青少年的大腦

Teenage Brains

青少年的大腦

Beautiful Brains

美麗的大腦

Moody. Impulsive. Maddening. Why do teenagers act the way they do? Viewed through the eyes of evolution, their most exasperating traits may be the key to success as adults.

喜怒無常、衝動任性、惹人討厭。青少年為什麼會如此行事呢？從進化的角度來看，他們讓人惱火的特點中大多數可能是成年時取得成功的關鍵。

By David Dobbs

撰文：大衛·多布斯（David Dobbs）

Photograph by Kitra Cahana

攝影：克伊楚娃·卡阿納（Kitra Cahana）

Although you know your teenager takes some chances, it can be a shock to hear about them.

雖然你明知道十幾歲的孩子會冒險，但是聽說了他們做的有些事你還是會感覺很震驚。

One fine May morning not long ago my oldest son, 17 at the time, phoned to tell me that he had just spent a couple hours at the state police barracks. Apparently he had been driving "a little fast." What, I asked, was "a little fast"? Turns out this product of my genes and loving care, the boy-man I had swaddled, coddled, cooed at, and then pushed and pulled to the brink of manhood, had been flying down the highway at 113 miles an hour.

不久之前，在五月份的一個美好的早晨，我的大兒子（那時他17歲）打電話告訴我說，他剛在州警察所度過了幾個小時。很顯然，他車開得「有點兒快了。」我問，什麼叫「有點兒快」？結果，這個稟承了我的基因和關愛的產兒，這個我曾包在襁褓里、嬌慣著、對他柔聲細語的而如今已跌跌撞撞來到成年邊緣的小男人，竟然以每小時113英里的速度在高速公路上飛馳。

"That"s more than a little fast," I said.

「那可不是什麼有點兒快，」我說。

He agreed. In fact, he sounded somber and contrite. He did not object when I told him he"d have to pay the fines and probably for a lawyer. He did not argue when I pointed out that if anything happens at that speed—a dog in the road, a blown tire, a sneeze—he dies. He was in fact almost irritatingly reasonable. He even proffered that the cop did the right thing in stopping him, for, as he put it, "We can"t all go around doing 113."

他表示同意。事實上，他的聲音聽起來又沮喪又後悔。我說他得自己付罰金，如果必要的話還要自己付律師費，他都沒有表示反對。我說如果以那種速度開車時發生了什麼事——路上出現一條狗，爆胎，打了個噴嚏——他可能就沒命了，他也沒有爭辯。實際上，他通情達理得都讓人惱火。他甚至說那個制止他的警察做得很對，因為，正如他指出的，「大家不能總是每小時開113英里。」

He did, however, object to one thing. He didn"t like it that one of the several citations he received was for reckless driving.

但是，他對一件事表示反對。他討厭「魯莽駕駛」（對他指控的罪名之一）的說法。

"Well," I huffed, sensing an opportunity to finally yell at him, "what would you call it?"

「哦，」我被激怒了，覺得終於逮著機會沖他大吼了，「那你管這叫什麼？」

"It"s just not accurate," he said calmly. "?"Reckless" sounds like you"re not paying attention. But I was. I made a deliberate point of doing this on an empty stretch of dry interstate, in broad daylight, with good sight lines and no traffic. I mean, I wasn"t just gunning the thing. I was driving.

「那不夠準確，」他冷靜地說。「『魯莽』聽起來就像你根本沒過腦子。但我不是這樣。我慎重考慮過這件事，選擇了州際間一條空蕩蕩的高速公路，頭上是大大的太陽，路上視線良好，根本沒車經過。我的意思是說，我並沒有開槍射擊。我只是開車而已。」

"I guess that"s what I want you to know. If it makes you feel any better, I was really focused."

「這就是我想對你說的。如果能讓你感覺好點兒，我就算說到點兒上了。」

Actually, it did make me feel better. That bothered me, for I didn"t understand why. Now I do.

事實上，那番話的確讓我感覺好多了。但它也讓我很困擾，我不明白為什麼會那樣。現在我懂了。

My son"s high-speed adventure raised the question long asked by people who have pondered the class of humans we call teenagers: What on Earth was he doing? Parents often phrase this question more colorfully. Scientists put it more coolly. They ask, What can explain this behavior? But even that is just another way of wondering, What is wrong with these kids? Why do they act this way? The question passes judgment even as it inquires.

我兒子的高速冒險經歷引出了一個問題，這也是探究青少年人群的人們長久以來一直在追問的一個問題：他到底在做什麼？父母們的措詞通常更加豐富多彩。而科學家會相對冷靜些。他們問，如何解釋這種行為？但就算只是換了一種提問方式，這些孩子到底怎麼了？為什麼他們會這樣？提問時，問題本身就是種評判。

Through the ages, most answers have cited dark forces that uniquely affect the teen. Aristotle concluded more than 2,300 years ago that "the young are heated by Nature as drunken men by wine." A shepherd in William Shakespeare"s The Winter"s Tale wishes "there were no age between ten and three-and-twenty, or that youth would sleep out the rest; for there is nothing in the between but getting wenches with child, wronging the ancientry, stealing, fighting." His lament colors most modern scientific inquiries as well. G. Stanley Hall, who formalized adolescent studies with his 1904 Adolescence: Its Psychology and Its Relations to Physiology, Anthropology, Sociology, Sex, Crime, Religion and Education, believed this period of "storm and stress" replicated earlier, less civilized stages of human development. Freud saw adolescence as an expression of torturous psychosexual conflict; Erik Erikson, as the most tumultuous of life"s several identity crises. Adolescence: always a problem.

