The Defining Decade by Meg Jay (Review & Book Notes)

Tomas Laurinavicius
Updated on April 24, 2024

Your twenties matter more than you think, the decisions you make in your twenties define your life more than any other decisions you will make later in your life. You must choose your partners, family and career.

The Defining Decade is a must-read for every twentysomething looking to improve their life. Here are my notes from the book.

Author: Meg Jay

Originally published: 2012

Pages: 272

Genre: Psychology

Goodreads rating: ⭐️ 4.11/5

👉 Buy The Defining Decade on Amazon

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In this book, Meg Jay, a clinical psychologist and a professor at the University of Virginia, explains how the decade between your 20s and 30s is the most defining period of your life. She argues that the choices you make during this decade will have a profound impact on the rest of your life.

Biggest lesson: Your twenties are the most impactful years and you can build a great life by being intentional.

Notes & Quotes

Your twenties matter. Eighty percent of life’s most defining moments take place by age thirty – five. Two – thirds of lifetime wage growth happens in the first ten years of a career. More than half of us are married, or dating, or living with our future partner, by age thirty. Personality changes more during our twenties than at any time before or after. The brain caps off its last growth spurt in the twenties. Female fertility peaks at age twenty – eight.

It might even seem like adulthood is one long stretch of autobiographically consequential experiences — that the older we get, the more we direct our own lives. This is not true.

As a clinical psychologist who specializes in adult development, I have seen countless twentysomethings spend too many years living without perspective.

People imagine that to do therapy with twentysomethings is to listen to the adventures and misadventures of carefree people, and there is some of that. But behind closed doors, my clients have unsettling things to say: I feel like I’m in the middle of the ocean. Like I could swim in any direction but I can’t see land on any side so I don’t know which way to go.

I didn’t know I’d be crying in the bathroom at work every day.

There are fifty million twentysomethings in the United States, most of whom are living with a staggering, unprecedented amount of uncertainty.

Uncertainty makes people anxious, and distraction is the twenty-first – century opiate of the masses.

We think that by avoiding decisions now, we keep all of our options open for later — but not making choices is a choice all the same.

In almost all areas of development, there is what is called a critical period, a time when we are primed for growth and change, when simple exposure can lead to dramatic transformation. Children effortlessly learn whatever language they hear before the age of five. We develop binocular vision between three and eight months of age. These critical periods are windows of opportunity when learning happens quickly. Afterward, things are not so easy.

The twenties are that critical period of adulthood.

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.

Most important, identity capital is what we bring to the adult marketplace. It is the currency we use to metaphorically purchase jobs and relationships and other things we want.

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.

Twentysomething unemployment is associated with heavy drinking and depression in middle age even after becoming regularly employed.

The one thing I have learned is that you can’t think your way through life. The only way to figure out what to do is to do — something.

Everything can change in a day. Especially if you put yourself out there.

True interconnectedness rests not on texting best friends at one a.m., but on reaching out to weak ties that make a difference in our lives even though they don’t have to. When weak ties help, the communities around us — even the adult community that twentysomethings are warily in the process of entering — seem less impersonal and impenetrable. Suddenly, the world seems smaller and easier to navigate.

“He that hath once done you a kindness will be more ready to do you another than he whom you yourself have obliged.”

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.

It’s good to be good. There is a “helper’s high” that comes from being generous. In numerous studies, altruism has been linked to happiness, health, and longevity — as long as the help we give is not a burden.

Part of aging well is helping others, and twentysomethings who turn to weak ties for help give them a chance to do good and feel good — unless what they ask for is overwhelming.

Make yourself interesting. Make yourself relevant. Do your homework so you know precisely what you want or need. Then, respectfully, ask for it.

Uncertainty will always be part of the taking – charge process. — Harold Geneen, businessman

Ian told me his twentysomething years were like being in the middle of the ocean, like this vast, unmarked body of water. He couldn’t see land in any direction, so he didn’t know which way to go. He felt overwhelmed by the prospect that he could swim anywhere or do anything. He was equally paralyzed by the fact that he didn’t know which of the anything would work out. Tired and hopeless at age twenty – five, he said he was treading water to stay alive.

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.

Being confused about choices is nothing more than hoping that maybe there is a way to get through life without taking charge.

Unthought knowns are those things we know about ourselves but forget somehow. These are the dreams we have lost sight of or the truths we sense but don’t say out loud. We may be afraid of acknowledging the unthought known to other people because we are afraid of what they might think. Even more often, we fear what the unthought known will then mean for ourselves and our lives.

“Not making choices isn’t safe. The consequences are just further away in time, like in your thirties or forties.”

The best is the enemy of the good. — Voltaire, writer/philosopher

Part of realizing our potential is recognizing how our particular gifts and limitations fit with the world around us. We realize where our authentic potential actually lies.

Goals direct us from the inside, but shoulds are paralyzing judgments from the outside. Goals feel like authentic dreams while shoulds feel like oppressive obligations. Shoulds set up a false dichotomy between either meeting an ideal or being a failure, between perfection or settling. The tyranny of the should even pits us against our own best interests.

