If you’ve ever looked at your siblings and wondered, “How did we come from the same two people?”—or if you’ve ever suspected that you were accidentally swapped for a more athletic model at birth—Dr. Nancy Segal has the data to make you feel both vindicated and deeply unsettled.
Segal has spent her career proving that while your parents might have given you a roof and a curfew, your DNA gave you your weirdest habits. Here is a breakdown of why her work suggests we are essentially walking genetic biological blueprints with very little choice in the matter.
The “Jim Twins” and the Myth of Individual Choice
You can’t talk about Segal’s work without mentioning the “Jim Twins.” Separated at birth and reunited at age 39, James Lewis and James Springer discovered they had both:
- Owned a dog named Toy.
- Married a woman named Linda, divorced her, and then married a woman named Betty.
- Named their first sons James Alan and James Allan, respectively.
- Suffered from tension headaches and bit their fingernails to the nub.
- Enjoyed math and were hobbiest woodworkers.
- Both were deputy sheriffs, smoked Salem cigarettes, and drove a light blueChevrolet.
If you are an only child, this sounds like the plot of a poorly written sci-fi movie. But for Segal, it’s just Tuesday. It raises the hilarious but haunting question: Are you actually “choosing” your spouse, or is there a sequence of nucleotides in your 11th chromosome that is really into women named Betty?
The Firemen and the “Pinky Under the Can”
One of Segal’s favorite modern anecdotes involves the “Fireman Twins.” These brothers met for the first time in their 30s. Not only were they both volunteer firefighters, but they also worked similar day jobs—one installed fire suppression systems, and the other installed burglar alarms.
The kicker? They both arrived at their first meeting with a very specific, idiosyncratic way of drinking beer: they held the can with their pinky finger tucked underneath the bottom. When Segal asked why, they didn’t have a reason—they just said it felt “right.”
This is what researchers call the “Gene-Environment Correlation.” It’s the idea that we don’t just land in an environment; we select one. If your genes make you a risk-taking, altruistic person who hates spilled beer, there is a statistically high probability you will end up holding a hose in one hand and a Budweiser in the other.
The Non-Shared Environment: The Ultimate Cosmic Joke
Perhaps the most “insulting” part of Segal’s research for hardworking parents is the concept of the Non-Shared Environment (NSE).
For years, we assumed that “Shared Environment”—the house you lived in, the books on the shelf, the “family values” preached at dinner—was what made siblings similar. Segal’s work, particularly with “Virtual Twins” (unrelated children of the same age raised in the same home), proved the opposite. These kids have the most shared environment possible, yet they grow up to be no more alike than two strangers at a bus stop.
The Non-Shared Environment is the “noise” of life that only happens to one sibling, and it’s often where the comedy lies:
- 1Same Home, Different Planet: Two kids can grow up with the same father and describe two different men. One child, genetically wired to be gregarious, sees Dad as a “fun-loving guy.” The other child, wired for sensitivity, sees that same man as a “loud, overbearing giant.” You aren’t reacting to your parents; you are filtering them through your DNA.
- Parental Differential Treatment (Or: It’s the Kid’s Fault): We often blame parents for having a “favorite,” but Segal’s research suggests that kids actually train their parents. A “cuddly” baby genetically triggers more hugs. A “difficult” baby triggers more discipline. The parents think they are in charge, but the kids’ DNA is actually writing the parenting manual.
- The Power of Randomness: NSE includes things parents can’t control—like a specific 4th-grade teacher who liked your brother better, or the time you got your head stuck in a banister while your sister was at camp. These tiny, unshared ripples in time do more to shape your personality than ten years of expensive family vacations to Disney World.
Why This is Good (and Terrifying) News
Dr. Segal’s work offers a strange kind of comfort. If you have a child who refuses to eat anything but dinosaur-shaped nuggets and insists on wearing their socks inside out, you can probably stop blaming your parenting.
The takeaway from decades of Segal’s research is this: We are not blank slates. We are more like a pre-programmed software update, and our parents are just the hardware installers. This doesn’t mean we don’t have a choice in how we behave, but it does mean we are working with a very specific set of tools.
So, the next time you find yourself doing something incredibly specific and strange—like flushing the toilet both before and after use (an actual trait found in one set of twins)— just smile and remember: You aren’t weird. You’re just following the instructions your DNA wrote before you were even born.
By: Krista Carpenter, MS, LPC