古往今來，大多數答案中提及了影響青少年的獨一無二的神秘力量。2300多年前，亞里士多德（Aristotle）指出「年輕人受天性驅使，就如醉漢受酒精驅使。」威廉·莎士比亞（William Shakespeare）《冬天的故事》（The Winter"s Tale）中，牧羊人希望「孩子與成人之間不再有差別，年輕人以後能夠安眠，因為二者之間本無差別，只有成人才會與孩子通姦、對古人不敬、偷竊、打仗。」他的輓歌也影響了現代大多數的科學探索。1904年，斯坦利·霍爾（G. Stanley Hall）在《青春期》一書中正式提出了他對青春期的研究成果：青春期的心理狀態以及與生理學、人類學、社會學、性、犯罪、宗教和教育的關係，他認為，這個「狂風暴雨」時期是人類發展早期階段、文明程度不高狀態的重現。弗洛伊德（Freud）認為青少年體現了一種痛苦的精神性慾的衝突，而艾里克·埃里克森（Erik Erikson）認為青少年處於人生各階段身份認同危機中最混亂的一個時期。青少年：反正就是麻煩。

Such thinking carried into the late 20th century, when researchers developed brain-imaging technology that enabled them to see the teen brain in enough detail to track both its physical development and its patterns of activity. These imaging tools offered a new way to ask the same question—What"s wrong with these kids?—and revealed an answer that surprised almost everyone. Our brains, it turned out, take much longer to develop than we had thought. This revelation suggested both a simplistic, unflattering explanation for teens" maddening behavior—and a more complex, affirmative explanation as well.

這種想法一直持續到20世紀晚期，研究者開發了腦成像技術，通過這種技術可以看到青少年大腦中的許多細節，從而得以追蹤大腦的生理髮展和活動方式的情況。成像工具提供了一種新的提問方式——這些孩子怎麼了？——也給出了一個幾乎讓所有人都感到意外的答案。結果表明，我們大腦的發展需要花費比我們以為的長得多的時間。這個新發現指出，對於青少年惹人討厭的行為，無論是直白的、過分簡單化的解釋還是肯定的、過分複雜化的詮釋，兩者沒什麼分別。

The first full series of scans of the developing adolescent brain—a National Institutes of Health (NIH) project that studied over a hundred young people as they grew up during the 1990s—showed that our brains undergo a massive reorganization between our 12th and 25th years. The brain doesn"t actually grow very much during this period. It has already reached 90 percent of its full size by the time a person is six, and a thickening skull accounts for most head growth afterward. But as we move through adolescence, the brain undergoes extensive remodeling, resembling a network and wiring upgrade.

20世紀90年代，第一次對青少年發展中的大腦進行全方位掃描（這是國家衛生研究院（NIH）的一個項目）旨在研究100多名青少年的成長過程，結果顯示，我們的大腦在12歲到25歲之間經歷了一次大規模的重組。這段時期，大腦並沒有怎麼長大。人6歲時，腦容量就已經達到成人的90%，之後頭部發育主要就是頭骨增厚。但是，隨著我們度過青春期，大腦進行大面積重組，就像一次網路和線路升級。

For starters, the brain"s axons—the long nerve fibers that neurons use to send signals to other neurons—become gradually more insulated with a fatty substance called myelin (the brain"s white matter), eventually boosting the axons" transmission speed up to a hundred times. Meanwhile, dendrites, the branchlike extensions that neurons use to receive signals from nearby axons, grow twiggier, and the most heavily used synapses—the little chemical junctures across which axons and dendrites pass notes—grow richer and stronger. At the same time, synapses that see little use begin to wither. This synaptic pruning, as it is called, causes the brain"s cortex—the outer layer of gray matter where we do much of our conscious and complicated thinking—to become thinner but more efficient. Taken together, these changes make the entire brain a much faster and more sophisticated organ.

剛開始，大腦的軸突（一種長的神經結構，神經元通過它向另一個神經元傳遞信號）在周圍那層叫作髓鞘（大腦中的白色物質）的脂肪物質的包裹下逐漸絕緣化，最終，軸突的傳輸速度能增大一百倍。與此同時，樹突（樹枝狀細胞突起，神經元通過它接受鄰近軸突發出的信號）更加細化，而且最常使用的突觸（小的化學連接體，軸突和樹突通過它傳遞信息）數量增多功能增強。而那些很少使用的突觸就開始萎縮。這種所謂的突觸剪除使得大腦皮層（外層灰色物質，我們的意識和複雜的思考都是在那兒進行的）變薄但效率提高。綜上所述，這些變化使整個大腦變成一個更靈活更精密的器官。

This process of maturation, once thought to be largely finished by elementary school, continues throughout adolescence. Imaging work done since the 1990s shows that these physical changes move in a slow wave from the brain"s rear to its front, from areas close to the brain stem that look after older and more behaviorally basic functions, such as vision, movement, and fundamental processing, to the evolutionarily newer and more complicated thinking areas up front. The corpus callosum, which connects the brain"s left and right hemispheres and carries traffic essential to many advanced brain functions, steadily thickens. Stronger links also develop between the hippocampus, a sort of memory directory, and frontal areas that set goals and weigh different agendas; as a result, we get better at integrating memory and experience into our decisions. At the same time, the frontal areas develop greater speed and richer connections, allowing us to generate and weigh far more variables and agendas than before.

