What makes us who we are?

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Professor David Linden. Photo by David Linden.

As any parent will attest, raising kids  comes with many challenges and questions. Perhaps the most fundamental:  nature versus nurture. Research now shows that there may be more components to that question. Are the experiences that we have even before we are born actually more important in defining our individuality? Beyond hereditary and experience, what else makes us who we are? 

KCRW’s Jonathan Bastian talks with Professor of neuroscience at Johns Hopkins University and author of “Unique: The New Science of Human Individuality.” 

The following interview excerpts have been abbreviated and edited for clarity. 

KCRW: When it comes to human individuality, why is it more complex than nature versus nurture?

David Linden: “‘Nature versus nurture’ is really fun to say, it has a good beat, and you can dance to it. But it's a terrible expression and it really doesn't get to the heart of the issue. I'm fine with nature standing in for heredity. That's just poetic language. But it gives the idea that nurture, which means how your parents and your community raised you as a child, constitutes everything that you don't inherit genetically from your parents. That's important and it is just so untrue. 

There's so many broader things that impinge upon us to form us as individuals. So I would replace ‘nurture’ with the word ‘experience.’ And by that, I mean experience in the broadest possible sense, not just social experience, and not just experience that can be written in the memory. But the diseases your mother fought off while she was carrying you in utero, the foods you ate as a child, the bacteria that colonize your gut, the day length and temperature when you were young. All these things form us. 

So experience must be a much broader category. And then the ‘versus’ is a problem, too. Because it's not all ‘versus,’ it's not all push and pull between one and the other. They often interact and conspire together. For example, if you're fortunate enough to be born quick and coordinated, you're very likely to practice sports and get better at them. That's heredity and experience reinforcing each other, not in opposition to each other.”

When you think of siblings who grow up in the exact same environment, why does one want to go out and explore, and the other likes to stay home?

Linden: “The trait of novelty seeking is approximately 40% heritable, which leaves a whole lot for other things.Those other things include not just how your parents raised you and what happened in the community, but also what happened when you're being carried in utero. But also importantly, there is a healthy dose of just pure randomness. 

So here's what I mean. If you take newborn identical twins, monozygotic twins, they have the same DNA, they grew together lying right next to each other in the womb. And they're born and you look at them. They're not really identical either in their appearance or in their temperament. If you go look inside with a medical scanning machine, you might find one has a liver that's 30% larger than the other. 

There are animals that are always born as identical sibs. For example, the nine banded armadillo is born as identical quadruplets. Yet, if you compare the nine banded armadillo siblings, already at birth they are showing lots of different things. 

The reason is because the DNA isn't a blueprint. It is not a precise map that instructs exactly where your cells go and what they do and what they connect with down to the finest finest detail. Rather, it's a set of general instructions. It might say to a group of neurons in your brain, ‘Hey you guys, about half of you cross the midline and half grow this other way.’ So in one twin that might be 40% and in the other 60%. So there's a big dose of randomness, and that's the third factor beyond heritability and experience that makes us individuals.” 

Explain what happens to a child in utero. What influence does temperature play, or whether or not your mother was sick? 

Linden: “This is something that's relevant to our present COVID situation. … During massive influenza outbreaks, like the 1918 pandemic flu, we know that women who were carrying babies in the winter of 1918-1919, when their children were born and grew up, those kids had a fourfold higher incidence of schizophrenia and roughly a fourfold higher incidence of autism than the general population, meaning that it went up from roughly 1% to roughly 4%. That's an enormous effect. 

We have some evidence from studies done on mice suggesting how that might happen. So when the mother is fighting off the infection, her body secretes a chemical called interleukin 17. It's an immune system signaling molecule, which passes through the placenta and affects the developing brain of the fetus, particularly if the fetus is just exactly at the right stage to be susceptible to that.  We don't know what's going to happen with the babies whose moms were fighting off COVID when they were in utero, and it's going to be really interesting to find out.” 

Does this shed more light on the importance of the period in which the mother is pregnant? 

Linden: “It’s a crazy time, so crazy that we're taught that our cells in our body all come as combinations from our mother and father. But that's not true. If you're a woman who's ever been pregnant, your body is colonized by the fetal cells of the fetuses you carried. They can take up residence in your body and in your brain and last your entire life. 

Likewise, if you have a twin, there's a good chance that your twin cells transfused, via your mother, into you and have been retained through your life. You can even, if you are a younger sibling, have cells from an older sibling that passed on to your mother, stayed there for a while, and then got passed to you during the pregnancy in which you developed. So yes the time in utero is really rife with all kinds of excitement and ways to impact and to create human individuality.” 

Credits

Guest:
David Linden - Professor of neuroscience at the Johns Hopkins University School of Medicine - @david_j_linden

Host:
Jonathan Bastian

Producer:
Andrea Brody