How a study that began with two strangers named Jim changed everything behavioral scientists thought they knew — and what 40 more years of data confirmed.
In the summer of 1979, two men named Jim met for the first time. They were 39 years old. They had been separated at four weeks of age, adopted by different families in Ohio, and had grown up not knowing the other existed. When Jim Lewis and Jim Springer finally reunited, researchers scrambled to document the coincidences: both had married women named Linda, then remarried women named Betty. Both had named their firstborn sons James. Both drove the same model of car. Both had dogs named Toy.
The “Jim twins” launched one of the most consequential research programs in behavioral genetics. And Twins Magazine was there from the very beginning.
What twins raised apart reveal about human nature is this: when identical twins grow up in completely different families, cultures, and countries, they still turn out remarkably similar — in personality, intelligence, interests, and even medical history. Genetics shapes more of who we are than most people assumed in 1984, and the evidence has only strengthened in the four decades since.
Then: What We Reported in 1984
In our very first issue — July/August 1984 — Dr. Nancy L. Segal wrote the inaugural Research column for Twins Magazine. She was then a post-doctoral fellow at the University of Minnesota, working directly on the Minnesota Study of Twins Reared Apart under Professor Thomas J. Bouchard Jr. The study had been running for five years. The dataset stood at 38 identical pairs and 14 fraternal pairs. The findings were preliminary.
What Segal reported was already striking. Identical twins raised in separate households were more similar to each other in IQ, personality profiles, and medical history than fraternal twins raised in the same home. One identical pair had grown up on opposite sides of the world — one raised in Trinidad by a Jewish father, the other in Germany by a Catholic mother — and their personality scores came back nearly identical. The investigators, Segal wrote, were “quite impressed.”
But the study was still young. Bouchard’s goal was 100 pairs of identical twins and 50 pairs of fraternals. Progress, as Segal noted in that first column, was being made all the time.
What Changed: The Landmark 1990 Findings
Six years after that first column, Bouchard and his team — including Segal as a co-author — published their landmark paper in Science. By then, more than 100 sets of reared-apart twins had completed the study’s intensive “twin week”: a full schedule of medical exams, psychological batteries, brain wave analyses, and more than 15,000 questions answered over six days.
The 1990 findings were unambiguous. About 70% of the variance in IQ was found to be associated with genetic variation, and on multiple measures of personality, temperament, occupational interests, and social attitudes, identical twins reared apart were about as similar to each other as identical twins reared together. The environment these twins grew up in — the families, the schools, the cultures — had far less influence on who they became than anyone had predicted.
This did not mean environment was irrelevant. The paper was careful on this point. Genes are the recipe; environment is the cook.. The researchers proposed that genetic differences likely shape development partly by influencing which environments individuals seek out or are drawn toward — a concept now central to modern behavioral genetics.
The study’s funding history has drawn scrutiny. The Pioneer Fund — a private organization founded in 1937 with documented roots in the American eugenics movement — provided roughly $2.3 million to the Minnesota team, making it the study’s largest single private backer during its most productive years. Bouchard consistently maintained the fund never influenced methodology or results, and the core findings on IQ and personality heritability have since been replicated by independent research with no connection to the Pioneer Fund. The funding source is worth knowing; it doesn’t change what the data showed.
What Stayed the Same: The Archive’s Instincts Were Right
Reading Segal’s 1984 column now, what’s remarkable is how well the preliminary data held up. The IQ findings, the personality similarities, the medical parallels — all of it was confirmed and extended by the full study. The original 52-pair dataset was pointing in exactly the right direction.
The study’s core logic — that greater similarity between identical twins than fraternal twins signals genetic influence — remains the foundation of behavioral genetics today. Forty years of replication across different populations and methods has not undermined it.
What the Research Means for Twins and Their Families
For twins and their families, the Minnesota findings carry something that can feel both reassuring and unsettling: a lot of who you are was shaped before you walked through your front door.