這個成熟期一直持續到青春期（以前人們以為在小學階段就已經大部分都完成了）。20世紀90年代以來所做的腦成像顯示，無論是從大腦後部到前部，還是從靠近腦幹（執行古老的更基本的行為功能，比如，視力、運動和基本生命過程）的區域到進化得更新更複雜的思維區域，發生的生理變化是以慢波形式移動的。胼胝體（連接大腦左右兩半球，負責傳遞對高級大腦功能有重要意義的信息）在穩步增厚。海馬體（有點像計算機內存）和額葉（負責設定目標並權衡利弊）之間的聯繫也增強了，結果，我們做決定時就能更好地整合已有的記憶和經驗。與此同時，額葉的速度加快聯結也更豐富，我們就能比以前處理更多的變數和日常事項。

When this development proceeds normally, we get better at balancing impulse, desire, goals, self-interest, rules, ethics, and even altruism, generating behavior that is more complex and, sometimes at least, more sensible. But at times, and especially at first, the brain does this work clumsily. It"s hard to get all those new cogs to mesh.

這個發展過程正常化以後，我們就能更好地協調衝動、慾望、目標、自私、規則、道德甚至是利他、反應生成行為（這更為複雜，至少是更理智）。但有時候，尤其是剛開始時，大腦並不擅長做這些。各部分很難協調運轉。

Beatriz Luna, a University of Pittsburgh professor of psychiatry who uses neuroimaging to study the teen brain, used a simple test that illustrates this learning curve. Luna scanned the brains of children, teens, and twentysomethings while they performed an antisaccade task, a sort of eyes-only video game where you have to stop yourself from looking at a suddenly appearing light. You view a screen on which the red crosshairs at the center occasionally disappear just as a light flickers elsewhere on the screen. Your instructions are to not look at the light and instead to look in the opposite direction. A sensor detects any eye movement. It"s a tough assignment, since flickering lights naturally draw our attention. To succeed, you must override both a normal impulse to attend to new information and curiosity about something forbidden. Brain geeks call this response inhibition.

皮茲堡大學（University of Pittsburgh）的精神病學教授貝亞特里斯·盧娜（Beatriz Luna）運用神經成像技術研究青少年的大腦，她通過一種簡單測試說明了這種學習曲線。盧娜掃描了孩子、青少年和二十多歲人在執行非眼跳任務（有點兒像是一種只用眼睛玩的電子遊戲，需要讓自己不去看突然出現的亮光）時的大腦。每當屏幕上其他位置出現閃爍的光，你就要去看屏幕中央出現的紅色十字游標。你收到的指令是不要去看亮光而要去看完全不同的方向。感測器會監測眼球運動。這項任務很難，因為人的注意力很容易被閃爍的光所吸引。為了完成任務，你必須不去理會想處理新信息的自然衝動以及對被禁止事物的好奇。大腦天才管這叫反應抑制。

Ten-year-olds stink at it, failing about 45 percent of the time. Teens do much better. In fact, by age 15 they can score as well as adults if they"re motivated, resisting temptation about 70 to 80 percent of the time. What Luna found most interesting, however, was not those scores. It was the brain scans she took while people took the test. Compared with adults, teens tended to make less use of brain regions that monitor performance, spot errors, plan, and stay focused—areas the adults seemed to bring online automatically. This let the adults use a variety of brain resources and better resist temptation, while the teens used those areas less often and more readily gave in to the impulse to look at the flickering light—just as they"re more likely to look away from the road to read a text message.

10歲孩子表現不理想，45%的時間都無法完成任務。青少年做得好一些。實際上，15歲的孩子如果有意識地抵制誘惑，他們就能做得和成人一樣好，70%到80%的時間都能得分。但是，盧娜的發現中最有趣的並不是得分。而是她在人們做測試時的所做的腦部掃描。與成年人相比，青少年很少使用大腦的以下區域：監控行為、發現錯誤、做計劃和保持專註，而成年人好像是「自動上線」。成年人運用大腦的各種資源有效地抵制誘惑，而較少使用那些區域的青少年通常更容易屈從於去看閃爍的光的衝動——正如他們更有可能不看路而去讀簡訊。

If offered an extra reward, however, teens showed they could push those executive regions to work harder, improving their scores. And by age 20, their brains respond to this task much as the adults" do. Luna suspects the improvement comes as richer networks and faster connections make the executive region more effective.

但是，如果有額外獎勵，青少年就能更努力運用那些執行區域，從而提高得分。20歲時，人的大腦對該任務的反應更加接近成年人。盧娜認為，更加豐富的神經網路和更加快速的連接速度使得執行區域更加有效。

These studies help explain why teens behave with such vexing inconsistency: beguiling at breakfast, disgusting at dinner; masterful on Monday, sleepwalking on Saturday. Along with lacking experience generally, they"re still learning to use their brain"s new networks. Stress, fatigue, or challenges can cause a misfire. Abigail Baird, a Vassar psychologist who studies teens, calls this neural gawkiness—an equivalent to the physical awkwardness teens sometimes display while mastering their growing bodies.

