The Defining Decade by Meg Jay: Summary & Notes

Rated: 8/10

Available at: Amazon

ISBN: 0446561754

Related: 12 Rules for Life, Tribe of Mentors


I was pleasantly surprised by this book.  It’s an easy read, a mix of psychology research and anecdotal experience, and touches on all the key points of how to live in your twenties.  I found lots of instances where I’ve had the same thought patterns as her patients, which made it extremely relevant.

Recommended for anyone in their late teens, twenties, or parents with kids around that age group, as it will be invaluable for both.  I’ll be gifting it to lots of my friends in the near future.


The Defining Decade

  • In a study of life-span development, researchers found important events that determined the years ahead were most heavily concentrated during the twenties.
  • About 80 percent of life’s most significant events take place by age 35.
  • The most substantial and lasting events - those that led to career success, family fortune, personal bliss or lack thereof - developed across days or weeks or months with little immediate dramatic effect.  
  • ie. we may not recognize the most impactful events in our lives as they happen.


  • When a lot has been left to do, there is enormous thirtysomething pressure to get ahead, get married, pick a city, make money, buy a house, enjoy life, go to graduate school, start a business, get a promotion, save for college and retirement, and have two or three children in a much shorter period of time.


Identity Capital

  • Identity capital is our collection of personal assets. It is the repertoire of individual resources that we assemble over time. These are the investments we make in ourselves, the things we do well enough, or long enough, that they become a part of who we are. Some identity capital goes on a résumé, such as degrees, jobs, test scores, and clubs. Other identity capital is more personal, such as how we speak, where we are from, how we solve problems, how we look.
  • Twentysomethings who take the time to explore and also have the nerve to make commitments along the way construct stronger identities. They have higher self-esteem and are more persevering and realistic. This path to identity is associated with a host of positive outcomes, including a clearer sense of self, greater life satisfaction, better stress management, stronger reasoning, and resistance to conformity.
  • The longer it takes to get our footing in work, the more likely we are to become, as one journalist put it, different and damaged. Research on underemployed twentysomethings tells us that those who are underemployed for as little as nine months tend to be more depressed and less motivated than their peers—than even their unemployed peers. But before we decide that unemployment is a better alternative to underemployment, consider this: Twentysomething unemployment is associated with heavy drinking and depression in middle age even after becoming regularly employed.

The Strength of Weak Ties

  • Our strong ties feel comfortable and familiar but, other than support, they may have little to offer. They are usually too similar—even too similarly stuck—to provide more than sympathy. They often don’t know any more about jobs or relationships than we do.
  • Weak ties feel too different or, in some cases, literally too far away to be close friends. But that’s the point. Because they’re not just figures in an already ingrown cluster, weak ties give us access to something fresh. They know things and people that we don’t know. Information and opportunity spread farther and faster through weak ties than through close friends because weak ties have fewer overlapping contacts.
  • When I encourage twentysomethings to draw on the strength of weak ties, there is often a fair amount of resistance: I hate networking or I want to get a job on my own or That’s not my style are common reactions. I get it, but that doesn’t change the fact that, as we look for jobs or relationships or opportunities of any kind, it is the people we know the least well who will be the most transformative. New things almost always come from outside your inner circle.

The Ben Franklin Effect

  • We imagine that if people like us, then they do us favors because this is how it works in the urban tribe. But the Ben Franklin effect, and subsequent empirical studies, show it works the other way around with people we know less well.
  • If weak ties do favors for us, they start to like us. Then they become even more likely to grant us additional favors in the future. Franklin decided that if he wanted to get someone on his side, he ought to ask for a favor. And he did.
  • I would advise the same approach today as you ask your own weak ties for letters of recommendation, suggestions or introductions, or well-planned informational interviews: Make yourself interesting. Make yourself relevant. Do your homework so you know precisely what you want or need. Then, respectfully, ask for it. Some weak ties will say no. More than you think will say yes. The fastest route to something new is one phone call, one e-mail, one box of books, one favor, one thirtieth birthday party.

The Unthought Known

  • There is a certain terror that goes along with saying "My life is up to me." It is scary to realize there’s no magic, you can’t just wait around, no one can really rescue you, and you have to do something. Not knowing what you want to do with your life—or not at least having some ideas about what to do next—is a defense against that terror. It is a resistance to admitting that the possibilities are not endless. It is a way of pretending that now doesn’t matter. Being confused about choices is nothing more than hoping that maybe there is a way to get through life without taking charge.

The Search for Glory and the Tyranny of the Should

  • Contrary to what we see and hear, reaching your potential isn’t even something that usually happens in your twenties—it happens in your thirties or forties or fifties.

