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Institute of Medicine (US) and National Research Council (US) Committee on the Science of Adolescence. The Science of Adolescent Risk-Taking: Workshop Report. Washington (DC): National Academies Press (US); 2011.

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The Science of Adolescent Risk-Taking: Workshop Report.

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3Biobehavioral Processes

The overview of common risk behaviors presented in Chapter 2 highlights two questions: Why are certain types of risk-taking more prevalent among adolescents than other age groups? And why do some adolescents engage in more risk-taking than others and suffer more negative effects? The research on individual risk behaviors provides strong reasons to think that common factors may cut across multiple problem areas. Findings from several fields offer insights into the bio-behavioral processes that influence adolescents and how they may vary among individuals.

THE DEVELOPING ADOLESCENT BRAIN

One possible explanation for the risks adolescents take is that their brains work differently from those of younger children or adults. The availability of new technologies (structural and functional magnetic resonance imaging and diffusion tensor imaging) has allowed researchers to trace changes in the size and shape of brain structures, to link those changes with behavior and observable development, and even to track emerging connections between brain structures and development (Casey et al., 2005). This research has expanded understanding of the development of different regions of the brain, which are responsible for selected functions, actions, and behaviors, and to draw connections between brain development and behavior. Linda Patia Spear and B. J. Casey both explored developmental processes that occur during this period, each focusing on different ways that brain development relates to adolescent risk-taking.

Adolescence Across Species

Developments in the brain relate to important features of adolescence, not only among humans but also among other mammals, Spear explained. The gradual transition from dependence and immaturity to relative independence and maturity is one that virtually all mammalian species experience. Humans and other species need to develop the skills necessary to survive as adults and to reproduce.

During this transition phase, mammals experience many hormonal and physiological changes, such as growth spurts and puberty, and they tend to display certain behaviors that are typical of the age. Spear noted that human behavior and brain function are significantly more complicated than those of other mammals—and also cautioned against interpreting these observed phenomena as evidence of biodeterminism, because many other factors affect human development and behavior. Nevertheless, across species, adolescents tend to show increases in preference for socializing with their peers, which researchers think may be adaptive behavior that helps individuals develop social skills, supports the skills they will need as adults, and helps them prepare to survive without parental protection. Adolescents in a variety of species also show increases in novelty-seeking and risk-taking, which, for humans, often are expressed through the behaviors discussed in Chapter 2. Researchers have posited, however, that the propensity to seek novelty and take risks may be adaptive in several ways. For males in particular, these impulses may improve the odds of reproductive success. They may foster acceptance among peers, and they may help the species avoid inbreeding by making males, females, or both more likely to leave their home territory by the time they are sexually mature, so they can seek mates elsewhere and avoid inbreeding.

The biological changes that occur in mammals also include puberty, a period when a cascade of hormonal activity, beginning with the release of gonadotropin from the hypothalamus gland, culminates in the release of the gonadal hormones estrogen and testosterone. These hormones, in turn, have a variety of effects on the body and on behavior, Spear explained. At the same time, however, equally dramatic changes in the brain are taking place.

Brain Structures

Spear pointed out that the basic structures of the brain are relatively ancient from an evolutionary perspective. Thus, virtually all mammalian species share not only these structures, but also the timing of the structural changes that occur in the brain as the individual matures. Researchers have found, for example, a decrease of up to 50 percent in the number of synaptic connections among neurons in different regions of the brain during adolescence. In general, researchers think that an overproduction of synapses occurs early in life, which is then followed by gradual pruning. The pruning that occurs during adolescence is thought to be more selective than earlier pruning—based on a “use it or lose it” principle— and contributes to the fine-tuning of brain connections necessary for adult cognition. It is also possible that this stage of pruning provides an important opportunity for the brain to be molded by the individual’s environment.

Researchers have also documented an increase in the death of neurons and their support cells, which is likely to be associated with a decrease in gray matter and an increase in white matter.1 The white matter is important because it helps quickly connect distant regions of the brain and therefore also supports the emergence of adult-type thinking. This selective pruning of connections among neurons is accompanied by a decline in the brain’s need for energy. Spear noted that in general the brain is the “most expensive organ in the body, in terms of energy requirements.” The lower demand that comes with a reduced number of synapses and a larger proportion of white matter (which is more efficient than gray matter) is more comparable to an adult brain.

