unused pathways.

The first areas of the brain to be pruned are those involving primary functions, such as motor and sensory areas. The areas of the brain involved in more complex processes lose matter later in development. These include the lateral and prefrontal cortices, among other regions. Some of the most developmentally significant changes in the brain occur in the prefrontal cortex, which is involved in decision making and cognitive control, as well as other higher cognitive functions. During adolescence, myelination and synaptic pruning in the prefrontal cortex increases, improving the efficiency of information processing, and neural connections between the prefrontal cortex and other regions of the brain are strengthened. This leads to better evaluation of risks and rewards, as well as improved control over impulses. Specifically, developments in the dorsolateral prefrontal cortex are important for controlling impulses and planning ahead, while development in the ventromedial prefrontal cortex is important for decision making. Changes in the orbitofrontal cortex are important for evaluating rewards and risks.

Three neurotransmitters that play important roles in adolescent brain development are glutamate, dopamine and serotonin. Glutamate is an excitatory neurotransmitter. During the synaptic pruning that occurs during adolescence, most of the neural connections that are pruned contain receptors for glutamate or other excitatory neurotransmitters. Because of this, by early adulthood the synaptic balance in the brain is more inhibitory than excitatory.

Dopamine is associated with pleasure and attuning to the environment during decision-making. During adolescence, dopamine levels in the limbic system increase and input of dopamine to the prefrontal cortex increases. The balance of excitatory to inhibitory neurotransmitters and increased dopamine activity in adolescence may have implications for adolescent risk-taking and vulnerability to boredom (see Cognitive development below).

Serotonin is a neuromodulator involved in regulation of mood and behavior. Development in the limbic system plays an important role in determining rewards and punishments and processing emotional experience and social information. Changes in the levels of the neurotransmitters dopamine and serotonin in the limbic system make adolescents more emotional and more responsive to rewards and stress. The corresponding increase in emotional variability also can increase adolescents' vulnerability. The effect of serotonin is not limited to the limbic system: Several serotonin receptors have their gene expression change dramatically during adolescence, particularly in the human frontal and prefrontal cortex.