這些研究解釋了為什麼青少年會有令人惱火的前後矛盾的行為方式：早餐時令人愉悅，晚餐時令人討厭；星期一是一副精明能幹的派頭，星期六變得就像魂游天外似的迷茫。他們普遍缺乏經驗，還在學習如何使用大腦的新神經網路。壓力、疲勞或挑戰都會引發失敗。瓦薩大學（Vassar）研究青少年的心理學家阿比蓋爾·貝爾德（Abigail Baird）把這叫做神經笨拙（與青少年掌控成長中的身體時會表現出來的生理上的笨拙類同）。

The slow and uneven developmental arc revealed by these imaging studies offers an alluringly pithy explanation for why teens may do stupid things like drive at 113 miles an hour, aggrieve their ancientry, and get people (or get gotten) with child: They act that way because their brains aren"t done! You can see it right there in the scans!

腦成像展現出的這些緩慢而不均勻的發展弧線提出一種簡明卻很有吸引力的解釋，說明了青少年為什麼會做些愚蠢的事，比如車速達到每小時113英里，冒犯傳統，讓人懷孕或懷孕：他們會這麼做是因為他們的大腦根本沒做事！這一點你在屏幕上看得很清楚！

This view, as titles from the explosion of scientific papers and popular articles about the "teen brain" put it, presents adolescents as "works in progress" whose "immature brains" lead some to question whether they are in a state "akin to mental retardation."

正如大量科學論文和科普文章的標題所示，這個觀點把青少年描繪成「在成長中工作」，其「不成熟的大腦」導致一些人懷疑他們是否「類似於智力缺陷」。

The story you"re reading right now, however, tells a different scientific tale about the teen brain. Over the past five years or so, even as the work-in-progress story spread into our culture, the discipline of adolescent brain studies learned to do some more-complex thinking of its own. A few researchers began to view recent brain and genetic findings in a brighter, more flattering light, one distinctly colored by evolutionary theory. The resulting account of the adolescent brain—call it the adaptive-adolescent story—casts the teen less as a rough draft than as an exquisitely sensitive, highly adaptable creature wired almost perfectly for the job of moving from the safety of home into the complicated world outside.

但是，你現在看的內容告訴你了另一種關於青少年大腦的不同的科學解釋。在過去的五年左右，即使「在成長中工作」的說法傳入我們的文化中，對青春期大腦進行研究的這門學科也學會了更加全面地考慮問題。一些研究者開始站在一種更積極、更討好的角度看待近期大腦和基因的研究成果，這明顯是受到了進化理論的影響。關於青少年大腦的最終描述（稱其為適應性青少年的描述）不再將其看作一個草稿而是看作一個相當敏感、具有高度適應性的生物體，他們幾乎等不及要離開家的安全港灣到外面複雜的世界去。

This view will likely sit better with teens. More important, it sits better with biology"s most fundamental principle, that of natural selection. Selection is hell on dysfunctional traits. If adolescence is essentially a collection of them—angst, idiocy, and haste; impulsiveness, selfishness, and reckless bumbling—then how did those traits survive selection? They couldn"t—not if they were the period"s most fundamental or consequential features.

這種觀點很可能有利於青少年。更重要的是，它符合了生物學自然選擇的最基本規則。選擇對有功能障礙的特性非常不利。如果青少年本質上是這種集合體——焦慮、愚蠢、輕率，衝動、自私和不計後果、笨手笨腳——那麼這些特性是如何經過選擇而存在的呢？如果它們不是這個時期最基本或最重要的特性，就不可能存在。

The answer is that those troublesome traits don"t really characterize adolescence; they"re just what we notice most because they annoy us or put our children in danger. As B. J. Casey, a neuroscientist at Weill Cornell Medical College who has spent nearly a decade applying brain and genetic studies to our understanding of adolescence, puts it, "We"re so used to seeing adolescence as a problem. But the more we learn about what really makes this period unique, the more adolescence starts to seem like a highly functional, even adaptive period. It"s exactly what you"d need to do the things you have to do then."

答案是，那些令人惱火的特性並未真正描繪出青少年的個性；它們只是我們關注最多的東西，因為它們激怒了我們或者說把孩子們置於危險境地。威爾康奈爾醫學院（Weill Cornell Medical College）的神經科學家凱西（B. J. Casey）花了將近十年時間對大腦和基因進行研究，讓我們得以了解青春期，他指出「我們太過習慣把青少年當作麻煩了。但是，我們對到底是什麼令這段時期如此獨特這一問題研究得越多，青春期就越像是一個高度功能化甚至頗具適應性的時期。他們需要去做他們不得不做的事。」

To see past the distracting, dopey teenager and glimpse the adaptive adolescent within, we should look not at specific, sometimes startling, behaviors, such as skateboarding down stairways or dating fast company, but at the broader traits that underlie those acts.

過去無論是把青少年看作精神渙散迷迷糊糊的，還是將其看作具有內在適應性的，我們都不應該只留意那些偶爾出現的令人吃驚的特定行為，比如踩著滑板從樓梯上滑下來或與約會對象分分合合，而應該注意那些隱藏在這些行為背後的更具普遍性的品質。

Let"s start with the teen"s love of the thrill. We all like new and exciting things, but we never value them more highly than we do during adolescence. Here we hit a high in what behavioral scientists call sensation seeking: the hunt for the neural buzz, the jolt of the unusual or unexpected.

讓我們從青少年對刺激感的熱愛開始。人都喜歡新鮮刺激的事物，但只有青春期時才如此看重這些。這裡我們上升到了一個高度，行為科學家叫做感覺尋求：尋求精神刺激、罕見或意外帶來的強烈感受。

Seeking sensation isn"t necessarily impulsive. You might plan a sensation-seeking experience—a skydive or a fast drive—quite deliberately, as my son did. Impulsivity generally drops throughout life, starting at about age 10, but this love of the thrill peaks at around age 15. And although sensation seeking can lead to dangerous behaviors, it can also generate positive ones: The urge to meet more people, for instance, can create a wider circle of friends, which generally makes us healthier, happier, safer, and more successful.