The changes that take place in the adolescent brain are specific to particular regions—those that are most important for modulating behavioral responses to reward and affective behavior. Control over these behaviors is likely to influence risk-taking. The prefrontal cortex, which undergoes significant change during adolescence, is the site of executive control functions that start emerging early in life and continue to develop into adulthood. Spear described these cognitive controls as top-down systems that are critical in allowing the individual to exert control over a range of responses. They help modulate sensitivity to different kinds of rewards, identify the significance of stimuli, and exert control over impulses and emotional and social responses—the bottom-up brain systems.

Casey also highlighted the significance of the fact that development occurs at different rates in different parts of the brain. The development of the prefrontal cortex is gradual and is not complete until well into adulthood. This aspect of brain function has been a focus for many researchers but by itself does not completely explain the behavior patterns adolescents exhibit. The relationship between the prefrontal cortex and the limbic system—the area that supports emotion and many behavioral tendencies, as well as long-term memory—has received increasing attention. The limbic system develops on a steeper curve than the prefrontal cortex, as shown in Figure 3-1, so that the disparity between these two regions is greatest during adolescence. The result can be an imbalance that may favor behaviors driven by emotion and response to incentives over rational decision making. It is this imbalance—not just the protracted development of cognitive control alone—that contributes to the prevalence of risk-taking in adolescents (Casey et al., 2008).

FIGURE 3-1. Different developmental trajectories.

FIGURE 3-1

Different developmental trajectories. NOTE: Differential development of limbic subcortical relative to prefrontal control regions leads to imbalance in brain systems that may favor incentive/emotion driven over rational behaviors. SOURCE: Casey et al., (more...)

Risk-Taking

Casey noted that risk-taking is a complex construct that involves more than sensation-seeking and inadequate impulse control—which themselves are often wrongly viewed as indistinguishable. Researchers have found a steady improvement in impulse control from childhood to adulthood, as shown in Figure 3-2, yet risk-taking still increases during adolescence (even though the definition of “risky” is dependent in part on the age of the individual engaged in the behavior). Why? One reason is that other factors, such as emotions and the incentives provided by environmental cues, also affect risk-taking. She pointed to research suggesting that sensitivity to rewards, such as money, food, or peer approval, can influence behavior even when individuals are not conscious of responding to these influences (Galvan et al., 2005). Adolescents and adults may be similarly responsive to potential rewards (a limbic region function), she explained, but adolescents have less control over the urge to seek a reward that may have negative effects (a prefrontal cortex function). Casey suggested that the development of the parts of the brain that respond to rewards (the limbic system) is on a different trajectory from those that may override unwise choices.2

FIGURE 3-2. Impulse control as a function of age.

FIGURE 3-2

Impulse control as a function of age. SOURCE: Casey et al., 2008. Data were collected as part of a National Institute of Drug Abuse grant no. R01DA018879 to B. J. Casey at Weill Cornell Medical College.

Spear noted that a range of studies of specific brain regions has shown the differences in the responses of adult brains and adolescent brains to stimuli, as well as perceptions of risk and reward. For example, adolescents seem more influenced by stressful, exciting, or emotionally charged situations when making decisions. As a result, they may find a variety of drugs more rewarding than adults do—perceiving more enhanced social facility when under the effects of alcohol, for example. They may also be less sensitive to the adverse effects of these substances; some evidence indicates that they may experience less gross behavioral change in response to intoxication and less hangover after imbibing, for example. This general tendency in adolescents may be exacerbated by certain genetic traits—with the result that an individual who uses substances in early adolescence, when sensitivity to negative effects is lowest and stresses are high, may have heightened susceptibility to later problems because of the action of the alcohol or drug on the developing brain.

Casey pointed out that a variety of differences among individuals— including biological predispositions and differences in the pace of development of different regions of the brain—also influence risk-taking behavior. She noted that researchers have identified differences in the way even young children respond to situations that reward self-control and delayed gratification, for example, and that these differences tend to persist into adulthood (Eigsti et al., 2006; Mischel et al., 1989). At the same time, adolescents differ from adults in their capacity to override their impulses when they are in emotionally charged situations. That is, adolescents may be perfectly able to reason that a decision is not prudent but feel powerless to resist the impulse, whereas when adults make imprudent decisions, it is because they have identified reasons in support of the decision. Casey reported that brain research has associated this trait with an increased activity level in the nucleus accumbens (a region associated with reward, pleasure, and other emotional responses) of the adolescent brain compared with both children and adults. Studies have linked heightened activity in this region to an increased likelihood of taking risks and decreased likelihood of perceiving negative consequences from risks.