The Customized Life

  • If the first step in establishing a professional identity is claiming our interests and talents, then the next step is claiming a story about our interests and talents, a narrative we can take with us to interviews and coffee dates. Whether you are a therapist or an interviewer, a story that balances complexity and cohesion is, frankly, diagnostic. Stories that sound too simple seem inexperienced and lacking. But stories that sound too complicated imply a sort of internal disorganization that employers simply don’t want.
  • No matter what company or program someone applies to, a sort of game goes on. Interviewers want to hear a reasonable story about the past, present, and future. How does what you did before relate to what you want to do now, and how might that get you to what you want to do next? Everyone realizes most applicants don’t actually know what their careers will look like. Even the ones who think they do often change their minds.


An Upmarket Conversation

  • Popular magazines portray a twentysomething culture dominated by singles who are almost obsessed with avoiding commitment. But behind closed doors, I hear a different story. I have yet to meet a twentysomething who doesn’t want to get married or at least find a committed relationship.
  • The most recent studies show that marrying later than the teen years does indeed protect against divorce, but this only holds true until about age twenty-five. After twenty-five, one’s age at marriage does not predict divorce.

The Cohabitation Effect

  • As Jennifer spoke, one assumption was easy to spot: Living together is a good test for marriage. This is a common misperception.
  • But couples who live together first are actually less satisfied with their marriages and more likely to divorce than couples who do not. This is what sociologists call the cohabitation effect.

Sliding, Not Deciding

  • Moving from dating to sleeping over to sleeping over a lot to cohabitation can be a gradual slope, one not marked by rings or ceremonies or sometimes even a conversation. Couples often bypass talking about why they want to live together and what it will mean.
  • To understand why, it helps to know that the cohabitation effect is technically a pre-engagement cohabitation effect, not a premarital cohabitation effect. Couples who live together before marriage but after becoming engaged, who combine their lives after making a clear and public commitment, are not any more likely to have distressed or dissolved marriages than couples who do not cohabitate before marriage. They do not suffer from the cohabitation effect.


  • Lock-in is the decreased likelihood to search for other options, or change to another option, once an investment in something has been made. The initial investment, called a setup cost, can be big or small.
  • Switching costs—or the time, money, or effort it requires to make a change—are more complex. When we make an initial investment in something, switching costs are hypothetical and in the future, so we tend to underestimate them.
  • Cohabitation is loaded with setup and switching costs, the basic ingredients of lock-in. Moving in together can be fun and economical, and the setup costs are subtly woven in.
  • There are things you can do to lessen the cohabitation effect. One is, obviously, don’t cohabitate before an engagement. Since this is not an entirely realistic suggestion, researchers also recommend getting clear on each person’s commitment level before you move in, and anticipating and regularly evaluating those constraints that may keep you from leaving even if you want to. There are also other ways to test a relationship besides moving in, including doing a wider variety of activities together than dating and sex.

Being in Like

  • Studies have repeatedly found that couples who are similar in areas such as socioeconomic status, education, age, ethnicity, religion, attractiveness, attitudes, values, and intelligence are more likely to be satisfied with their relationships and are less likely to seek divorce.
  • Finding someone like you might seem easy, but there is a twist—not just any similarity will do. Dating and married couples do tend to be similar to each other in attractiveness, age, education, political views, religion, and intelligence.
  • The problem is, while people are good at matching themselves and others on relatively obvious criteria, such as age and education, it turns out that these qualities are what researchers call deal breakers, not match makers.
  • Deal breakers are your own personal sine qua non in relationships. They are qualities—almost always similarities—you feel are nonnegotiable. The absence of these similarities allows you to weed out people with whom you have fundamental differences.
  • One match maker to consider is personality. Some research tells us that, especially in young couples, the more similar two people’s personalities are, the more likely they are to be satisfied with their relationship. Yet personality is how dating, and even married, couples tend to be least alike. The likely reason for this is, unlike deal breakers, personality is less obvious and not as easy to categorize. Personality is not about what we have done or even about what we like. It is about how we are in the world, and this infuses everything we do.