At a time when many twentysomethings yearn for somewhere to call home and have no idea where they might be in ten years, choosing a place can be incredibly useful.

But different is simple. Like the easiest way to explain black is to call it the opposite of white, often the first thing we know about ourselves is not what we are — it’s what we aren’t.

“You need to work. Your life needs to work.”

As a twentysomething, life is still more about potential than proof. Those who can tell a good story about who they are and what they want leap over those who can’t.

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.

This was a problem because while schools and companies want originality and creativity, they want communication and reasoning even more.

As we build a career, it seems there is a book, class, degree, consultant, or service available at every turn. Maybe that’s as it should be, because careers are important. But along the way, because of these very choice points, there is so much room for revision that developing your career in no way compares to choosing a partner or spouse. Maybe this was what David Brooks meant when he said that whom you marry is the most important decision in your life.

Every time somebody on Facebook changes their status to engaged or married, I panic. I’m convinced Facebook was invented to make single people feel bad about their lives.

Besides, like with work, good relationships don’t just appear when we’re ready. It may take a few thoughtful tries before we know what love and commitment really are.

Other things may change us, but we start and end with family. — Anthony Brandt, writer

Too often, being successful when you are young is about survival. Some people are good at hiding their troubles. They are good at “falling up.”

Today, we see marriage as a commitment between two individuals. Western culture is generally individualistic, prizing independence and self – fulfillment in almost all areas. We emphasize rights over duties and choice over obligation. This extends especially to marriage.

Every problem was once a solution.

Life stories with themes of ruin can trap us. Life stories that are triumphant can transform us.

When you write a story, there are probably some good instincts there, but you’re blinded by the feelings of the moment. When you look back on a story later, you can be more objective. A story you wrote might have made sense to you at the time you wrote it, but it has to make sense for everyone who reads it. You can see where it doesn’t make sense.

People love those who are like themselves. — Aristotle, philosopher

What counts in making a happy marriage is not so much how compatible you are, but how you deal with incompatibility. — Leo Tolstoy

Traveling in a third – world country is the closest thing there is to being married and raising kids.

the more they are able to understand each other.

Two people who are similar are going to have the same reactions to a rainy day, a new car, a long vacation, an anniversary, a Sunday morning, and a big party.

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.

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.

The Big Five


practical, conventional, prefers routine, skeptical, rational, shies away from new things open to new experiences, intellectually curious, creative, imaginative, adventurous, insightful


relaxed about standards, easygoing, can be careless, spontaneous, prone to addiction disciplined, efficient, organized, responsible, dutiful, self – directed, thorough, can be controlling


likes solitary time, shy, reserved, energized by being alone, quiet, independent, cautious, aloof outgoing, enthusiastic, active, novelty – seeking, gets energy from interactions with others, talkative

AGREEABLENESS uncooperative, antagonistic, suspicious, has trouble understanding others cooperative, kind, affectionate, friendly, compassionate, trusting, compliant, understanding


not easily bothered, secure, takes things at face value, emotionally resilient tense, moody, anxious, sensitive, prone to sadness, worries a lot, quickly sees the negative

Neuroticism, or the tendency to be anxious, stressed, critical, and moody, is far more predictive of relationship unhappiness and dissolution than is personality dissimilarity.

“But sometimes differences are just differences. They can even be strengths.”

“You’ll never know with complete certainty. That’s why marriage is a commitment, not a guarantee.”

“The same way you make any decision. You weigh the evidence and you listen to yourself. The trick for you is going to be to listen to what matters, not to every single thing that makes you dissatisfied or anxious.”

Life can only be understood backward, but it must be lived forward. — Søren Kierkegaard, philosopher

The more you use your brain, the more brain you will have to use. — George A. Dorsey, anthropologist

Being smart in school is about how well you solve problems that have correct answers and clear time limits. But being a forward-thinking adult is about how you think and act even ( and especially ) in uncertain situations.

In a use-it-or-lose-it fashion, the new frontal lobe connections we use are preserved and quickened; those we don’t use just waste away through pruning. We become what we hear and see and do every day. We don’t become what we don’t hear and see and do every day. In neuroscience, this is known as “survival of the busiest.”

Twentysomethings who don’t use their brains become thirtysomethings who feel behind as professionals and as partners — and as people, and they miss out on making the most of life still to come.

The brain even has a built-in novelty detector, a part that sends chemical signals to stimulate memory when new and different things happen. We know from research studies that when people view slides of ordinary objects (such as a house) and bizarre objects (such as a zebra head attached to a car), the viewers are more likely to remember the bizarre.

People are more likely to remember highly emotional events, such as times when they were happy or sad or embarrassed.

As we age, we feel less like leaves and more like trees. We have roots that ground us and sturdy trunks that may sway, but don’t break, in the wind.

Inaction breeds fear and doubt. Action breeds confidence and courage. If you want to conquer fear, do not sit home and think about it. Go out and get busy. — Dale Carnegie, writer and lecturer

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.