The timing of these various changes in the brain means that they play an important role in the experience of adolescence. Spear suggested, however, that a dynamic process occurs in which developing activities in different regions of the brain become more strongly interrelated and linked over time. That is, they do not follow an inevitable sequential pattern—and they are probably influenced by one another and by the experiences the individual has while they are occurring. The adolescent brain reacts differently to stimuli than the adult brain. The combination of exaggerated sensitivity to the rewards offered by many high-risk behaviors, a reduced sensitivity to adverse effects, and the insufficient power of immature frontal cognitive control all contribute to adolescent risk-taking. Since the neural underpinnings of adolescent behavior are likely to vary significantly in the course of adolescence, Spear suggested, it is important to recognize that approaches to managing or preventing risky behaviors may need to be tailored to different ages. Helping young people find safer ways to explore risks, for example, may work well with younger adolescents, whereas with older ones it may be preferable to help them strengthen their emerging capacity for cognitive control. Casey highlighted the importance of considering interactions among the environmental and genetic factors that may contribute to risk-taking and resilience.

Discussion

Daniel S. Pine and Elizabeth J. Susman raised a number of questions about the implications of understanding functions of the adolescent brain. Although basic science is many years away from producing diagnostic tests or other tools that would simplify diagnosis or intervention, it does provide the basis for new thinking about adolescent risk-taking. The cross-species research and other studies may enhance understanding of developmental sensitivities from a circuitry-based perspective, which could lead to many other valuable ideas for interventions to test in humans.

Similarly, the insights about the changes in brain responses to rewards that occur during adolescence link well with findings from studies of the peripheral processes related to stress, Susman noted. Research on stress has identified regulation of the stress response—a reduction in the normal physiological stress response—in children who are displaying problem behavior. Because of a reduction in the release of cortisol or other physiological components of the stress system, children who are highly disruptive or show symptoms of conduct disorder have reduced heart rates and other stress responses in stressful situations. In other words, consistent with earlier theories of sensation-seeking (e.g., Zuckerman, 1993), these individuals do not experience the appropriate arousal and reactivity and therefore they engage in risky behavior to somehow increase their reactivity or sense of pleasure. Susman suggested that the developmental changes Spear described could partly account for these differences between adolescents and adults—and this possibility suggests important links between central and peripheral processes of the brain. Additional research is needed to explore such questions as how the timing of puberty might interact with brain development (discussed below) and possible gender differences in the development of the reward system.

SELF-REGULATION

Although adolescents are physically strong and healthy, their rates of injury and death increase by 200 percent from childhood to late adolescence. The primary reason, Ronald E. Dahl explained, is the difficulties they have controlling their behavior and emotions. Whether the issue is accidents, homicide, depression, alcohol, substance use, violence, reckless behaviors, eating disorders, or health problems related to risky sexual behaviors, he suggested, the development of self-regulatory processes is key to understanding it.

As Spear explained earlier, adolescence is a time during which humans (and other species) are prone to explore and to seek novelty. The social context, however, has an important influence on how those impulses are acted on. Dahl noted that adolescence itself has changed over the past 150 years, biologically, socially, and culturally. Children are growing faster and to larger adult sizes than ever before, and they are reaching reproductive and physical maturity at earlier ages (Panter-Brick and Worthman, 1999). Adolescence once might have lasted 2 to 4 years; but based upon our understanding of pubertal processes, neurodevelopmental changes, social development, and other elements of adolescent development, it now may last a decade or more. The onset of adolescence, linked to the onset of puberty, is characterized by:

  • Increased romantic motivation and interest in sexuality,
  • Increased emotional reactivity and intensity,
  • Changes in circadian rhythms,
  • Increased appetite during periods of rapid growth,
  • Increased risk of depression, and
  • Increased sensation-seeking.

Dahl used the metaphor of “igniting passions” to capture the tendency for young people to become passionate about their goals and the links between their goals and their social identity.

Emerging empirical evidence suggests specific neurobehavioral changes occurring in the systems of emotion and motivation that help account for these characteristics, Dahl suggested. Looking specifically at sensation-seeking, he noted first that it is important to parse exactly what it means. Is it reward-seeking, a craving for excitement or higher arousal? Is it an urge for novel experiences? Is it a willingness to tolerate stressing sensations in order to be admired and achieve status? Dahl pointed out important differences between sensation-seeking—an appetitive drive, a willingness to take risks to attain novel, varied, and stimulating experiences and feelings—and impulsivity, which he described as a tendency to take quick actions without engaging in careful thought in advance. He noted research indicating that impulsivity follows a more or less linear decline from age 10 to age 30. In contrast, sensation-seeking increases between ages 10 and 15. In general, sensation-seeking seems to reach its peak at the time of puberty, especially in males (Martin et al., 2002; Steinberg, 2008).

