“Let's say you have an axe. Just a cheap one, from Home Depot,” opens the horror-comedy novel John Dies at the End. “On one bitter winter day, you use said axe to behead a man.” This blow splinters the axe's handle – so the story goes – so you get the hardware store stick a new handle on the blade.
The repaired axe sits in your garage until one day the next spring, when you damage the blade while fending off “a foot-long slug with a bulging egg sac on its tail,” which requires another trip to the store to replace the axe-head. Unfortunately, when you arrive home, you're greeted by the enraged reanimated corpse of the man you beheaded last year. He takes a long look at the weapon you're holding, and he screams, “That's the same axe that beheaded me!”
The question is, Is he right?
It's a riddle with a long and distinguished pedigree, dating at least as far back as ancient Greece, where the historian Plutarch posed essentially the same question by telling a story involving an aging ship, an adventurous crew, and a notable absence of vengeful zombies. Philosophers through the ages have regarded the riddle as a sort of dead end, because any given person's answer hinges not on any actual attribute of the axe or ship in question, but on how the answerer chooses to define the word “same.” In one sense, if we swap the ancient Greek ship for a zombie-slaying axe, we're not even posing the “same” riddle.
Consider, by way of comparison, a set of identical twins. Although plenty of anecdotes attest that sets of twins often share the “same” tastes in music, pets and mates – even if they've spent most of their lives apart – they still remain distinct individuals, and may have converged on those tastes for surprisingly different reasons. Parents and close friends, meanwhile, can tell one twin from his brother or her sister by tiny differences in behavior – one might be a little more adventurous than the other, or more introverted, or pickier about food; and over a lifetime, these small difference tend to add up to more significant ones.
In other words, despite the genes – and the womb – shared by a set of identical twins, they can't share everything in common. Each twin lives in an inner mental world that's distinct from every other on earth – and what's more, researchers are learning that tiny variations in brain development begin to lay the foundation of each twin's individual personality long before birth.
And as a new study has discovered, those prenatal variations add up to distinct personalities even in animals engineered to be genetically identical – twins that are, in effect, exact clones of one another. These results raise pointed questions not only about our framing of the old “nature versus nurture” debate, but also about what it means, precisely, to be an individual.
Look back far enough in history, and you'll find that the concept of “individuality” was once synonymous with that of the soul. For millennia, religious leaders taught that God (or the gods, or one's karma) placed a specific soul into one's body sometime around the moment of conception or birth, infusing that body not only with breath and animation, but also with a specific self, unique among all others and distinct from the bodily vessel that contained it.
Even in the midst of such dualistic thinking, though, philosophers warred over the questions of how much “self-stuff” descended directly from the heavens, and how much of it was shaped by a person's life here on earth. Confucius wrote, “By nature, men are nearly alike; by practice, they get to be wide apart” – but in the West, Plato and Aristotle launched the debate in earnest by taking stances at opposite ends of it: Plato with his notion of the individual as a perfect heavenly form marred by earthly struggles; Aristotle with his idea of the newborn mind as a blank slate.
Echoes of this old Greek argument were still ringing when Shakespeare wrote The Tempest, though the debate had grown more nuanced over the intervening centuries. When Shakespeare's character Prospero describes the monster Caliban as “a born devil, on whose nature nurture can never stick,” he's alluding to the idea that an individual's nature must somehow be shaped by social interaction – and coining the phrase “nature/nurture” while he's at it.
In Shakespeare's day, official Church doctrine taught that at least some “innate ideas” – those of God, of virtue and of absolute truth – existed in the soul from the moment of birth, and possibly even in the womb. It wasn't for another century or so that a philosopher like John Locke could freely and openly describe the mind as a “tabula rasa” – a blank slate on which experience inscribes a personality – while the more cynical Thomas Hobbes could argue that the only truly innate idea is probably selfishness. And it wasn't until the nineteenth century, when scientists like Gregor Mendel and Francis Galton began describing heredity in precise mathematical terms, that the “nature” part of the equation really came under the microscope.
Ever since James Watson and Francis Crick explained the structure and behavior of DNA, most scientists have understood that “genes load the gun, but environment pulls the trigger.” In other words, DNA doesn't actually encode your individual traits; it encodes long lists of instructions for building various kinds of biological structures if – and only if – the right circumstances arise. In programming terms, much of your DNA could be described as a nested hierarchy of “IF-THEN” statements, each waiting for the signal to execute the code within. And many of those signals come not from the DNA itself, but from the environment in which it unzips.
When we speak of a “gene for Parkinson's disease,” for example, we're not talking about a gene that literally encodes instructions for producing the disease's symptoms; we're talking about certain genetic mutations – tiny alterations in DNA code – that can help create those symptoms under certain unfortunate circumstances. These kinds of correlations become even murkier when we speculate about “psychopath genes” or “genes for depression.” As much as Shakespeare's character Prospero might disagree, psychologists have found that a nurturing childhood environment can soften even the most “devilish” genetic nature.