Hockenbury Discovering Psychology 5th txtbk

396 CHAPTER 9 Lifespan DevelopmentFOCUS ON NEUROSCIENCEThe Adolescent Brain: A Work in ProgressFor many adolescents, the teenage years, especially the earlyones, seem to seesaw between moments of exhilaration andexasperation. Impressive instances of insightful behavior arecounterbalanced by impulsive decisions made with no considerationof the potential risks or consequences. How can erraticadolescent behavior be explained?For many years, the unpredictable behavior and mood swings ofadolescents were explained as being due to “raging hormones.”However, researchers have actually found little connectionbetween hormone levels and adolescent behavior. As researcherRonald Dahl (2003) explains, “High levels of sex hormones arenot the cause of emotional problems in adolescents. Many adolescentswith peak hormone levels experience no emotional difficultiesat all.” Rather than raging reproductive hormones, theexplanation seems to lie within the adolescent brain.To track changes in the developing brain, neuroscientists JayGiedd, Elizabeth Sowell, Paul Thompson, and their colleagueshave used magnetic resonance imaging (MRI) since the early1990s to repeatedly scan the brains of normal kids andteenagers. One striking insight produced by their studies is thatthe human brain goes through not one but two distinct spurtsof brain development—one during prenatal development andone during late childhood just prior to puberty (Giedd & others,1999; Gogtay & others, 2004a; Lenroot &Giedd, 2006).ZITSEarlier in the chapter, we described hownew neurons are produced at an astonishingrate during the first several months ofprenatal development—so much so that bythe sixth month of prenatal development,there is a vast overabundance of neurons inthe fetal brain. During the final monthsof prenatal development, there is fiercecompetition among the neurons to makeconnections and survive. In the neuronalequivalent of “survival of the fittest,” those neurons that don’tmake connections are eliminated. This process is called pruning.During the years of infancy and early childhood, the brain’souter gray matter continues to develop and grow. The tapestryof interconnections between neurons becomes much more intricateas dendrites and axon terminals multiply and branch toextend their reach. White matter also increases as groups of neuronsdevelop myelin, the white, fatty covering that insulatessome axons, speeding communication between neurons.Outwardly, these brain changes are reflected in the increasingcognitive and physical capabilities of the child. But in the brainitself, the “use-it-or-lose-it” principle is at work: Unused neuroncircuits are being pruned. While it may seem counterintuitive,the loss of unused neurons and neuronal connections actuallyimproves brain functioning by making the remaining neuronsmore efficient in processing information.By 6 years of age, the child’s brain is about 95 percent of itsadult size. This well-documented fact led to the mistaken beliefthat brain development is essentially complete by late childhood.But the longitudinal MRI studies of normal kids and adolescentsrevealed something very surprising—a second wave of gray matteroverproduction just prior to puberty, followed by a secondround of neuronal pruning during the teenage years (Sowell &others, 2004).Zits cartoon, © Zits Partnership. Reprinted withspecial permission of King Features Syndicate.menarche in girls. Why would such factors influence the timing of puberty?Although researchers are trying to pinpoint the exact mechanisms, part of theanswer is that stressful family events increase many of the same hormones that areinvolved in activating puberty. On the other hand, positive family environments areassociated with later physical development (see Ellis, 2004; Ellis & Essex, 2007;Romans & others, 2003).Early Versus Late MaturationAdolescents tend to be keenly aware of the physical changes they are experiencingas well as of the timing of those changes compared with their peer group. Most adolescentsare “on time,” meaning that the maturational changes are occurring atroughly the same time for them as for others in their peer group.However, some adolescents are “off time,” experiencing maturation noticeablyearlier or later than the majority of their peers. For girls, early maturation seems tocarry a greater risk for a variety of negative health and psychological outcomes. For