Where Does "Identity" Come From?

 (You, Your Brain, and the Nature vs. Nurture Debate)

Imagine your life is a tape, and we rewind it. All your accomplishments, awards, and graduations are erased. Your experiences move in reverse, and you grow smaller and smaller, as you were as a child. Your adult teeth turn back into baby teeth, and eventually retract completely, while all of your traits and quirks start to fade away. Pretty soon language goes away too, and you’re no longer you, but potential you. The tape continues rewinding, halving colonies of your cells until finally we arrive at the amazing singular miracle: the one cell that will evolve to become you.

Now, the question is this: what happens when we press “play” again? The common battle between nature versus nurture arises – the question that psychologists and scientists everywhere are prodding, probing, and researching to figure out. Are your talents, traits, and personal characteristics deeply embedded in your genes? Is what makes you you implanted in the DNA of our cells? Or could things completely change who we are with a few simple nudges? To put it simply – how much of your fate do you believe depends on your genes, your surroundings, or merely just by chance?

Since we can’t rewind time, Julia Freund and her colleagues found another way to better answer this question in a simple but remarkable recent study. To test the nature-versus-nurture phenomenon, Freund and her investigators placed genetically identical mice in a common environment, and tested to see whether systematic behavioral differences still could emerge. By answering “yes,” it would mean that there are sources of behavioral variability (intrinsic individuality, if you will) that could be unaccounted for by a combination of common genes and a common environment.

For the experiment, 40 genetically identical mice were placed in an “enriched” environment, where Freund monitored their behavior for a period of three months (which, for mice, is about 10-15% of their entire lifespan). The enriched environment was about 36 square feet, engineered to include multi-tiered platforms, nesting boxes, and interconnected tubes in order to promote exploratory behaviors in the mice that would not exist in a normal confined cage. What makes this study different from one of human twins is that by using mice, the subjects’ movements could be recorded with extraordinary detail over a major period of their lifespan. A radiofrequency ID transponder was placed on every mouse, measuring their every movement, chase, and sedentary period.

In order to measure the differences in behavior between the mice, the investigators used a gage called “roaming entropy.” Roaming entropy captures how often you get out, and with how much variety – so basically if you are someone who just darts back and forth (say from your dorm room to Van Pelt…) your roaming entropy is low. But if you’re the type of person who could pretty much be anywhere at any given time, you have a high roaming entropy. At the beginning of the study, the mice all had fairly similar roaming entropies… however, as the weeks progressed, the population diverged significantly, with some mice being much more exploratory and active than others. If you take the tendency to explore as a characteristic trait, then this is obviously one that elaborates and changes over time in a way that isn’t strictly determined by genes or the environment.

However, the most interesting part of the study arose when Fruend and her team examined the changes in brain activity that went along with the changes in exploratory behavior. Before the experiment was over, the mice were injected with a compound that selectively incorporates itself into dividing cells. This basically means that the compound can show researchers which neurons are formed in adulthood, and which neurons the mice were born with. While most of our neurons are formed during early development, there are a good number of well-studied brain areas that continuously produce new neurons throughout our adult lives.

Surprisingly, the mice that were the most exploratory throughout the study (who exhibited the most outgoing behavior) were also those who experienced the greatest production of adult-born neurons. While we can’t say this particularly proves anything, the results are still pretty intriguing. Even after your genes are set in stone from birth, and the majority of your environmental surroundings are laid firm throughout your early development, your brain maintains the raw potential to grow its own new neurons. The investigators of this study propose that these neurons are involved in tailoring and tuning our behaviors, implying that the way we live our lives may make us who we are.

So, how does this happen? We don’t actually know. No disrespect intended to these researchers, but any experiment addressing such a controversial, profound, and metaphysically-tangled problem as the nature-vs.-nurture debate is going to generate more questions than answers. It could be that epigenetic changes, where experience modifies gene expression, gives rise to completely different life paths. It can also be questioned just how substantial the differences in roaming entropy could actually be, and whether it was actually statistically significant. Regardless of the specifics and questions left to be answered, this experiment is a reminder that our lives are truly a work in progress. Whether it is our genes, our environments, or generation of adult born neurons, the nature versus nurture debate is yet to be solved. But it seems that if we are living out our lives as a sort of tape, then it’s a tape in which the tracks can be tweaked as they’re read, as our genes can be modified as we live. As your brain is shaped by your life and vice versa, there is so much room for chance and noise – room for you to become you.