Literally, confidence means “with trust.” In research psychology, the more precise term is self – efficacy, or one’s ability to be effective or produce the desired result. No matter what word you use, confidence is trusting yourself to get the job done — whether that job is public speaking, sales, teaching, or being an assistant — and that trust only comes from having gotten the job done many times before.

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 work of K. Anders Ericsson, a research psychologist who is probably the expert on, well, expertise. In years of study, he and his colleagues have looked at surgeons, pianists, writers, investors, darts players, violinists, and other types of talent. They have found that a large part of what makes people good — and even great — at what they do is time in. For the most part, “naturals” are myths.

Employed twentysomethings are happier than unemployed twentysomethings.

Feeling better doesn’t come from avoiding adulthood, it comes from investing in adulthood. These are the years when we move from school to work, from hookups to relationships or, in Sam’s case, from couches to apartments. Most of these changes are about making adult commitments — to bosses, partners, leases, roommates — and these commitments shift how we are in the world and who we are inside.

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.

The availability heuristic is a mental shortcut whereby we decide how likely something is based on how easy it is to bring an example to mind.

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.

In 1970, one in ten fortysomething women were childless. Today, one in five are. It’s true that more women and men are childless by choice. Being a parent is nothing to be idealized. As meaningful as it can be, it is also unrelenting hard work. It can be an emotional stretch.

So I went for my MRI and it was a really fucking scary thing. Being cooped up in that magnet coffin with all that whirring and banging. There was an alarm sound that kept going off. The machine was the only thing in this big sterile room, and the operator sat in a booth on the other side of the wall.

It was seven thirty in the morning and really cold. They gave me headphones with music to drown out some of the noise, and it was on a preset station. Ozzy Osbourne was playing, believe it or not. There was a time when that would have been funny to me. But it was just ironic or pathetic. Nothing could have felt more irrelevant to my life at that moment than Ozzy Osbourne.

I was really scared of what they were going to find. And the funny — no, sad — thing was my life didn’t flash before my eyes. Not at all. I’m thirty – eight years old and there were, like, two things I had in my mind — the way my little son’s hand feels when I hold it and how I didn’t want to leave my wife behind to do it all on her own. What seemed plain to me was that I wasn’t scared of losing my past. I was scared of losing my future. I felt like almost nothing in my life mattered up until just a few years ago. I realized that all the good stuff is still to come.

I was so sick and panicked that I might never see my son ride a bike, play soccer, graduate from school, get married, have his own kids. And my career was just getting good. Nothing is wrong, thank God. But this has made me face some things.

I saw my regular doctor a couple of days after the MRI, and I told her she needed to keep me going for a good twenty years at least. She said she sees that a lot now. When people had their kids at twenty – two, it was pretty much a given you’d be around to finish what you started. Nobody worried about it.

Now she says a lot of parents come in and say, “Hey, I need to be healthy at least until my kids are off in college. Please be sure I make it that long.” How screwed up is that? What I can’t figure out, and what I feel like I am grieving a little, is why I spent so many years on nothing. So many years doing things and hanging out with people that don’t even rate a memory. For what? I had a good time in my twenties, but did I need to do all that for eight years?

Lying there in the MRI, it was like I traded five years of partying or hanging out in coffee shops for five more years I could have had with my son if I’d grown up sooner. Why didn’t someone drop the manners and tell me I was wasting my life?

To achieve great things, two things are needed: a plan, and not quite enough time. — Leonard Bernstein, composer

Decades and several similar studies later, we now know that the brain has difficulty keeping time across long, unpunctuated intervals. We condense unmarked time. The days and years pass, and we say, “Where did the time go?”

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.

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.

“You’re twenty – six. When are you going to turn toward law school?” I asked with my pencil ready. “I don’t know exactly. Your timeline is making me nervous,” she laughed, “so I hate to commit to next year or something. But definitely by thirty. I definitely will not be bartending at thirty.”

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.

There is a sign just outside of Rocky Mountain National Park that reads in big, bold letters: MOUNTAINS DON’T CARE. It is a sign about preparedness, and it goes on to educate mountain-goers about lightning, avalanches, and proper equipment.

Adulthood is sort of like that. There are things that just are what they are. The smartest thing to do is know as much about them as you can.

It is almost a relief to imagine that these years aren’t real, that twentysomething jobs and relationships don’t count. But a career spent studying adult development tells me this is far from true. And years of listening closely to clients and students tells me that, deep down, twentysomethings want to be taken seriously, and they want their lives to be taken seriously. They want to know that what they do matters — and it does.

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.

👉 Buy The Defining Decade on Amazon

🎧 Listen for free on Everand (plus 1+ million other books)

Tomas Laurinavicius

Hi! I'm Tomas, a writer and growth marketer from Lithuania, living in Spain. I'm always involved in multiple projects driven by my curiosity. Currently, I'm a marketing advisor at Devsolutely and a partner at Craftled, building Best Writing and Marketful. Let's connect on X and LinkedIn.