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How likely are you?

CHILDREN are generally fascinated by tales of how they came to be. Even young ones can often grasp the mind-boggling implication if the events of the story leading up to their existence had been any different: they wouldn鈥檛 be there to hear it.

Your you-ness is a precarious thing. Rerun the experiment of you with a different sperm and egg from the same people, and 鈥測ou鈥� would be as different from your current self, genetically, as siblings are from one another. If the egg were the same, but through some random fluctuation a different sperm won the race, you would also be distinctly different. For a start, depending on whether the sperm bore an X or a Y chromosome, you could have ended up another sex. 鈥淭hat鈥檚 a pretty big difference, right there,鈥� says , a neuroscientist at Johns Hopkins University in Maryland and author of Unique: The new science of human individuality.

The potential for being a different you didn鈥檛 stop once destiny set your founding sperm and egg on their collision course, either. A lot of what makes you what you are is down to how your brain is connected. But your DNA doesn鈥檛 encode a precise wiring diagram: it is more like a rather hand-wavy recipe or set of instructions. Even genetically identical twins don鈥檛 end up with the same neuronal network. 鈥淎 pool of cells in the developing brain might receive instructions that say: 鈥楢bout half of you move across the midline of the brain鈥�, 鈥� says Linden. 鈥淚n one twin, 40 per cent of the cells might cross and in the other twin, 60 per cent.鈥�

Then there is mutation. As cells of the developing embryo, and later fetus, multiply and DNA is duplicated, mistakes are made and inherited by the cells鈥� descendants. These mutations are known to contribute to autism and conditions such as schizophrenia. It is plausible they influence core personality traits too.

After birth, the question of what makes us who we are has long been characterised as 鈥渘ature or nurture鈥�, or genes versus upbringing. Today, we know it isn鈥檛 such a simple dichotomy. Most of our characteristics are shaped by both nature and nurture, intertwined in intricate ways.

After we are born, our brains are constantly reshaped by our everyday experiences, an idea known as neuroplasticity. To take the most extreme example, if children are abused or neglected, it can affect them long-term 鈥� but so can good or neutral experiences. How you use your brain changes its structure as well. Some professional musicians, for example, have a distinct bulge that can be seen with the naked eye at autopsies in a part of the brain that controls movement. 鈥淚t turns out that what makes you 鈥榶ou鈥� is every conversation, every experience you鈥檝e ever had,鈥� says , a neuroscientist at Stanford University in California.

That鈥檚 before we even factor in how foreign bodies inside us influence our moods and emotions (see 鈥�Where are your boundaries?鈥�). All these factors make your existence, your appearance, your feelings, your quirks and your foibles, vanishingly improbable. It may sound trite, but you truly are unique.