I couldn’t believe how different our identical twin boys are sometimes. For two kids raised exactly the same, they absolutely have different personalities. But then our third child turned out completely different too, and I realized it can’t just be us. I looked into it, and not only is this fairly common in twins, but science may actually have an explanation for this phenomenon.

Our three kids have numerous similarities like general silliness and a penchant for cucumbers. But they are also as different as can be in other ways. One of the twins is a social butterfly while the other is shy when meeting people the first time, then quickly their best friend later. One can’t get enough of wearing muscle shirts and shorts even when it’s in the 40s, while the other demands pants unless it’s over 70. One would chose to color rainbows all day long if he could while the other would be off playing with friends on the playground. Some days it feels like they aren’t even related.

The baby, though only a year old is incredibly different too. While the twins needed movement and constant rocking to nap as babies, he will completely put himself to sleep in his crib after one minute but refuses to nap in a stroller while it was the only place we could get the twins to sleep. The baby has a nut allergy and only wants meat while the twins would (and have) scarf down a full pint of tomatoes or a loaf of bread. Of course the baby grew up in different situations, but some of these differences seem like more than just a product of slightly different conditions.

However, scientists are now finding that these little differences may, as the result of a series of feedback loops, actually cause these large differences in behavior. Behavioral studies with children as well as in studies with mice show some evidence that small differences in personality at birth and early on can result in slightly different experiences, which then spiral into very different personalities that diverge over time. While twins may be very similar at birth, they may experience slightly different things in life that cascade into these differences.

For example, one twin might spend a little more time with an extroverted family member, friend, or teacher, and thus become slightly more extroverted. Because they’re slightly more extroverted, they will be more likely to make friends with, or even just spend slightly more time with other extroverted children. Conversely, the other might begin to shy away from this to distance themself from their sibling, or through other chance encounters and experiences, align themself with introverted people, sustaining the same kind of feedback loop that drives them even further away. It’s not clear how much of this may be driven by small natural differences in personality, and how much is truly up to these little experiential differences, but it seems likely that this combination plays a large role in development.

It’s natural to think we don’t treat our children differently as well. As parents, or grandparents, or friends, we try to treat kids equally to demonstrate fairness. Everyone should play by the same rules and everyone should get the same chance. In reality though, there are of course little differences. I know for a fact that I give one of the twins a little more rope with some behavior like rough housing because it’s less common for him. The other is less likely to be in trouble for doing other things like teasing his brother because he’s funnier about it. The baby gets away with a ton of things because he’s figured out if he smiles and laughs, he won’t be in trouble. He also just hasn’t been on our nerves as much as the older boys.

It’s likely that these little differences in how we treat them also add up over time to encourage different behavior and what we perceive as personalities in them. One grows to learn that he is allowed certain behavior while for the other it’s forbidden. In our attempt to even things up as parents and encourage the same set of rules and behaviors, we are actually grooming divergent ones. All of this feel reminiscent of chaos theory. The more we seek control, the more things will spiral out and have unintended consequences from these feedback loops.

For me at least, it’s too easy to think of kids as pre-programmed by their DNA and genes, and that environment only slightly shapes them. These studies reaffirm what I notice in the real world, that while nature may establish the baseline, nuture makes a huge difference, especially over time. Three kids growing up in the exact same household can end up with dramtically different personalities and traits. It may feel like we’re doing nothing different between them, but it’s pretty obvious when one is out riding a bike trying to keep up with kids five years older to seem cool, another is digging for worms to save with the flowers, and the third is trying to crawl over to feed the dog his goldfish. Then we go inside and the twins fight about whether they should build a tower or play cars while the baby tries to smash it all. Then after all of that, the twins hug the baby in a sandwich and screetch “twinsies!”

We may feel in control as parents, but nature and science have different things in mind for us. Kids will grow up differently no matter what we do. When we think we’re controling the situation, we’re actually probably making things even more different. As much as we think our children are getting the same equal experiences and opportunities, little differences snowball over time. Much like a butterfly flapping its wings halfway around the world, little things can have massive effect over time. We can’t keep everything exactly the same for our kids, but, why would we actually want them to have the exact same personalities anyway?