尋求感覺不一定是衝動。你可能做了計劃——延緩張傘跳傘或飛車——而且非常謹慎，就像我兒子做的一樣。衝動從10歲左右開始，通常延續終生，但這種對刺激感的熱愛在15歲就達到頂峰。雖然感覺尋求會引發危險行為，但它也有積極的一面：比如，想認識更多的人能夠形成更大的朋友圈子，這種行為通常會讓我們更健康、更幸福、更安全也更成功。

This upside probably explains why an openness to the new, though it can sometimes kill the cat, remains a highlight of adolescent development. A love of novelty leads directly to useful experience. More broadly, the hunt for sensation provides the inspiration needed to "get you out of the house" and into new terrain, as Jay Giedd, a pioneering researcher in teen brain development at NIH, puts it.

這可能解釋了為什麼對新事物保持開放態度（不過有時可能是殺貓事件）仍然是青少年發展中的一個亮點。獵奇帶來了有用的經驗。正如傑伊·吉德（Jay Giedd）（國家衛生研究所（NIH）研究青少年大腦的一位先驅研究者）所說，從更廣泛的角度來看，尋求感覺為「走出家門」進入新環境提供了必要的激勵。

Also peaking during adolescence (and perhaps aggrieving the ancientry the most) is risk-taking. We court risk more avidly as teens than at any other time. This shows reliably in the lab, where teens take more chances in controlled experiments involving everything from card games to simulated driving. And it shows in real life, where the period from roughly 15 to 25 brings peaks in all sorts of risky ventures and ugly outcomes. This age group dies of accidents of almost every sort (other than work accidents) at high rates. Most long-term drug or alcohol abuse starts during adolescence, and even people who later drink responsibly often drink too much as teens. Especially in cultures where teenage driving is common, this takes a gory toll: In the U.S., one in three teen deaths is from car crashes, many involving alcohol.

冒險在青春期也達到最高峰（也許還有最激烈地冒犯傳統）。法庭審訊青少年的冒險行為比其他任何時候都多。這一點也在實驗室得到驗證，青少年在控制實驗中更傾向於冒險，無論是紙牌遊戲還是模擬駕駛都是如此。這表明，在現實生活中，大約15到25歲之間，各種冒險行為和危險結果都達到頂峰。這個年齡段死於意外的幾乎在各方面（除了工作事故之外）都占很高比率。長期濫用毒品或酗酒大都開始於青春期，甚至那些後來節制飲酒的人在青春期通常會飲酒過度。特別是在允許青少年駕駛的文化背景中，冒險使其付出了慘痛的代價：在美國，三分之一的青少年死於車禍，其中有些是酒後駕駛。

Are these kids just being stupid? That"s the conventional explanation: They"re not thinking, or by the work-in-progress model, their puny developing brains fail them.

這些孩子很蠢嗎？傳統的解釋是：他們不過腦子或只是個正在發展中的模型，微不足道的發展中的大腦讓他們失敗連連。

Yet these explanations don"t hold up. As Laurence Steinberg, a developmental psychologist specializing in adolescence at Temple University, points out, even 14- to 17-year-olds—the biggest risk takers—use the same basic cognitive strategies that adults do, and they usually reason their way through problems just as well as adults. Contrary to popular belief, they also fully recognize they"re mortal. And, like adults, says Steinberg, "teens actually overestimate risk."

但是，沒有證據支持這些說法。坦普爾大學（Temple University）專門研究青少年的發展心理學家勞倫斯·斯坦伯格（Laurence Steinberg）指出，即使在14到17歲那個最具冒險精神的年齡段，青少年運用的基本認知策略與成年人也一樣，他們通常通過問題思考道理，這就跟成年人是一樣的。同人們的一般認識相反，他們根本不是不知死活。斯坦伯格說，跟成年人一樣，「青少年只是低估了危險。」

So if teens think as well as adults do and recognize risk just as well, why do they take more chances? Here, as elsewhere, the problem lies less in what teens lack compared with adults than in what they have more of. Teens take more risks not because they don"t understand the dangers but because they weigh risk versus reward differently: In situations where risk can get them something they want, they value the reward more heavily than adults do.

所以，如果青少年跟成年人的思維方式一樣，也知道危險，那他們為什麼還要去冒險？這就跟其他方面是一樣的，問題不在於跟成年人相比他們缺少了什麼，而在於他們多了些什麼。青少年傾向於冒險，不是因為他們不知道危險而是因為他們對風險和回報的看法與成年人不同：當冒險能讓他們得到自己想要的東西時，他們就會把回報看得更重要（與成年人相比）。

A video game Steinberg uses draws this out nicely. In the game, you try to drive across town in as little time as possible. Along the way you encounter several traffic lights. As in real life, the traffic lights sometimes turn from green to yellow as you approach them, forcing a quick go-or-stop decision. You save time—and score more points—if you drive through before the light turns red. But if you try to drive through the red and don"t beat it, you lose even more time than you would have if you had stopped for it. Thus the game rewards you for taking a certain amount of risk but punishes you for taking too much.