Dahl pointed out both individual and developmental differences in the ways humans react to this sort of experience, and he explored the possible explanations. As discussed earlier, the neural systems that govern motivation and emotions are in a dynamic state during adolescence. Cross-cultural studies have supported the notion that adolescents need to learn to master high-intensity situations—to show courage and master fear—in order to prepare for adult responsibilities. Dahl suggested, however, that in contemporary Western society there is a maturation gap. With puberty happening earlier than ever before, sensation-seeking impulses are activated earlier relative to the slow and gradual development of cognitive control and the capacity for self-control. Dahl sees the balance between the affective load (the cluster of factors that increase stress on adolescents) and the sources of regulatory control—both young people’s internal capacities and external controls on behavior—as a very delicate one. Figure 3-3 depicts this balance. Many factors can tip the balance in one direction or another: challenges or disadvantages in the family or broader environment, strong support structures in the family, or any of a host of individual differences (discussed further below).

FIGURE 3-3. Balance between affective load and sources of regulatory control.

FIGURE 3-3

Balance between affective load and sources of regulatory control. SOURCE: Dahl, 2009.

To illustrate the way in which the balance between stresses and regulation can tip, Dahl described the issues surrounding sleep in adolescents. At this age, he explained, changes in the circadian rhythms tend to make adolescents prefer to stay up later at night and sleep later in the morning. Because they are experiencing rapid growth and development, they also tend to need more sleep than they had in late childhood. Adolescents in contemporary Western society are also very busy with sports, homework, and many other activities. They have many social and electronic distractions in their lives and in their bedrooms, and they have considerable freedom to select their bedtimes. As a result, Dahl estimates that 30 to 50 percent of U.S. adolescents typically do not get adequate sleep. The consequences can include missed school time; sleepiness and decreased motivation; irritability; and difficulty with self-control of attention, emotion, and behavior. Insufficient sleep can also have direct effects on learning and memory consolidation; affect the metabolism in ways that promote obesity; increase the risk of using alcohol, nicotine, and other substances; and increase the risk of depression. The effects of being moderately sleep-deprived and imbibing moderate amounts of alcohol are about the same, Dahl explained, and together these two factors significantly increase the risk of impaired driving, for example.

More broadly, however, the sleep issue demonstrates how a biological change in adolescence can lead to a spiral of negative effects with potentially very significant consequences. It is the social context that has amplified the problem, he argued. Generations ago, when evening entertainment options and other distractions were far fewer, adolescents’ preference for altered sleeping patterns presumably had far less effect on their lives. In the current context, however, the result is more likely to be significant sleep deprivation, which may interact with other small changes that occur during adolescence (e.g., sensation-seeking, emotional volatility). The social context can amplify the effect of these changes because young people may have greater opportunity to take risks. All of this suggests to Dahl that improved understanding of the mechanisms that affect sensation-seeking, cognitive control, and emotional regulation could yield valuable insights for intervention. He stressed that it is important to remember, however, that although adolescence is a period of intensity when the developing sense of self is sculpted by context and experience, it is also a time when adolescents can be idealistic and passionate about positive goals, whether in sports, literature, the arts, or politics and can begin trying to change the world in positive ways.

PUBERTY AND NEUROENDOCRINE CHANGES

Biologically, puberty is the developmental phase at which humans first become capable of begetting or bearing children, and this development is governed by the brain, as Susman explained. The brain is responsible for reproductive maturation, physical growth, and the behavior changes that are associated with puberty. These changes involve complex processes, and, Susman observed, this means not only that there is room for considerable individual variation, but also that adolescents and their environments influence their own development. Puberty is initiated and governed by the hypothalamus, the brain structure that controls metabolic processes and secretes neurohormones. As puberty begins, the hypothalamus activates the hypothalamic-pituitary-gonadal axis, which stimulates the release of ovarian and testicular hormones (estrogen and testosterone), and the effects are observable in rapid growth and the development of secondary sexual characteristics. Researchers are not certain exactly what turns on this process, Susman explained, but several factors seem to play a role: genes, neuroendocrine changes, and environmental factors. Obesity in girls, for example, is associated with early puberty; toxic substances in the environment may either delay or precipitate it; and some research has suggested that family influences may even play a role.

