top of page
  • Writer's pictureMark Walmsley

The Defining Decade by Meg Jay Summary

Updated: Mar 8, 2021

The Defining Decade Book

This book implores late teenagers and young adults in their 20s to get on with their life.

It is a frontal assault on the 30-is-the-new-20 culture which suggests you can live loose and free in your 20s without consequence.

Meg Jay chronicles the social, professional, family, psychological, and physiological importance of striving in your 20s to the rest of your life. Key insights:

  1. Your 20s matter a great deal; success in your 40s and 50s is built on the efforts of your 20s

  2. Build identify capital (~CV) by leveraging the power of weak connections

  3. Don’t live together before marriage; instead use 3rd world travel to find your partner

  4. Being alike in terms of education, age, ethnicity, values, etc is important for relationships

  5. The more similar your personalities, the more likely you are to be satisfied with your partner

  6. Beware the tyranny of ‘should’ – externally generated goals that we don’t align with

  7. Adopt a growth mindset – one that believes almost anything is possible with effort

  8. Learn to take criticism – it gets easier with age so start early!

  9. More options does NOT equal more happiness. You really can’t be anything you want so choose

  10. Plan the critical years of life from 20-35 or else you miss out or overload your early 30s

Book details

Full title: The Defining Decade: Why Your Twenties Matter and How to Make the Most of Them Now. By Meg Jay.

Length: 272 pages, or 5 hours and 28 mins on Audible

Buy the book (USA): Amazon (book, Kindle, Audible)

Buy the book (AUS): Amazon (book, Kindle, Audible), Booktopia (book, eBook, audio-book)

Key insight 1: Your 20s matter, a LOT!!

Let’s start with a mathematical explanation of the importance of taking your 20s seriously based on the phenomenon of compound growth. I think we can all agree that life is life cumulatively where each successive year builds on the achievements of the prior year. This is shown below.


The solid black line shows compound growth at 5% from age 6 resulting in a notional achievement of 1400 (the units don’t matter, focus on the principle). The grey dashed line shows our trajectory if we grow normally from age 6 to 20, stop growing from age 20 to 30, and then resume at 5% till age 60. Here we end up at with a notional achievement of 855 – just 61% of the constant growth level. Think of this gap as reflecting an income gap, professional experience/qualification gap, or just a life development gap. But in reality it is worse.

As Jay outlines in The Defining Decade, our 20s are actually the period of our second and final brain growth phase. From Jay:

“As ‘neurons that fire together, wire together,’ the jobs we have and the company we keep are rewiring our frontal lobes – and these same frontal lobes are, in turn, making our decisions in the office and on Saturday nights. Back and forth it goes, as work and love and the brain knit together in the 20s to make us into the adults we want to be in our 30s and beyond.
Or not. Because our twenties are the capstone of this last critical period… The post 20 something brain is still plastic, of course, but the opportunity is that never again in our lifetime will the brain offer up countless new connections to see what we make of them. Never again will we be so quick to learn new things. Never again will it be so easy to become the people we hope to be.
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.’”

Your 20s as ‘survival of the busiest.’ That’s a good way to think of it. But let’s make add this concept of reduced potential after our 20s to taking a learning/growth gap in our 20s. In the chart below we add a third line (light grey, dotted). Here we have growth at 5% from age 6 to 20, and then no growth from 20 to 30. Now we have growth from age 30 to 60 at 4% instead of 5% resulting in end achievement of 640 – just 42% of the constant growth baseline scenario. Having a ‘gap decade’ hurts you twice over.


Whilst that 20s gap decade is looking pretty expensive at 5% annual growth, imagine the difference if you worked really hard in your 20s and achieved higher growth rates? The last word on this concept goes to Jay:

“20 somethings who use their brains by engaging with good jobs and real relationship are learning the language of adulthood just when their brains are primed to learn it. They learn to get along and get ahead, and this makes them happier and more confident. They learn to be forward thinking before life’s defining moments are in the rear-view mirror. 20 somethings who don’t use their brains become 30 somethings who feel behind as professionals and as partners, and as people; and they miss out on making the most of life still to come.”