The Big Five

  • One of the simplest and most widely researched models of personality is what is called the Big Five. The Big Five refers to five factors that describe how people interact with the world: Openness, Conscientiousness, Extraversion, Agreeableness, Neuroticism. Just by reading about the Big Five and considering your own behavior, it is pretty easy to tell whether you fall on the high end or the low end, or somewhere in the middle, of the five dimensions.
  • The Big Five is not about what you like—it is about who you are, it is about how you live. The Big Five tells us how you wake up in the morning and how you go about doing most anything. It has to do with how you experience the world and, as a result, how others experience you. This is important because, when it comes to personality, wherever you go, there you are.
  • Consider that where we are on the Big Five is about 50 percent inherited. This means that you came into this world with roughly half of who you are already in place, because of genes, prenatal influences, and other biological factors. While you learn to interact with the world somewhat differently as experiences make their mark, personality remains relatively stable over time.
  • When you figure out your highs, mediums, and lows, you have a general profile of your personality, one that should describe your behavior across different situations and times. You can do the same for anyone that you know well, or are starting to know well, and this will bring into relief how similar—or dissimilar—your personalities are. There is no right or wrong personality, there is just your personality and how it fits with the personalities of other people. While it is not better or worse to be high or low or in the middle of the dimensions of the Big Five, it is often the case that we like or dislike people because of the way their extremes compare to our own.
  • Being on the high end of the Neuroticism dimension is toxic for relationships.

The Brain and the Body

Forward Thinking

  • Forward thinking doesn’t just come with age. It comes with practice and experience. That’s why some twenty-two-year-olds are incredibly self-possessed, future-oriented people who already know how to face the unknown, while some thirty-four-year-olds still have brains that run the other way.

Calm Yourself

  • Twentysomethings take these difficult moments particularly hard. Compared to older adults, they find negative information—the bad news—more memorable than positive information—or the good news. MRI studies show that twentysomething brains simply react more strongly to negative information than do the brains of older adults. There is more activity in the amygdala—the seat of the emotional brain.
  • When twentysomethings have their competence criticized, they become anxious and angry. They are tempted to march in and take action. They generate negative feelings toward others and obsess about the why: Why did my boss say that? Why doesn’t my boss like me? Taking work so intensely personally can make a forty-hour workweek long indeed.
  • Knowing what to overlook is one way that older adults are typically wiser than young adults. With age comes what is known as a positivity effect. We become more interested in positive information, and our brains react less strongly to what negative information we do encounter. We disengage with interpersonal conflict, choosing to let it be, especially when those in our network are involved.
  • Twentysomethings and their active amygdalae often want to change their feelings by changing their jobs. They quit work that becomes messy or unpleasant, or they storm in and complain to their bosses’ bosses, not realizing that their bosses’ bosses’ amygdalae are unlikely to be as worked up as their own. If Danielle left her job, she would feel better for a time. But quitting would also only confirm her fear: that she was a poseur who didn’t belong in a good job anyway.
  • Research shows that people who have some control over their emotions report greater life satisfaction, optimism, purpose, and better relationships with others.

Outside In

  • Growth mindset: believe that people can change, and success is something to be achieved.
  • Fixed mindset: belief that something is inherent, or fixed (in this case, innate confidence in work).
  • Decades of research in schools tells us that a fixed mindset gets in the way of success.
  • Confidence doesn’t come from the inside out. It moves from the outside in. People feel less anxious—and more confident—on the inside when they can point to things they have done well on the outside.
  • Real confidence comes from mastery experiences, which are actual, lived moments of success, especially when things seem difficult. Whether we are talking about love or work, the confidence that overrides insecurity comes from experience. There is no other way.
  • For work success to lead to confidence, the job has to be challenging and it must require effort. It has to be done without too much help. And it cannot go well every single day. A long run of easy successes creates a sort of fragile confidence, the kind that is shattered when the first failure comes along. A more resilient confidence comes from succeeding—and from surviving some failures.
  • The real challenge of the twentysomething years is the work itself. Ten thousand hours is five years of focused, full-time work (40 hours × 50 work weeks a year = 2,000 hours a year × 5 years = 10,000 hours) or ten years of less-targeted work (20 hours × 50 work weeks a year = 1,000 hours a year × 10 years = 10,000 hours). My ten thousand hours were seven years of graduate school.

Getting Along and Getting Ahead

  • Our personalities change more during the twentysomething years than at any time before or after.
  • Numerous studies from around the world show that life starts to feel better across the twentysomething years. We become more emotionally stable and less tossed around by life’s ups and downs. We become more conscientious and responsible. We become more socially competent. We feel more agreeable about life and more able to cooperate with others. Overall, we become happier and more confident.
  • In our twenties, positive personality changes come from what researchers call getting along and getting ahead. Feeling better doesn’t come from avoiding adulthood, it comes from investing in adulthood.
  • The investments we make in work and love trigger personality maturation. Being a cooperative colleague or a successful partner is what drives personality change. Settling down simply helps us feel more settled. Twentysomethings who don’t feel like they are getting along or getting ahead, on the other hand, feel stressed and angry and alienated.
  • Even simply having goals can make us happier and more confident—both now and later. In one study that followed nearly five hundred young adults from college to the mid-thirties, increased goal-setting in the twenties led to greater purpose, mastery, agency, and well-being in the thirties.
  • Outside of work, commitments to others also foster change and well-being. Studies in the United States and Europe have found that entering into stable relationships helps twentysomethings feel more secure and responsible, whether these relationships last or not. Steady relationships reduce social anxiety and depression as they help us feel less lonely and give us the opportunity to practice our interpersonal skills.
  • Being single while you’re young may be glorified in the press, but staying single across the twenties does not typically feel good. A study that tracked men and women from their early twenties to their later twenties found that of those who remained single—who dated or hooked up but avoided commitments—80 percent were dissatisfied with their dating lives and only 10 percent didn’t wish they had a partner.
  • Being chronically uncoupled may be especially detrimental to men, as those who remained single throughout their twenties experienced a significant dip in their self-esteem near thirty.

