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Why Are No Two People Alike? (Part 1)

“My solution to the mystery is that three perpetrators are involved:
three mental systems that go about their business in different ways.
Together, these three can answer the hows, whys, whens, and wheres of personality development.”

 Judith Rich Harris


Becoming Human

What makes us…us? What makes one person open, honest, and enthusiastic, and another ornery and closed-minded? Why do some of us love risk-taking and some not? What causes us to be so alike…and yet so different?

The “nature versus nurture” debate is probably as old as modern humanity. There’s something related to our consciousness which makes us wonder whether our personality is pre-determined or perhaps whether our parents, peers, culture, nation, or experiences are the determining factor.

We still wonder, but thanks to Darwin and all that came after him, we don’t have to speculate as much as we once did. The modern study of behavioral genetics has shown us that the answer is, unequivocally, both. Our genes play an enormous role in how we turn out as children and full-grown adults, and the evidence is unmistakable.

In fact, oddly enough, we can see through carefully done research that about half of personality variation can be explained genetically. Take two identical twins and their personalities will be similar to the extent of their identical genes. This surprises no one.

The surprise is that two identical twins are so damn different! Think about it: They share all of their genetics, they probably grew up in the same house with the same set of parents, with the same books on the wall and the same TV watching habits, went to the same schools at the same time, had similar groups of friends…and yet two completely separate personalities emerge. How?

The Nurture Assumption

Judith Rich Harris may have the best answer, and it’s in her book No Two Alike, an amazing contribution to modern thought. What Harris — a former author of child development textbooks turned super-synthesizing social scientist — wanted to know was: Why do we all turn out with unique personalities? What really drives the differences?

She had begun to answer this in her deeply controversial 1998 book The Nurture AssumptionThat book proposed her group socialization theory, the idea that children are mostly socialized by their peers, not their parents. By socialized, we mean — how do children learn the way to behave and operate within their culture? How to speak the right way, act the right way, play the right way, say the right things, and so on? The idea that parents had the primary influence had become fashionable in the 20th century Western world, thanks to Freud.

But contrary to popular belief, Harris explained, whether identical siblings were raised by the same set of parents or by two different sets, years of research proved they’d be no more or less alike than their genetic connection would predict. Likewise, two siblings put up for adoption end up no less alike, on average, than if they’d been raised in the same home. Identical twins are more alike than regular siblings in general, but the reason is ultra simple: They share more genes!

This meant, Harris explained, that parenting doesn’t have an effect on adult personality which isn’t already explained by genetic factors, any more than Chinese parents can give their child a Chinese accent if they raise him or her in Minnesota. It’s a bit hard to see on the surface, but many traits we think are due to parenting are simply due to the genetics shared between parent and child. Before behavioral genetics showed the genetic component, the two were regularly confounded, making much of the “research” on development worthless.

What does have an effect, besides genetics, is the peer group and culture children the child was raised in. And so besides genetics, it is the groupneighborhood, social group, and subculture of the child that matters, not whether their parents were kind or scolding, attentive or inattentive, soft or hard, or any other style of parent. Just like the child of Chinese immigrants would take on the Minnesota accent, they’d also take on the social behavior of their peers as well. Harris showed that people simply do not depend on direct input from their parents to become successful adults, as hard as it is to believe. (Although parents can have indirect influence in a number of ways, most obviously by moving the child to different areas and cultures.)

For this revelation and investigation into human development, the debunking of what Harris called the Nurture Assumption — that parents can mold the personalities of their children — Charlie Munger said that Ms. Harris “has not lived in vain.”

But that still left a big question: Since group socialization tends to make people more similar to others in their identified group, what accounts for personality differences, even among identical twins hanging in the same social circle? How do we end up with a group of “conforming individualists,” as Harris calls us? Why are some people trustworthy, and some not? Why are some more law-abiding and some less so? Why are some friendly and some mean?


The difficulty in figuring out the answer is illustrated by a story Harris tells about a pair of identical twins:

Conrad and Perry McKinney, age fifty-six, were featured in an article in the Boston Globe titled “Two Lives, Two Paths.” Born and reared in New Hampshire, the twins did everything together in their earlier years. They attended the same schools, sat in the same classrooms. Academically they were average students, but they were troublemakers. Eventually their teachers got fed up with their shenanigans and the twins were separated: Perry was held back in fifth grade, Conrad was promoted to sixth. That, according to the Globe reporter, was where their paths diverged. Conrad went on to graduate from high school; Perry dropped out in eleventh grade. Today, Conrad is a successful businessman–as it happens, he runs a private detective agency. Perry…well, Perry is a homeless alcoholic “who sleeps amid trash under a bridge,” by the Piscataqua River in New Hampshire.