Your temperament, your interests, even aspects of your health trajectory — these carry a strong heritable component. That doesn’t make your upbringing irrelevant. It means the relationship between genes and environment is more complex than a tug-of-war. Rather than making twins alike, environment tends to make them different. Identical twins raised apart stay remarkably similar despite their different homes; identical twins raised together still diverge in meaningful ways. What genes appear to organize is not a fixed outcome but a range of likely responses to the world.
For parents of twins, this reframes what “doing everything right” actually means. It means creating conditions where each child’s individual nature can develop — not trying to make two people the same, or assuming they’re entirely products of what you do.
Segal, who spent her post-doctoral years on the Minnesota study and later documented its full arc in Born Together — Reared Apart (Harvard University Press, 2012), has written that meeting the twins and watching their relationships evolve over the course of a single week was a thrilling experience — and that most reared-apart twins were profoundly gratified to finally meet a biological family member. The science and the human story were never really separate.
Where the Research Stands Today
The Minnesota study officially concluded in 1999 after two decades of data collection. The 70% IQ heritability figure has held up well against subsequent meta-analyses, though researchers now recognize that heritability estimates vary by age, trait, and population — and that epigenetics adds a layer of complexity the Minnesota study couldn’t fully address. Genes are not destiny; they are probability.
Segal herself has continued the work. She is currently studying Chinese twins adopted internationally by different families — a new natural experiment in an era of assisted reproductive technology and changing adoption practices. Twins continue to be separated today, she has noted, driven in part by families who cannot support multiple births and by the legacy of policies like China’s former One-Child Policy. Every new case is scientifically informative in its own way.
The question Galton posed in 1875, that Segal took up in 1984, and that Bouchard’s team answered in 1990 — how much of who we are comes from our genes? — turns out not to have a single number as its answer. It has a relationship. And twins, in their similarities and their differences, remain the best window we have into it.
Read more about what makes twins diverge even when they share the same DNA: Same DNA, Different People: Why Twins Diverge.
Frequently Asked Questions
What did the Minnesota Study of Twins Reared Apart find?
The study found that identical twins raised in completely separate families were remarkably similar in IQ, personality, interests, and medical history — nearly as similar as identical twins raised together. About 70% of IQ variation in the sample was linked to genetic differences. The study ran from 1979 to 1999 and included more than 100 sets of reared-apart twins.
How much of personality is determined by genetics?
The Minnesota study found a strong heritable component for most personality traits. But the researchers were clear that this doesn’t make environment irrelevant. Genes and environment interact continuously; what genes appear to influence is partly which environments individuals seek out and respond to. The finding is not that environment doesn’t matter — it’s that it works differently than expected.
Are identical twins raised apart still similar to each other?
Yes, consistently. Identical twins raised in separate families, sometimes in different countries and cultures, show striking similarities in temperament, interests, cognitive profiles, and even health history. The degree of similarity is comparable to identical twins raised together — which is the study’s most striking finding.
What was the Jim twins story?
Jim Lewis and Jim Springer were identical twins adopted separately at four weeks old in Ohio. Reunited at age 39 in 1979, they discovered an extraordinary list of parallels: both named their firstborn sons James, drove the same model of car, had dogs named Toy, and experienced similar headache patterns and health histories. Their reunion prompted Bouchard to launch the Minnesota study.
Does being raised in a different environment make twins more different?
The Minnesota research suggests that environment tends to make twins different rather than alike — which is the reverse of what most people expect. When identical twins grow up apart in different environments, their genetic similarities persist regardless. This points to genes playing a larger organizing role in development than the shared environment does in most circumstances.
This piece revisits research and guidance originally published in Twins Magazine in 1984. The original content has been updated to reflect the full arc of the Minnesota Study of Twins Reared Apart, including the landmark 1990 findings and subsequent research. What changed: the preliminary 1984 dataset of 52 pairs has since grown to 100+ pairs, and the findings on IQ and personality heritability have been confirmed and independently replicated.