Key insight 2: Identity capital, and the power of weak ties

Jay introduces the concept of identity capital as follows:

“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.
Identify capital is how we build ourselves – bit by bit, over time. 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.”

In an era where more and more people have college degrees, 20 somethings need to take the job that offers the most identity capital. And to be clear sitting around at home waiting for the ideal job or taking dead-end jobs doesn’t count. What counts? Perhaps working as a PA to a CEO or film producer where you’ll make industry contacts, learn how the industry works, and learn how to manage competing demands and solve complex problems. And a million other things from your industry of choice that you won’t learn making coffee at Starbucks. So when out of school or college take the job that results in the most career capital, not necessarily the one where you’ll earn the most.

A 2013 Columbia research study indicated that the average American knows 600 people, an earlier study suggested 290 people. Let’s split the difference on the low side and say it is 400 people. Now with one degree of separation you have access to 160,000 people who have a weak tie to you via one of your friends. Jay says these people are very important to your future success as they open up new networks, relationships, and opportunities. They are key to professional success, and potentially relationship success in a way close ties are not (our close friends usually have each other as contacts limiting the network effect). From Jay:

“Information and opportunity spread farther and faster through weak ties than through close friends because weak ties have fewer overlapping contacts. Weak ties are like bridges you cannot see all the way across, so there is no telling where they might lead.”

The other advantage of weak ties (extended networks) is that they force us to think and talk in ways our close ties don’t. Jay again:

“Whether we are talking about career ideas or our thoughts on love, we have to make our case more fully. In this way, weak ties promote, and sometimes even force, thoughtful growth and change.”

So be attentive to the opportunity weak ties represent. Say yes when someone offers to introduce or connect you to their acquaintances whether for professional or personal reasons. And when talking with them, be deliberate in your thinking and speech.

Key insight 3: Don’t live together before marriage; instead use 3rd world travel to find your partner

Are you more likely to ultimately divorce if you marry at 15, 25, or 35? The answer is yes for teenage marriages, but after 25 the chance of divorce doesn’t change. So there’s no reason to delay. What about living together before marriage – to ‘try before you buy’? Research suggests this isn’t a good idea unless you are publicly engaged to be married. More from Jay:

“It is the couples who live together before being clearly and mutually committed to each other who are more likely to experience poorer communication, lower levels of commitment to the relationship, and greater marital instability down the road.”

Why is this the case? It happens when couples slide into marriage, rather than make a positive decision, what Jay calls “sliding not deciding,” This happens when couples live together out of convenience or to save money, then eventually deciding to get married, especially if you’ve just turned 30. But the criteria to cohabitate are be lower than marriage, so a marriage resulting from cohabitation may lead to a less than perfect marriage.

So what is the solution? Jay says travelling to a third world country for several weeks to stress test the relationship. From Jay:

“Travelling in a third-world country is the closest thing there is to being married and raising kids. You have glorious hikes and perfect days on the beach. You go on adventures you would never try, or enjoy alone. But you also can’t get away from each other. Everything is unfamiliar. Money is tight or you get robbed. Someone gets sick or sunburned. You get bored. It is harder than you expected, but you are glad you didn’t just sit home.”

This sounds a faster and cheaper way to determine if a potential partner is right for you.

Other insights The Defining Decade

4. Being ‘alike’ is important. 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. However, these are deal-breakers, not matchmakers.

5. Personality – an important matchmaker. Research indicates the more similar a couples’ personality is, the more likely they are to be satisfied with their relationship – especially in young couples. Personality is not about what we’ve done, or what we like; it is about how we engage with, and operate in the world.