Small changes in circumstance, many of which are random, can have dramatic effects on one’s life, making many experiments totally unethical. We can’t just sort out twins and send some of them to the ghetto and some to Palo Alto and see what happens — we’re reliant on what we can observe in natural experiments.

In No Two Alike, Harris does this by corralling information from a wide variety of sources including developmental psychology, evolutionary psychology, cognitive science, and genetics, dispatching a number of popular red herrings in the search for a theory of why human personality turns out the way it does. By elimination and investigation, the theory she lands on seems not only plausible, but probable.

The Modular Mind

Her theory derives from what modern evolutionary psychologists have come to call the “modular mind” — the idea that the mind is made up of specific, useful, mechanisms to carry out a variety of functions, all put there through a long process of natural selection. Our mental tools allow us to see, hear, taste, feel, learn, speak, and do lots of other things that we need to survive and thrive. Some of these are present in other species and some are not: It depends how highly developed they are. (For example, ants can certainly see and taste, although crudely, but cannot speak or learn non-programmed behavior.)

As Harris sees it, from the perspective of human personality development, it doesn’t all happen in one simple way. Our modular minds have at least three separate but interrelated systems, working at turns separately and together to produce social success — one of the prime goals of a human being. (We are, after all, a highly social species.)

One passage illustrates why this is so important:

“Why,” asked the psychologist William James in 1890, “are we unable to talk to a crowd as we talk to a single friend?” Why do we shrug when we hear of hundreds of people killed in an earthquake but weep when we see a photo of one injured child? Why is it that getting to know and like someone doesn’t necessarily causes us to think well of the group to which he or she belongs–a disjunction revealed by the ineffectiveness of the protest, “Some of my best friends are Jews?”

The answer is that there are multiple systems in the mind for processing information. We have, I propose, two different mental mechanisms designed to process and store information about people. One collects data on individuals, the other on groups or social categories–types or classes of people. Criminal justice and law enforcement are (or should be) based on information processed by the first mechanism; war and bigotry are outcomes of the second. These mechanisms belong to different mental systems.

Besides the two systems that help us learn about others, we have a uniquely human third: A system that helps us learn about ourselves. Let’s take each in turn and see what we can learn and why they matter in the development of unique human personalities.

The Relationship System

Harris calls the Relationship System our “people information lexicon”. It’s how we know how to identify and deal with specific people. As humans, we need to know not just that girls at school tend to be mean if we don’t wear makeup, but that Jane will say something particularly nasty and that Sally will say something particularly nice. The mental system for categorizing the “girls our age at school” isn’t the same as the one that knows the difference between Jane and Sally.

We have a number of “modules” that roll up into a Relationship System: Our ability to recognize faces, voices, and scents; our ability to know what’s a familiar face — one that belongs to someone we know; the ability to put specific names to people and things and recall them, and hundreds more.

Harris uses the analogy of a Rolodex:

We have thousands and thousands of mental storage sites for people-information. Each is associated with a particular individual; each contains (or is linked to) other information we have about that individual. Picture a mental lexicon with a page for each individual you know, with slots for the face, name, and whether he or she is a close relative; plus other information such as occupation, plus memories of the experiences you’ve had with him or her.

There may also be an emotional marker, indicating how you feel about this person. The contents of some slots may be hard to read; other slots may never have been filled in. A page can be set up in the lexicon even if you’ve never set eyes (or ears) on the individual it refers to. You collect and store information on characters you read about in novels or hear about from other people. Folks you’ve never met may have a page in their lexicon for you!

This “lexicon” of information, constantly updated, gives us what we need to deal with individual people and figure out how to interact with them. Although we generalize and categorize people we don’t know, once we do get to know them specifically (even at a distance) we start filling in details and set up a page in the lexicon. We don’t have to be motivated to do this for a specific reason — we just do it automatically.

The evolutionary purpose of the people lexicon is as clear as the evolutionary purpose of pair-bonding: to enable us to behave appropriately toward different individuals, depending on what we have learned about them. To enable us to tailor our behavior to the nature of the relationship we have with each. The baby lifts up its arms to its mother but not to the stranger, even if the stranger is the right age and sex. The child learns to avoid the bully but to seek out other kids in the neighborhood. People stop doing favors for people who never pay them back, unless they are close relatives.


