What Makes You, You?

It sounds like the sort of question that belongs in a philosophy lecture. It is also the kind of thing a six-year-old might ask from the back seat of a car before immediately changing the subject to dinosaurs. Yet simple questions are often the hardest to answer, and this question has occupied philosophers and scientists for centuries.

Ask a few people, and the answers tend to arrive quickly: your DNA, your brain, your memories, or perhaps your personality. Each explanation seems convincing until you look a little closer. Identical twins can share almost all of their DNA yet develop into noticeably different people. Our bodies are home to vast communities of microorganisms, and the brain itself is constantly shaped by what we learn and experience.

The closer researchers look, the harder it becomes to isolate one feature and declare that’s what makes a person who they are. Instead, every single answer seems to raise a hundred more questions. To understand what makes you you, we may need to look through several different lenses, each revealing a different part of the story.

The starting point: your genome

If you had to locate the instructions for building you, DNA would seem like a pretty sensible place to start. Packed into almost every cell is a genome inherited from your biological parents, containing genes that influence everything from blood type to hair colour and susceptibility to certain diseases. Yet describing DNA as a blueprint can be misleading. Blueprints imply a finished design. Biology is rarely that tidy. Most human characteristics emerge through interactions between many genes and the environments in which they operate.

Identical twins make that especially clear. Monozygotic twins develop from the same fertilised egg and therefore begin life with essentially the same DNA sequence. Yet anyone who knows a pair of identical twins will know that “identical” has its limits. Differences can emerge in appearance, health and behaviour, and some become more pronounced with age. If the genetic instructions are so similar, where exactly do those differences come from?

Part of the answer lies in how the genome is regulated. Epigenetics refers to molecular processes that affect how genes are regulated without changing the underlying DNA sequence. In a landmark 2005 study, Fraga and colleagues examined identical twins and found that younger pairs had relatively similar epigenetic patterns, while older twins tended to show greater differences (1). The researchers focused on chemical modifications involved in gene regulation, including DNA methylation, a process that can affect whether particular genes are more or less active. These differences were especially noticeable among twins who had spent less of their lives together or had adopted different lifestyles. Their DNA remained almost indistinguishable, but the way it was being regulated had begun to diverge. 

There is a temptation to imagine genes as a set of instructions that unfold in exactly the same way every time. The evidence, however, suggests something less predictable. Recent work has increasingly highlighted the role of epigenetic processes in shaping individual differences, even among people with highly similar genomes (2). DNA matters enormously, but it does not operate in isolation from development, environment or chance. Identical twins begin with almost the same genome, yet they do not necessarily arrive at the same destination. For a question as personal as what makes someone who they are, that is a difficult fact to ignore. 

Where do you end?

Where, exactly, does the boundary of a person lie? Whatever makes you you, it seems reasonable to assume that it is contained somewhere inside your own body. Even that boundary turns out to be less tidy than it sounds. The gut alone is home to dense communities of bacteria, archaea, fungi and other microorganisms, many of which spend their lives in close biochemical conversation with us. They help break down components of our food, interact with the immune system and produce molecules that our own cells can respond to. 

Some of this conversation takes place along what researchers call the gut-brain axis, a network of connections linking the digestive system and the nervous system. Signals can travel between the gut microbiota and the brain through neural pathways, immune responses and chemical messengers circulating in the bloodstream. In their 2025 review, Ohara and Hsiao describe this communication as a two-way exchange rather than a one-sided chain of command (3). That does not mean that a particular bacterium is secretly responsible for your taste in music or your inability to enjoy coriander. Much of the research linking the microbiome to human behaviour is still developing, and many findings come from animal studies or reveal associations rather than direct causes.  Still, the broader implication is difficult to ignore - the brain belongs to a body in constant communication with organisms that are, genetically speaking… not us.

Things, unfortunately, do not become more straightforward once we enter the skull. Azevedo and colleagues estimated in 2009 that the human brain contains roughly 86 billion neurons, but the number is almost less interesting than what those cells do together (4). No single neuron contains a childhood memory, a political opinion or your inexplicable attachment to one particular song. Neuroscientists have long known that learning can alter the strength of synaptic connections, a phenomenon known as neuroplasticity (5). Every new skill, conversation or experience leaves some trace in the architecture of the brain.  When neurons are repeatedly activated together, the connections between them can become stronger, making certain patterns of activity more likely in the future. Over time, these small changes accumulate. Information that once seemed impenetrable can become almost automatic. Most students have experienced the peculiar transformation of spending weeks convinced a topic makes no sense, only to catch themselves explaining it to someone else a month later. Not every connection is reinforced, though. Pathways that are rarely used may gradually weaken, allowing the brain to adapt to new experiences without endlessly holding onto every detail. Much of this remodelling happens quietly and unfolds across thousands of neural interactions rather than in dramatic moments of revelation.