6. Beware the tyranny of ‘should’. Goals direct us from the inside, but shoulds are paralyzing judgments from the outside. Goals feel like authentic dreams while should feel like oppressive obligations. Shoulds set up a false dichotomy between either meeting an ideal or being a failure, between perfection or settling.

7. Adopt a growth mindset. Having a growth mindset is essential to accelerate your personal and professional success. With a growth mindset you see failures as opportunities to learn and grow; rather than proof of your incompetence or evidence the world is rigged against you.

8. Learn to take criticism – it gets easier with age. In our 20s we take criticism poorly, often personalizing the feedback and ignoring the message. This means we are less likely to reflect on the feedback and improve, and more likely to become resentful, which makes our relationships worse and reduces our success.

9. More options ≠ happiness. Research has shown we are better able to choose from 6 items on a menu, whilst 24 items has us paralyzed. And this is true of career/life choices. Whilst your parents told you could be anything it isn’t true. If you’re honest you really only have 6 or so things you can be great at – so list them, evaluate them, choose one, and get moving on it.

10. Plan ahead and do the math. You can’t delay career and post-graduate study till after 30; and at the same time hope to be married and have a family before 35. Does doing further study, working long hours at the new job, getting married, and having babies between 30 and 35 sound viable? Or fun? Probably not. Plan your life out, make hard decisions, and realize you need to get started earlier – that is, in your 20s.

Why you should read this book if you’re under 30

Do you want to be successful in your 30s, 40s and 50s? Most successful people you meet in these years set up their success by being focused in their 20s. Life is lived cumulatively, make the most of your 20s and you won’t regret it later on. By reading The Defining Decade you’ll have a good idea of how to get started. A final word from Jay:

“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.”

Relationship to other Eruditeable books

#1 – The Algebra of Happiness. This book covers some key topics from The Defining Decade around working hard in your 20s and partner choice.

#3 – Atomic Habits. This book shows you how set goals, and then build the personal systems to achieve them. Very useful for making the most of your 20s.

#5USA – Set for Life. This book shows you the three stages to go from broke to financial freedom; which unsurprisingly is helped by an early start to working, saving, and investing.

#5AUS – The Barefoot Investor. This book shows you how to set up and manage your finances in a way that will have you reaching financial independence and retiring well ahead of everyone else.

#6 – The Magic of Thinking Big. This book will open your mind to be able to see the many employment or promotion opportunities that are around you; and learn to think about them in ways that will help you achieve them.

#7 – Emotional Agility. This book deals with decoupling from negative emotionality such as that associated with workplace criticism.

#14 – Peak. While Jay discusses the importance of choosing a career and getting on with it, this book shows you how to think about how to becoming world class in that career.

#16 – The Personal MBA. Understanding how business works will be an essential skill for the keen-to-progress 20something. This book will be extremely helpful.

#19 – Gifts Differing. This book deals in detail with personality type systems and the differences between difference personality types.

#13 – 12 Rules for Life. This book deals covers some key topics from The Defining Decade around making friends, pursuing meaningful (not expedient) goals, listening to others, and being precise in solving problems.

#17 – Crucial Conversations. This book provides guidance on how to manage the difficult conversations that inevitably arise as 20 somethings navigate initial professional roles and relationships.

#24 – The Seven Principles for Making Marriage Work. While Jay discusses the importance of choosing the right partner, this book provides guidance on how to make the marriage thrive.

Book resources

About the author

Dr. Jay earned a doctorate in clinical psychology, and in gender studies, from the University of California, Berkeley. She earned a B.A. with High Distinction in psychology from the University of Virginia. Her work has appeared in numerous media outlets including the New York Times, Wall Street Journal, Los Angeles Times, USA Today, O Magazine, and on the BBC, NPR and TED. Meg Jay is a Clinical Psychologist and Associate Professor of Education at the University of Virginia. She maintains a private practice in Charlottesville.

External links

Youtube video

151 views0 comments


bottom of page