斯坦伯格通過電子遊戲很好地說明了這一點。遊戲中，你要在最短時間內驅車穿越城市。沿途有幾個交通信號燈。跟現實生活一樣，你靠近交通信號燈時，它有時會從綠燈變黃燈，讓你必須快速決定穿過或停下。如果你在變紅燈前開了過去，你用的時間會更少，得分也就更高。但如果你嘗試這麼做卻沒成功，那麼你就會比停下來等花費更多時間。因此，在這個遊戲中，冒險會得到獎勵，而過於冒險就會受到懲罰。

When teens drive the course alone, in what Steinberg calls the emotionally "cool" situation of an empty room, they take risks at about the same rates that adults do. Add stakes that the teen cares about, however, and the situation changes. In this case Steinberg added friends: When he brought a teen"s friends into the room to watch, the teen would take twice as many risks, trying to gun it through lights he"d stopped for before. The adults, meanwhile, drove no differently with a friend watching.

青少年獨自駕駛時，如斯坦伯格形象化地稱為「冷靜」的情況下（獨自在一間空房子里），他們冒險的機率與成年人相似。但是，如果增加了青少年在意的獎勵，情況就不同了。斯坦伯格把朋友加進來的情況下：當他帶著青少年的朋友到房間來觀看時，青少年冒險的機率提高了兩倍，在之前他會停下來的亮燈的地方現在會拚命衝過去。但，在有朋友觀看的情況下，成年人開車沒有什麼變化。

To Steinberg, this shows clearly that risk-taking rises not from puny thinking but from a higher regard for reward.

對斯坦伯格而言，這清楚地表明，冒險增加並非由於考慮不周而是由於回報增加。

"They didn"t take more chances because they suddenly downgraded the risk," says Steinberg. "They did so because they gave more weight to the payoff."

「他們傾向於冒更多險不是因為不重視危險，」斯坦伯格說。「這麼做只是因為他們更看重回報。」

Researchers such as Steinberg and Casey believe this risk-friendly weighing of cost versus reward has been selected for because, over the course of human evolution, the willingness to take risks during this period of life has granted an adaptive edge. Succeeding often requires moving out of the home and into less secure situations. "The more you seek novelty and take risks," says Baird, "the better you do." This responsiveness to reward thus works like the desire for new sensation: It gets you out of the house and into new turf.

斯坦伯格和凱西這樣的研究者相信，這種衡量成本和回報的適當冒險之所以存在，是因為在人類進化過程中，青春期冒險的意願順應了一種適應的優勢。成功通常需要走出家門進入不夠安全的環境。「你尋求越多的新奇和冒險，」貝爾德說，「你做得就越好。」這種對獎勵的反應就像對新感覺的渴望一樣在起作用：它讓你走出家門進入新的活動領域。

As Steinberg"s driving game suggests, teens respond strongly to social rewards. Physiology and evolutionary theory alike offer explanations for this tendency. Physiologically, adolescence brings a peak in the brain"s sensitivity to dopamine, a neurotransmitter that appears to prime and fire reward circuits and aids in learning patterns and making decisions. This helps explain the teen"s quickness of learning and extraordinary receptivity to reward—and his keen, sometimes melodramatic reaction to success as well as defeat.

正如斯坦伯格的開車遊戲所指出的，青少年對社會獎勵的反應很強烈。生理和演化理論都解釋了這種趨勢。生理上青少年對多巴胺（一種神經遞質，重要的獎賞迴路開關，對學習方式和做出決策起輔助作用）的敏感度達到頂峰。這有助於解釋青少年學習速度快而且對獎勵特別有感覺，而且他對成功的熱衷就如同對失敗的反應一樣，有時候會很過激。

The teen brain is similarly attuned to oxytocin, another neural hormone, which (among other things) makes social connections in particular more rewarding. The neural networks and dynamics associated with general reward and social interactions overlap heavily. Engage one, and you often engage the other. Engage them during adolescence, and you light a fire.

青少年的大腦同樣對催產素（另一種神經激素，（在各種其他事物中）使人從社會關係中感受到更多回報）很敏感。神經網路和動力系統（與一般獎勵和社會互動相聯繫）有很多相互交疊的地方。觸及一方面，通常就會涉及另一方面。青春期時觸及了它們，就等於點燃了一團火。

This helps explain another trait that marks adolescence: Teens prefer the company of those their own age more than ever before or after. At one level, this passion for same-age peers merely expresses in the social realm the teen"s general attraction to novelty: Teens offer teens far more novelty than familiar old family does.

這有助於解釋另一種青少年身上的特質：青少年喜歡和同齡人在一起而不喜歡和比他們大或小的人在一起。在某個層面上，這種對同齡人的激情只是表達了青少年在社會領域中普遍為新奇事物所吸引：比起熟悉的家人，青少年彼此之間更具新奇性。

Yet teens gravitate toward peers for another, more powerful reason: to invest in the future rather than the past. We enter a world made by our parents. But we will live most of our lives, and prosper (or not) in a world run and remade by our peers. Knowing, understanding, and building relationships with them bears critically on success. Socially savvy rats or monkeys, for instance, generally get the best nesting areas or territories, the most food and water, more allies, and more sex with better and fitter mates. And no species is more intricately and deeply social than humans are.

