Читать книгу Child Psychology - Jean-Pascal Assailly - Страница 42

Youth vulnerability can be linked to what we have called a “dual agenda” (Assailly 2019), or, to be more precise, the coordination of two trends.

On the one hand, a purely biological, physiological tendency called the “secular trend in puberty”, attested by biologists, physiologists and doctors. Compared with our ancestors, puberty, as observed by physiological and physical changes in the bodies of young boys and girls, appears earlier in life, on average two years earlier than in previous centuries, for both boys and girls.

This secular trend has stabilized and, although it is not completely explained, hypotheses are increasingly being put forward around hygiene, nutrition, chemical pollutants and endocrine disruptors, factors similar to the contemporary obesity epidemic. As puberty is mainly a hormonal triggering phenomenon, all of the factors affecting the hormonal life of the embryo (its first exposure to testosterone) and the adolescent (its main exposure to testosterone) may play a role in this earlier triggering.

On the other hand, a psychosociological phenomenon that can be described as the “increasing precocity of behavioral disorders”, attested by educators and psychologists, among others (“what previous generations did at 14 is now experienced at 12”), and which is reflected in the earlier average age of onset of all kinds of problems (such as regular and excessive consumption of alcohol, tobacco, use of illicit drugs and antisocial behavior).

It is only recently that neurobiologists and brain imaging specialists have helped us to better understand the relationships and interactions between these two historical developments and how risk-taking in adolescence is being constructed in the confrontation between two neurobiological agendas.

According to the dual systems model (Steinberg 2010) and the related maturational imbalance model (Casey 2015), adolescent risk-taking can be understood in terms of two distinct dimensions – incentive processing and cognitive control – that mature on separate time scales and are underpinned by distinct neurobiological circuitry.

From puberty onwards, the subject becomes more sensitive to rewards, novelty and sensation seeking, due to changes in the functioning of the dopaminergic system, a “socioemotional” system that involves limbic structures, the amygdala, the nucleus accumbens and other areas of the brain associated with judgments about the attractiveness of stimuli.

This stimulus-processing system develops rapidly in early to mid-adolescence; adolescents show greater reactivity in the ventral striatum in response to rewards than do children or adults and the extent of ventral striatum reactivity correlates with real-world risk-taking.

This system is activated by a strong secretion of testosterone during puberty, which is, of course, much higher in boys than in girls. So this “reward system” is activated earlier now, from the beginning of puberty.

In late adolescence, self-regulation is enhanced by a second system, the “cognitive control decision system”, located in the prefrontal areas (responsible for executive functions such as memory, attention, planning, inhibition and flexibility). This system regulates decisions about reward and peer pressure. Unfortunately, this control system does not mature until the age of 22, 23 or even 24.

For example, white matter volume increases until the mid-20s, particularly in brain areas involved in high-level cognitive control. Thus, this neural maturity gap is thought to result in increased sensitivity in adolescents to rewards and emotions without a concomitant increase in their ability to control their behavior.

The problem of adolescence is thus a problem of the agenda between these two systems, the first developing more rapidly than the second and producing that strong increase in the search for sensations and rewards that the adolescent can find in risk-taking, this being the means of satisfying this need. With the secular changes in the age of puberty, this also allows us to understand why risky behaviors and their potentially dangerous consequences are occurring earlier today.

Thus, the issue is not that adolescents are more impulsive than adults or that they seek more rewards than adults; the issue is that they do not sufficiently understand the associations between behavior and consequences (and the balance between benefits and costs: “If I have one more drink, I will be more euphoric but I might also get my license revoked”). This has been shown using functional brain imaging: the synaptic chains connecting the limbic to the prefrontal regions are not “finite” (“myelinated”, to use the technical term), so there is not enough synchronization between cognition and affect.

Brain imaging has also shown the influence of this coordination on resistance to peer pressure, a highly significant factor in risk-taking in young people. Adolescents who are more sensitive to peer pressure are those who activate the areas most involved in the perception of others’ actions, whereas subjects who are less sensitive to peer pressure are those with better connectivity between these areas and those of cognitive control in decision-making.

The transition from adolescence to adulthood thus seems to be a period of “tuning”, thanks to a better connectivity between the prefrontal and the limbic, which allows a better cognitive control of emotions. For as long as this adjustment is not made, adolescence will be a period of vulnerability, because the two cortical systems are in conflict during decision-making and risk-taking, and when one system predominates over the other – for example, the socioemotional system – immediate rewards will be too prevalent.

To conclude on this neurobiological agenda, it leads to the idea that risk-taking is a question of tempo, or, more precisely, of two different tempi, between the rapid socioemotional system of reward and the much slower system of cognitive control. Adolescents will (fortunately) not take risks all the time, but there will be times when emotions and peer pressure will activate the emotional system to such an extent that it will overwhelm the adaptive capacity of cognitive control.

This helps to explain why early puberty and early initiation of substance use behaviors are predictive risk factors, which occur at a time in life when the prefrontal and executive functions are not yet controlling the reward system.

However, adolescent behavior also depends on the social and relational context. To take just one example, English adolescents are killed half as much on roads as their French counterparts, with the same number of individuals and cars. While biology is the same, it is culture that explains the differences.

Furthermore, the discovery of the dual agenda by neurobiology may lead actors to two conflicting conclusions:

 – some may argue that the lack of maturation of prefrontal control means that nothing can be done and that adolescents must be protected in spite of themselves, by measures to reduce exposure to risk;

 – others will argue, on the other hand, that adolescence is a time of maximum plasticity for learning and experience and that intervention is needed. Furthermore, testosterone also increases social status sensitivity at puberty, not just reward sensitivity; this could allow for status to be played out in peer intervention and prevention.