Case Reports, anecdotal evidence, descriptive epidemiologic studies, psychoanalytic and neuroscientific studies converge to conclude that impulsivity, sensation seeking and risky behaviours are common during adolescence. Epidemiological studies put forward that not only adolescents (fifteen-eighteen years old) but also young adults (eighteen/fifty five years old) are at increased risks of dangerous behaviours. Structural and functional neuro-imaging studies have shown that neural circuitry undergoes major reorganization during adolescence and young adulthood, particularly in those regions of the brain relating to executive functions, the self and social cognition. Decreases in gray matter volume during adolescence mainly reflect a massive reduction in the number of synapses ; meanwhile, increases in white matter volume reflect improvement of connectivity between distant regions of the brain, studies showing that the brain undergoes major microstructural changes during adolescence, and indeed beyond. Two different approaches to the « tendency to act » during adolescence are expressed in both neurosciences and psychoanalysis. One of the most widespread conceptions hypothesizes that this tendency results from some deficiencies, namely a lack of mentalization, for psychoanalysts, or an inability of the prefrontal cortex to exert a mature control over the emotional brain – the limbic system – for neuroscientists. Hence adolescents could be dominated by their drive or their emotions. The other conception, that we follow and we develop, is that the relative unbalance between emotion and cognition during that period, if any, allows a fine tuning of the brain structure through adjustment of connectivity and functionality of the prefrontal regions. From an evolutionary perspective, the last brain regions to mature in a given adolescent are also the last brain regions to develop phylogenetically. Taking into account gregariousness and social conformism of adolescents, the « tendency to act », often described in pejorative terms referring to their consequences as « risky behaviors », helps to edify a social brain adapted to the adulthood of a new generation.

Adolescence, 2011, T. 29 n° 3, pp. 479-515.