By Philip David Zelazo, PhD
This is the third feature of a multi-part series on the topic of executive function. Dr. Zelazo is the Nancy M. and John E. Lindahl Professor at the Institute of Child Development, University of Minnesota.
Executive function improves considerably in early childhood. The differences between an infant and a preschooler are enormous. Whereas infants are largely stimulus-bound, reacting immediately to events near them, and oriented to the present, preschoolers may think about the past and plan for the future, considering several options and then selecting one. But preschoolers' ability to consciously control their thoughts, actions, and emotions is still severely limited, and quite often their knowledge about what they should do surpasses their ability actually to do it. They can be unable to do “the right thing.”
In a recent study, three-year-old children were asked to help an experimenter decide: Should she have one candy now or several candies when it was time to go home? Children typically told the experimenter that she should wait and take the larger reward, which is perfectly rational advice. But when children themselves were presented with the same options (one now or more later), they almost always took the smaller, but immediate, reward. Despite knowing what to do and giving good future-oriented advice to the experimenter, children acted impulsively, opting for immediate instead of delayed gratification.
Of course, this general phenomenon is hardly restricted to the preschool period. Indeed, many parents of older children may be wondering whether children ever grow out of it, and praying that they do. Well, most children do… eventually. The abilities that make it possible to act in accord with one’s knowledge, the abilities underlying executive function, follow an extremely protracted developmental course that extends well into adolescence and probably into early adulthood. (Of course, some people seem to struggle with executive difficulties all their lives, but that is a topic for another installment.) As we will see in the next installment of this series, the slow development of executive function corresponds closely to the equally slow development of the prefrontal cortex, the most anterior (farthest forward) part of the brain.
Rebels without a cause
Difficulties with executive function may provide valuable insights into the problems children face as they develop through the school-age years, but the importance of executive function is perhaps especially clear during adolescence, which the paediatrician Ron Dahl defines as “that awkward period between sexual maturation and the attainment of adult roles and responsibilities.” The period is awkward at least in part because puberty is associated with a variety of emotional and motivational changes, for example romantic passions and sexual desires, that present strong challenges to a teenager’s slowly developing executive skills.
From the lovesick hero of Goethe's The Sorrows of Young Werther to the sexually precocious girls in Catherine Hardwicke's film Thirteen, we have seen, again and again, characterizations of adolescents struggling to make sense of themselves as their lives unfold, seemingly beyond their control. The Greek philosopher Plato gave us an image of Reason as a charioteer attempting to rein in Passion, a powerful and untamed steed. Dr. Dahl updates the metaphor and applies it specifically to adolescence: Teens, he says, are like unskilled drivers trying to manoeuvre a car that has just been turbo-charged by puberty. These metaphors illustrate well how self-control, such as choosing to delay gratification, depends on a competition between the top-down influence of executive function and the bottom-up influence of desires, drives, impulses, and habits. Sometimes it is anybody's guess, including the teen's, which type of influence will win the competition, and sometimes the stakes are high (see table).
Some possible consequences of poor executive function in adolescence
- mood swings
- suicidal ideation
Risk taking/impulsive behaviour
- alcohol/drug use
- unprotected sex
- alcohol/drug abuse
- preoccupations with appearance
- poor academic planning
Measuring adolescent executive function in the laboratory
Laboratory measures of executive function are often sensitive to changes that occur in late childhood and adolescence, and what these measures show is that executive function develops continuously at least until late adolescence (see figure). This is true both for “cool,” cognitive measures of executive function that are operative in abstract reasoning and problem solving, and for more “hot,” emotional measures that are operative in personally meaningful situations.
Cool measures include the Stroop Colour Word Task, introduced in , and the Dimensional Change Card Sort (DCCS), introduced in . Performance on the Stroop task, which measures one’s ability to suppress the tendency to read the words and name the colour in which they are printed, improves from when children are proficient readers until at least age 17 years. So does performance on more difficult versions of the DCCS, a measure of one's ability to flexibly redirect or shift one's attention to a new dimension. Less is known about the development of hot executive function, but hot executive function may be even more relevant than cool executive function during the emotional storm and stress of adolescence.
One measure of hot executive function is the Iowa Gambling Task, which measures decision-making about uncertain and meaningful events. In this task, participants are presented with four decks of cards that, when turned, reveal a combination of gains and losses, measured in play money. Participants are given a “stake” with which to play and told to win as much as possible by choosing cards from any of the four decks, one card per trial. As it happens, two of the decks are alluring but risky, and ultimately disadvantageous: cards in these decks yield high rewards ($100) on every trial but occasionally and unpredictably cause whopping losses, resulting in a net loss over trials. The other two decks offer a slow and steady route to the accumulation of wealth: small rewards on every trial together with occasional small losses. Adolescents, like adults with damage to prefrontal cortex, often persist in choosing from the disadvantageous decks, and this seems to be especially true of teens with behaviour problems.
The rise and fall of executive function
Everyday, real-world indices of executive function clearly continue to develop in late adolescence and early adulthood. This is reflected, to some extent, in the relatively high rates of accidents and violent crimes, including homicide and suicide, that occur during this developmental period. It is with good reason that many rental-car companies refuse to rent cars to anyone under the age of 25 years: adults below this age are an unacceptable insurance risk. Thus, although performance on many laboratory measures of executive function appears to reach a “ceiling” by late adolescence, failing to show further improvement, it is possible that new measures will prove more sensitive to more subtle changes occurring beyond this age.
Even existing measures, however, are sensitive to changes in executive function that occur at the end of the lifespan, when difficulty exercising top-down, executive control over thoughts, actions, and emotions may reappear (see figure). For example, compared to young adults, elderly adults show a decline in performance on the Stroop task and on versions of the DCCS. Other well-known signs of aging, such as increased forgetfulness and unwanted intrusions in one’s speech, may be attributable to some extent to impaired executive function associated with the aging of prefrontal cortex.
Thus, the development of executive function appears to follow an inverted U-shaped curve when considered across the lifespan. Although other cognitive functions may show a similar developmental trajectory, there is some reason to believe that the curve is especially pronounced for executive function: functions that are the “last in” during childhood and adolescence may the “first out” during aging. But as we’ll see in the next installment of this series, which considers brain development and notes the important role that experience plays in the process, aging readers who continue to exercise their intellectual skills have reason for optimism.
In the next installment, Dr. Zelazo discusses how brain development affects executive function.