Puberty putatively plays an important role in risk-taking. Most risk behaviors are first evident at approximately the ages of 10 to 12 (Ge et al., 2006), when neuroendocrine changes are occurring. Elevated levels of testosterone have been particularly associated with aggressive risk-taking in boys; in girls, elevated testosterone is associated with the tendency to affiliate with deviant peers (Vermeersch et al., 2008). Both testosterone and estrogen are also recognized as having an energizing effect, and thus puberty has been described as a time of awakening to both pleasure and risk. In contemporary Western society, that frequently means experimentation with drugs and sex. Studies in animals have also supported a general association between elevated levels of the gonadal hormones and aggressive behavior (Sato et al., 2008). Researchers have suggested the possibility that individuals who experience puberty unusually early or late have a higher propensity for risk behaviors. Early puberty is a particular risk for girls, although the literature is less conclusive about effects for boys (Negriff and Susman, in press).

Establishing a causal link between hormones and risk-taking has been challenging, however, Susman explained. The practice of treating certain disorders in adolescents with testosterone or estrogen has permitted researchers to examine their effects on aggression, other behavior problems, cognition, and other phenomena. A study using a randomized control design of adolescents with delayed puberty who received hormones showed that some, but not all, of the subjects responded to the treatment, and that some showed increased aggression, although there were differences associated with both gender and dose (Belsky et al., 2007). Other research (Paus et al., 2008) has focused on a possible connection between the development of white matter in the brain (discussed earlier) and the timing of puberty and risk-taking and has found some support for this connection in males. Tarter and colleagues (2007) have also found that elevated levels of testosterone foster social dominance, which is associated with behavior that violates norms and, in turn, an increased proclivity for substance use.

Susman concluded that there are three converging lines of evidence on puberty that relate to risk-taking. First, puberty is a highly sensitive period for steroid-dependent brain organization. That is, as both Casey and Spear also observed, the brain undergoes significant transition during puberty, which seems to be significantly affected by environmental factors. Second, testosterone levels are quite variable during this stage. And finally, testosterone is linked to dominance, which is related to aggressive behavior, which may in turn also promote risk-taking. Although less attention has focused on the effects of estrogen, Susman noted, it is likely that this hormone also influences risk behaviors.

Susman identified a few implications of these findings for risk prevention. First, if parents understand the ways in which these neuroendocrine changes can affect their children’s behavior, they will be better prepared. Because early puberty in girls heightens the risk of pregnancy, for example, parents may need to address issues of sexuality earlier than they might expect. Pregnancy prevention programs, Susman argued, should also target much younger children than they currently do. Programs for 13- and 14-year-olds may be too late. Fourth graders are well into the onset of puberty, and although the younger girls may be at low risk for early pregnancy, they may be at higher risk for not using safe sex practices (e.g., not using condoms). Her final point was that adolescents have a considerable influence on the context in which they are developing—a point that is discussed further in later sections.

SUMMARY

The physical and biochemical development taking place during adolescence is complex, and the presenters highlighted not only ways that these processes affect behavior, but also ways they may interact with one another and with social influences on behavior. The imbalance between the gradual development of the prefrontal cortex, which, among other things, supports self-control, and the more rapidly developing limbic system, which, in turn, governs appetite and pleasure-seeking, helps to explain why adolescents are prone to seek novelty and take risks. At the same time, as young people reach puberty, they are faced with an array of social pressures as well as neuroendocrine changes that can affect their moods and focus their attention on sexuality and sensation-seeking. The average age for puberty has declined, and the gap between these developments and the development of the cognitive capacity for self-control is even greater than before. In modern Western cultures, many of the tempting risk behaviors are far less potentially beneficial than those for which humans may originally have adapted.

Spear explained that white matter in the brain is made up of collections of axons that are myelinated, that is, insulated by a fatty substance that appears white. It is thought that the myelination enhances transmission of signals across the brain.

See Casey et al. (2008) for a discussion of brain imaging studies related to this point.

Footnotes

1

Spear explained that white matter in the brain is made up of collections of axons that are myelinated, that is, insulated by a fatty substance that appears white. It is thought that the myelination enhances transmission of signals across the brain.

2

See Casey et al. (2008) for a discussion of brain imaging studies related to this point.

Copyright © 2011, National Academy of Sciences.
Bookshelf ID: NBK53414
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