So the relationship system contains many intricately connected parts. There’s a people-information acquisition device that constructs and stores a lexicon of people and provides the motivation to collect the information. There are regulatory mechanisms that make use of the information stored in the lexicon to guide behavior in different domains of social life and that provide their own motivations, the sex drive being an obvious example. Other specialized modules deliver input to the relationship system: they include the face-recognition module, a device that assesses kinship, and the mindreading mechanisms I described in the previous chapter. Whatever you are considering doing with another person–help them, mate with them, engage in trade with them, pick a fight with them–it is extremely useful to have an idea of what their intentions are and what they are thinking about you.

And so it goes. From birth, our lexicon is ready to go, ready to be filled in. We spend a lifetime gossiping, learning, thinking, interacting with, and watching others so that we can have successful relationships with them.

But we also begin to categorize fairly early, based on a deeper analysis of our lexicon. We start putting people into groups — adults, children, teenagers, girls, boys, teachers, students, and a million others, depending on context. Importantly, we also begin to categorize ourselves, and this is where the socialization process occurs.

The Socialization System

Why is it that children “hive off” into groups and seek to differentiate themselves from other groups? The usual high school groups are not uncommon all around the world — they may differ in makeup and interests, but all young (and old) people form some group or another, if given the opportunity.  During this grouping process, the child is socialized:

In the old days, a human’s life, too, depended on remaining a member of the group. But because human groups differ in culture, the behaviors necessary for group membership couldn’t all be built in–much had to be learned. The baby’s Job 2, therefore, is to learn how to behave in a way that is acceptable to the other members of his or her society. This is the process that developmentalists call “socialization.” It consists of acquiring the social behaviors, customs, language, accent, attitudes, and morals deemed appropriate in a particular society. 

Socialization makes children more alike–more similar in behavior to others of their age and gender. Therefore, socialization cannot solve the central mystery of this book: why people (even identical twins reared together) differ in personality and social behavior. But the socialization system is an essential part of the solution, because one of the things I have to explain is why children become both more alike and less alike while they are growing up. The ways in which they become more alike do not consist solely of language and customs. There is evidence that children become more alike even in the sorts of things that are measured on personality tests. 

This process must happen as a child grows up — they must prepare for adulthood outside the home. And to do that, the child must learn what is acceptable in the groups they are a part of, and they will be part of many. A young boy from Texas will be at times a boy, a male, a student, an employee, an American, a Southerner, an athlete, and a child, among many more. These all require somewhat different actions and behaviors. So we start categorizing as best as we can:

The first step for the child is to figure out the social categories that exist in his or her society. This task is equivalent to that of learning other kinds of categories: for example, chairs and fish. Like chairs and fish, categories of people have fuzzy boundaries. Is a three-legged stool a chair? Is a seahorse a fish? Is this person a boy or a man? Traditional societies often provide rites of passage to sharpen the boundaries between age categories, but industrialized societies seem to manage pretty well without them. What we haven’t gotten used to yet is the blurring of the boundary between male and female.

An interesting thing about fuzzy mental categories is that, although they tend to be hazy around the edges, they’re clear at the center. We have an image of what the ideal or prototypical member of each category should be, and it’s somewhere in the middle. When I say “man,” you don’t think of an eighteen-year old or an eighty-year old and you probably don’t picture him wearing a dress. When I say “bird,” you think of a robin or a sparrow, not an ostrich or vulture. The prototypical chair has four legs, a seat, and a back.

We build up all kinds of implicit knowledge about the world, and we do it like the relationship system — automatically and without thought. We categorize people the same way we categorize chairs and birds, though the idea of stereotyping is certainly unpopular. Until we actually have a sheet set up in the lexicon for an individual person, all we can do is categorize them. Once we do start to learn about them specifically, the two systems begin interacting. Let’s say we meet a woman named Susan. At first, we might classify her as “White, middle-aged woman who looks like a mother.” (Again, not purposely — it happens instantly and automatically.)

Once we go on a date with her though, Susan becomes no longer just a member of a category: She becomes Susan. And although we don’t immediately remove the categories, we let her entry in the lexicon begin to develop and dominate our thoughts about her. Sometimes the two systems conflict. (I don’t usually like white middle-aged women, but that Susan is alright!)


Let’s leave it there for now. In Part 2, next week, we’ll explore the rest of Harris’s theory, and tie it all together to try to understand the mystery of human personality.