但是，青少年為同齡人所吸引的另一個更為強大的原因是：投資對象是未來而非過去。我們進入一個由父母們組成的世界。但是我們將會活在由我們自己組成的世界裡，而且將在這個由我們的同齡人掌控和重塑的世界裡取得成功（或失敗）。認識、理解並且和他們建立關係對成功有著決定性的影響。比如，在老鼠或猴子之類的群居動物中，富有社會經驗的通常會得到最好的巢區或領域、最多的食物和水、更多的擁護者和與更優秀更健康的伴侶有更多的性活動。而且沒有哪種生物的社會關係比人類更加複雜和精密。

This supremely human characteristic makes peer relations not a sideshow but the main show. Some brain-scan studies, in fact, suggest that our brains react to peer exclusion much as they respond to threats to physical health or food supply. At a neural level, in other words, we perceive social rejection as a threat to existence. Knowing this might make it easier to abide the hysteria of a 13-year-old deceived by a friend or the gloom of a 15-year-old not invited to a party. These people! we lament. They react to social ups and downs as if their fates depended upon them! They"re right. They do.

這種具有決定性的人類特質使同伴關係不再是一件瑣碎小事而成了一件頭等大事。實際上，一些腦部掃描顯示，我們的大腦對同伴排斥的反應與對威脅身體健康或食物供應的反應一樣強烈。換句話說，在精神層面上，我們認為社會排斥是對生存的一種威脅。明白這一點就更容易接受一個13歲的孩子被朋友欺騙後表現出的歇斯底里或者是一個15歲的孩子沒接到聚會邀請時表現出的悶悶不樂。這些人！我們為之惋惜。他們對社交中的起起落落表現得好象自己的命運取決於那些似的！他們是對的。的確如此。

Excitement, novelty, risk, the company of peers. These traits may seem to add up to nothing more than doing foolish new stuff with friends. Look deeper, however, and you see that these traits that define adolescence make us more adaptive, both as individuals and as a species. That"s doubtless why these traits, broadly defined, seem to show themselves in virtually all human cultures, modern or tribal. They may concentrate and express themselves more starkly in modern Western cultures, in which teens spend so much time with each other. But anthropologists have found that virtually all the world"s cultures recognize adolescence as a distinct period in which adolescents prefer novelty, excitement, and peers. This near-universal recognition sinks the notion that it"s a cultural construct.

興奮、新奇、冒險、同伴相隨。這些特性不過意味著和朋友一起做一些愚蠢的新鮮事。但是，深入去看，你就會發現，無論是對個人而言還是對整個物種而言，這些特性（我們用來描述青少年的）使我們更具適應性。那就是為什麼這些特性（廣義上）毫無疑問地在人類幾乎各種文化中都有所體現，無論是現代社會還是原始部落。現代西方社會可能表現得更突出更明顯，那些青少年與同伴一起度過大量的時間。但人類學家發現，實際上世界上的所有文化都把青春期看作一個截然不同的時期，青少年喜歡新奇、刺激以及同伴。這個近乎普遍的認識弱化了一個概念，那就是青春期是一種文化構建的產物。

Culture clearly shapes adolescence. It influences its expression and possibly its length. It can magnify its manifestations. Yet culture does not create adolescence. The period"s uniqueness rises from genes and developmental processes that have been selected for over thousands of generations because they play an amplified role during this key transitional period: producing a creature optimally primed to leave a safe home and move into unfamiliar territory.

文化無疑塑造了青春期。影響了青春期的表達方式和可能持續的時間。文化還放大了青春期的表現。但是，青春期並不是由文化創造出來的。這個時期的獨一無二來自於基因和發展過程（這個過程經歷了數千代的進化），因為在這個關鍵的轉換時期青春期扮演了一種誇大的角色：催生一種最適宜準備離開安全的家進入不熟悉領域的生命體。

The move outward from home is the most difficult thing that humans do, as well as the most critical—not just for individuals but for a species that has shown an unmatched ability to master challenging new environments. In scientific terms, teenagers can be a pain in the ass. But they are quite possibly the most fully, crucially adaptive human beings around. Without them, humanity might not have so readily spread across the globe.

從家裡搬出去是人類做過的最困難的事，也是最危險的事，這不只是對個人而言，也是對整個物種而言，它表現出一種要掌控新的挑戰性環境的無與倫比的能力。從科學角度來看，青少年可能是個討厭鬼，但他們很可能就是最出色最徹底的環境適應者。沒有他們，人類可能就不會這麼容易地遍及整個地球。

This adaptive-adolescence view, however accurate, can be tricky to come to terms with—the more so for parents dealing with teens in their most trying, contrary, or flat-out scary moments. It"s reassuring to recast worrisome aspects as signs of an organism learning how to negotiate its surroundings. But natural selection swings a sharp edge, and the teen"s sloppier moments can bring unbearable consequences. We may not run the risk of being killed in ritualistic battles or being eaten by leopards, but drugs, drinking, driving, and crime take a mighty toll. My son lives, and thrives, sans car, at college. Some of his high school friends, however, died during their driving experiments. Our children wield their adaptive plasticity amid small but horrific risks.

無論具有適應性的青少年的這種觀點是多麼正確，都很難得到認同——父母們處理青少年那些特別惱人、執拗或絕對令人恐慌的時刻越多就越如此。讓人安心的是，人們可以把青少年令人擔憂的方面看作生物體在學習如何與周圍環境交涉的表現。但是，自然選擇是把雙刃劍，青少年的草率會帶來令人無法承受的後果。我們可能不用去冒在宗教戰鬥中被殺的危險或是被美洲豹吃掉的危險，但是毒品、酗酒、飛車和犯罪造成了重大的損失。我兒子沒車，他還好好地活著而且還在大學裡茁壯成長。但是，他高中時的一些朋友已經在嘗試飛車時死掉了。我們的孩子們在那些看似微小但卻可怕的冒險中運用著他們的適應環境的可塑性。

We parents, of course, often stumble too, as we try to walk the blurry line between helping and hindering our kids as they adapt to adulthood. The United States spends about a billion dollars a year on programs to counsel adolescents on violence, gangs, suicide, sex, substance abuse, and other potential pitfalls. Few of them work.