Every Body

  • What is about to follow are some sobering statistics about having babies after the age of thirty-five. Medicine has been called a science of uncertainty and an art of probability, and this holds especially true for reproductive medicine. It is an imperfect science, so not all pre-thirty-five women will easily have the babies they want, nor is it true that those over thirty-five will not. But there are some age-related changes that everyone who wants children would be better off understanding.
  • Researchers are beginning to find that older sperm may be associated with various neurocognitive problems in children, including autism, schizophrenia, dyslexia, and lower intelligence. For this reason, and for reasons we will discuss further into the chapter, both men and women ought to be thinking about the timing of babies.
  • Fertility, or the ability to reproduce, peaks for women during the late twentysomething years. Biologically speaking, the twenties will be the easiest time to have a baby for most women. Some declines in fertility begin at about thirty and at thirty-five, a woman’s ability to become pregnant and carry a baby to term drops considerably. At forty, fertility plummets.
  • Compared to their twentysomething selves, women are about half as fertile at thirty, about one-quarter as fertile at thirty-five, and about one-eighth as fertile at forty.
  • The first signs of decreased fertility are difficulty becoming and staying pregnant. Trying au natural—just having sex around the time of ovulation—a woman has about a 20 to 25 percent chance of conceiving during each cycle, up to about age thirty-five. So when you’re young it takes, on average, about four or five months of having sex to get pregnant.
  • One indicator of how difficult it can be to have a baby as we age is the cost. The average cost of a fertility intervention for a twentysomething couple is $25,000. By thirty-five, the cost is about $35,000. After age 35, as the obstacles to pregnancy increase, so does the price tag. At forty, couples who need fertility treatments will pay an average of $100,000 for one live birth.

Do the Math

  • This study brings to life, at least digitally, a core problem in behavior: present bias. People of all ages and walks of life discount the future, favoring the rewards of today over the rewards of tomorrow. We would rather have $100 this month than $150 next month. We choose the chocolate cake and the new outfit now and face the gym and the credit card bill later. This isn’t a twentysomething tendency.
  • But twentysomethings are especially prone to present bias.
  • In my practice, I notice that many twentysomethings—especially those who surround themselves with other twentysomethings—have trouble anticipating life. They need memento vivi—or ways to remember they are going to live.
  • Present bias is especially strong in twentysomethings who put a lot of psychological distance between now and later. Love or work can seem far off in time.
  • The problem with feeling distant from the future is that distance leads to abstraction, and abstraction leads to distance, and round and round it goes.
  • A timeline may not be a virtual reality chamber, but it can help our brains see time for what it really is: limited. It can give us a reason to get up in the morning and get going.
  • Our twenties are when we have to start creating our own sense of time, our own plans about how the years ahead will unfold. It is difficult to know how to start our careers or when to start our families. It is tempting to stay distracted and keep everything at a distance. But twentysomethings who live beyond time usually aren’t happy.


Will Things Work Out for Me?

  • "The best part about being my age is knowing how my life worked out." —Scott Adams, cartoonist
  • There is no formula for a good life, and there is no right or wrong life. But there are choices and consequences, so it seems only fair that twentysomethings know about the ones that lie ahead. That way, the future feels good when you finally get there. The nicest part about getting older is knowing how your life worked out, especially if you like what you wake up to every day. If you are paying attention to your life as a twentysomething, the real glory days are still to come.
  • The future isn’t written in the stars. There are no guarantees. So claim your adulthood. Be intentional. Get to work. Pick your family. Do the math. Make your own certainty. Don’t be defined by what you didn’t know or didn’t do.
  • You are deciding your life right now.

Want to get my latest book notes? Subscribe to my newsletter to get one email a week with new book notes, blog posts, and favorite articles.

Thank you! Your submission has been received!
Oops! Something went wrong while submitting the form.