 The organ sitting inside your skull today is physically continuous with the one you had as a child, but it has been remodelled by every year in between. If the microbiome challenges the idea that our bodies are entirely our own, neuroscience challenges the idea that the self is built upon a fixed biological foundation. The brain provides continuity across a lifetime, yet its networks are continually rewritten by the life they help us experience. 

The story you remember

Perhaps the most convincing evidence that you are still the same person you were ten years ago is that you can remember being them. A childhood birthday, the first day at a new school, an argument you wish you had handled differently: autobiographical memories stitch moments separated by years into something that feels like one life. Memory, though, does more than preserve the past.  Research by Wilson and Ross (2003) suggests that people often use memories of their former selves to understand who they are in the present (6). Looking back, we rarely encounter our younger selves as neutral observers. We view them instead through the lens of everything that has happened since. An awkward twelve-year-old may become evidence of personal growth, and a difficult experience may acquire a meaning that was invisible at the time. 

The trouble is that memory is a rather peculiar archivist. Remembering is not like opening a file that has been sitting untouched in storage. When an event is recalled, the brain reconstructs it from stored information, and that reconstruction can be influenced by later knowledge, expectations and the person we have since become. Decades of research into reconstructive memory, including work by Elizabeth Loftus, have shown that recollection can be altered, sometimes substantially, without the person doing the remembering being aware of it (7). Anyone who has found themselves reliving a painfully awkward moment from years ago while trying to fall asleep has experienced a version of this. The event itself may be long over, yet each return to it is coloured by who we are now, not who we were then.

That distinction matters. Imagine a twelve-year-old who spends an entire class presentation staring at the floor, convinced that every mistake is being noticed. The event itself remains fixed in the past. What changes however is its interpretation. Years later, the same memory may provoke amusement rather than embarrassment, or become evidence of how much confidence has been gained since.

Further research argues that autobiographical memory contributes to our self-concept by connecting individual experiences to broader ideas about who we are (8). This reflects a broader view in memory research that our recollections are not stored as isolated moments, but organised in ways that help maintain a sense of continuity across time (9). We do not carry every detail of our lives forward with equal weight. Some memories fade into the background, while others become reference points that help explain our values, ambitions or fears. Family stories and photographs often shape this process too, by preserving versions of ourselves that we might otherwise have forgotten.

A life may happen only once, but it can be remembered many times.  The curious thing is that each return to the same memory occurs from a slightly different vantage point that has been shaped by experiences and perspectives that did not exist when the memory was first formed.

So what makes you you?

After genetics, microbiomes, neural networks and memory, the most honest answer may be that there is no single place to look. What makes a person who they are appears to lie less in any individual component than in the way those components continually shape one another over time. The six-year-old asking “what makes you you?” may never receive a simple answer. Then again, some questions are interesting precisely because they refuse to have one.

References

1. Fraga MF, Ballestar E, Paz MF, et al. Differential epigenetic marks in monozygotic twins. Proc Natl Acad Sci U S A. 2005;102(30):10604-10609. doi:10.1073/pnas.0500398102

2. Webster AK, Phillips PC. Epigenetics and individuality: from concepts to causality across timescales. Nat Rev Genet. 2025;26:406-423. doi:10.1038/s41576-024-00804-z

3. Ohara TE, Hsiao EY. Microbiota-neuroepithelial signalling across the gut-brain axis. Nat Rev Microbiol. 2025;23(6):371-384. doi:10.1038/s41579-024-01136-9

4. Azevedo FAC, Carvalho LRB, Grinberg LT, et al. Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain. J Comp Neurol. 2009;513(5):532-541. doi:10.1002/cne.21974

5. Citri A, Malenka RC. Synaptic plasticity: multiple forms, functions, and mechanisms. Neuropsychopharmacology. 2008;33(1):18-41. doi:10.1038/sj.npp.1301559

6. Wilson AE, Ross M. The identity function of autobiographical memory: time is on our side. Memory. 2003;11(2):137-149. doi:10.1080/741938210

7. Loftus EF. Planting misinformation in the human mind: a 30-year investigation of the malleability of memory. Learn Mem. 2005;12(4):361-366. doi:10.1101/lm.94705

8. Grilli MD, Ryan L. Autobiographical memory and the self-concept. In: Lane RD, Nadel L, eds. Neuroscience of Enduring Change: The Neural Basis of Adaptation and Development. Oxford University Press; 2020:189-212. doi:10.1093/oso/9780190881511.003.0008

9. Conway MA, Pleydell-Pearce CW. The construction of autobiographical memories in the self-memory system. Psychol Rev. 2000;107(2):261-288. doi:10.1037/0033-295X.107.2.261