No Two People are Alike: The mystery of personality
Distilling books - Number 4
Welcome to Mental Garden. The following letter is part of our “Distilling Books” collection, in which we extract the most revealing ideas from literature. For the complete library, click here.
🏷️ Categories: Learning, Behavior, Social relationships, Biology.
“There are many books about ‘human nature,’ but very few about the important question of why humans differ from one another.”
“Even identical twins who grow up in the same household with the same genes end up with different personalities.”
“The nurture assumption is what I call the idea that what influences children's development—apart from their genes—is the way their parents raise them. After raising my two children and co-authoring three college textbooks on child development, I began to question this assumption. I have recently come to the conclusion that it’s wrong.”
We all know someone who is nothing like their family.
Maybe that person is you.
Even though you grew up under the same roof as your siblings and received the same parenting, you don’t fit in at all.
What makes you different?
Why, despite sharing experiences like upbringing and friends, do you stand apart?
Let’s take a look at a fascinating psychological theory that explains why parents are not the key players in shaping their children’s personalities—and what really has shaped your personality and your children's.
You’ll see that not even twins are necessarily alike… (and I’ve seen this case firsthand).
The Nurture Assumption
For years, the debate around personality has revolved around one key dilemma:
Is it nature or nurture that defines who we are?
For a long time, it was believed that parents were the primary factor shaping their children's behavior and personality—an idea rooted in Freudian theories and sustained for decades despite genetic research that contradicted it.
In contrast, Judith Rich Harris proposed a theory that completely changed the way we view parenting.
It’s not the parents—it’s the socialization with other children that shapes personality.
Group Socialization Theory
Not even twins have to be alike.
Harris studied identical twins and found that even though they share the same genetics and grow up in the same home, their personalities can be very different.
How is this explained?
Simple. They didn’t share the same social groups.
In fact, the biggest differences in personality stem from experiences outside the home—not from parenting. This concept is key to understanding why no two people are the same, even when raised in identical conditions.
Social groups play a crucial role.
As we grow, we integrate into different social circles beyond our family: groups of friends, classmates, and even people outside our usual circle and of different ages. In these groups, we learn to behave according to social norms and adapt our behavior to fit in and feel valued.
This is how each personality starts to become unique.
Harris explains that through daily interactions, each child gathers information about how to fit in and which behaviors are most valued in each context. So even if two siblings live in the same house, their different interactions with friends or social groups shape their personalities in unique ways.
To explain the process, Harris uses 3 systems:
Relationship System
Socialization System
Status System
Let’s start with the first one…
1. Relationship System
Imagine a mental filing cabinet filled with pages in your brain.
Each page holds information about someone you’ve interacted with, based on direct experiences and indirect learning from observing their interactions with others. Thanks to this kind of learning—what Harris calls a “people lexicon”—we associate names, remember faces, and feel emotions when recalling experiences with those individuals.
This classification goes beyond simply recognizing someone on the street.
It’s not just knowing that girls at school can be mean if you don’t dress in style—you also anticipate that María will make a rude comment about your “bad taste,” while Laura might compliment you on your originality.
Every social interaction adds more information to someone’s "page" in that mental filing cabinet.
The difference between one person and another lies in how they’ve filled those pages with social experiences—thus shaping their personality.
2. Socialization System
Socialization is key to personality development.
Through the Socialization System, children and teens learn to behave according to cultural norms and to either integrate or differentiate themselves within social groups. Over time, children begin to resemble their regular circle of friends.
For example, during adolescence, they often fall into categories like “athletes,” “intellectuals,” “artists,” or “popular kids,” based on shared interests. The socialization system helps people find their place and teaches them how to adapt to the expectations of their social environment.
Harris puts it simply:
“The job is to learn how to behave in ways that are acceptable to the other members of the group. The process consists of acquiring social behaviors, customs, language, accent, attitudes, and values that are considered appropriate in a particular group.” (Harris, 2007)
However, socialization doesn't explain all personality differences. Even if children grow more similar to their closest peers, there are still differences in behavior and personality that remain a mystery—implying the role of other systems, such as status.
3. Status System
Competition for status is inherent to human nature, and Harris suggests it can change our personality in the long term.
“The purpose of this system is to compete within the group—classic Darwinian competition. Competing with group peers means competing for status; striving to be the best among them.” (Harris, 2007)
The Status System helps us evaluate ourselves in relation to others, influencing how we interact and compete within groups. From childhood and adolescence, people start assessing their physical, social, and psychological traits to determine their place in the social hierarchy.
For example, Harris notes how a teenager might gain confidence due to their height—and how this can influence early personality development.
This system shapes our interactions based on our perceived strengths or weaknesses and influences self-esteem from an early age. Adolescent experiences related to physical or social traits—such as height, intelligence, or popularity—can make a lasting impact on who we become.
That’s Group Socialization Theory.
Parenting, while important, is not the decisive factor.
The influence of social groups, the learning of behaviors within those groups, and the drive to stand out in them—together with our genetics and what we learn at home—create the blend we call personality.
That’s why two siblings might be nothing alike.
Even if they’re twins.
✍️ Your turn: Do you know anyone who is completely different from their family or siblings? I do—a friend of mine has nothing in common with his parents or brother. I always thought it was because of his environment, which is very different from his brother’s. And I wasn’t wrong—his surroundings clearly shaped his personality.
💭 Quote of the day: “My solution to the mystery is that there are three causes: three mental systems that go about their business in different ways. Together, these three can answer the hows and whys of personality development.”
— Judith Rich Harris, No Two Alike.
See you soon! 👋
References 📚
Harris, J. R. (2007). No Two Alike: Human Nature and Human Individuality.