當然，我們作父母的也很困惑，我們想儘力把握好一個度，以在孩子適應成年期這條路上提供幫助和進行阻止。美國每年大概花十億美元製作節目建議青少年該如何處理暴力、幫派、自殺、性行為、藥物濫用以及其他潛在危險。但很少起作用。

Yet we can and do help. We can ward off some of the world"s worst hazards and nudge adolescents toward appropriate responses to the rest. Studies show that when parents engage and guide their teens with a light but steady hand, staying connected but allowing independence, their kids generally do much better in life. Adolescents want to learn primarily, but not entirely, from their friends. At some level and at some times (and it"s the parent"s job to spot when), the teen recognizes that the parent can offer certain kernels of wisdom—knowledge valued not because it comes from parental authority but because it comes from the parent"s own struggles to learn how the world turns. The teen rightly perceives that she must understand not just her parents" world but also the one she is entering. Yet if allowed to, she can appreciate that her parents once faced the same problems and may remember a few things worth knowing.

但是我們能夠幫上忙，而且的確幫上了忙。我們能夠阻止世界上的一些最糟糕的風險並且推動青少年去適應剩下的風險。研究表明，父母以一種柔和但堅定的手段參與指導青少年，與之保持聯繫但允許其獨立，他們的孩子通常會在現實生活中表現得更好。青少年想從朋友那裡學習大部分但並非全部內容。某種程度上，某些情況下（父母要做的就是確定在什麼情況下），青少年知道父母能提供某種智慧（這種知識之所以有價值不是因為父母的權威而是因為它是父母努力掌控變化的世界的經驗）。青少年準確地察覺到她必須理解的不只是父母的世界更是她自己將要進入的世界。但是，如果條件允許的話，她會意識到她父母也曾經面臨同樣的問題，也會記住一些有必要知曉的事。

Meanwhile, in times of doubt, take inspiration in one last distinction of the teen brain—a final key to both its clumsiness and its remarkable adaptability. This is the prolonged plasticity of those late-developing frontal areas as they slowly mature. As noted earlier, these areas are the last to lay down the fatty myelin insulation—the brain"s white matter—that speeds transmission. And at first glance this seems like bad news: If we need these areas for the complex task of entering the world, why aren"t they running at full speed when the challenges are most daunting?

由此引出了一個讓人疑惑的青少年大腦的最後一個不同之處，也是它的笨拙和非凡的適應性的最後一個關鍵點。這就是隨著青少年緩慢的成熟期最後完成發育的前額區所具有的持久可塑性。如早先提到的，這些區域是最後停止髓鞘（大腦中的白色物質，能提高信息傳輸速度）化的。乍一看，這好像是個壞消息：如果我們在進入社會時需要這些區域來執行複雜任務，那麼為什麼面臨最大挑戰時它們卻不能全速應戰呢？

The answer is that speed comes at the price of flexibility. While a myelin coating greatly accelerates an axon"s bandwidth, it also inhibits the growth of new branches from the axon. According to Douglas Fields, an NIH neuroscientist who has spent years studying myelin, "This makes the period when a brain area lays down myelin a sort of crucial period of learning—the wiring is getting upgraded, but once that"s done, it"s harder to change."

答案是，速度的獲得以靈活性的失去為代價。髓鞘化的外層能夠提高軸突的傳輸速度，但它也限制了軸突上新的分支生長。根據國家衛生研究院的神經科學家道格拉斯·菲爾茲（Douglas Fields）花了數年時間研究髓磷脂的結果來看，「這使得那個時期（大腦的一個區域停止產生髓磷脂的時候）成為學習的關鍵期——線路在升級，但升級一旦完成，就很難再發生改變。」

The window in which experience can best rewire those connections is highly specific to each brain area. Thus the brain"s language centers acquire their insulation most heavily in the first 13 years, when a child is learning language. The completed insulation consolidates those gains—but makes further gains, such as second languages, far harder to come by.

各個腦區獲得信息的渠道（通過這些渠道，經驗能最大限度地對神經網路進行重組）是非常具體的。因此，大腦語言中樞的絕緣化主要在頭13年中（也就是一個孩子學習語言的時期）完成。絕緣完成後已有收益得到鞏固，但新的收益（比如學習第二外語）就會受到妨礙。

So it is with the forebrain"s myelination during the late teens and early 20s. This delayed completion—a withholding of readiness—heightens flexibility just as we confront and enter the world that we will face as adults.

因此，前腦的絕緣化是在青少年晚期和二十歲初期完成的。這個完成過程的延遲（也延遲了準備就緒）增強了我們以成人身份進入世界時的靈活性。

This long, slow, back-to-front developmental wave, completed only in the mid-20s, appears to be a uniquely human adaptation. It may be one of our most consequential. It can seem a bit crazy that we humans don"t wise up a bit earlier in life. But if we smartened up sooner, we"d end up dumber.

這種長期的緩慢的從後到前的發展過程，直到25歲左右才結束，這好像是人類特有的一種適應能力。它可能也是我們最重要的適應能力之一。我們人類在生命早期並不明智，這聽起來有點瘋狂。但如果越早變聰明，我們最終就可能處于越愚蠢的境地。