Author Guide ·
Books Written by
Segal is a fraternal twin herself, and she has spent a forty-year academic career trying to answer the question that follows every twin around: how much of who we are was decided before birth? As director of the Twin Studies Center at Cal State Fullerton, she has published widely on the nature-versus-nurture debate, but her books are where she leaves the journal-article register behind and writes for a general reader — using real, often extraordinary cases of twins separated at birth, switched in hospitals, or deliberately split apart for research, to make an argument about how much of ourselves is written in before we ever choose anything.
By Lisanne Swart · 7 books · Psychology · Twin Research · Non-Fiction · Updated August 2026
- #1 Entwined Lives: Twins and What They Tell Us About Human Behavior
- #2 Indivisible by Two: Lives of Extraordinary Twins
- #3 Someone Else's Twin: The True Story of Babies Switched at Birth
- #4 Born Together—Reared Apart: The Landmark Minnesota Twin Study
- #5 Twin Mythconceptions: False Beliefs, Fables, and Facts About Twins
- #6 Accidental Brothers: The Story of Twins Exchanged at Birth and the Power of Nature and Nurture
- #7 Deliberately Divided: Inside the Controversial Study of Twins and Triplets Adopted Apart
PsychologyOn my shelf
Entwined Lives: Twins and What They Tell Us About Human Behavior
Segal’s first book for a general audience, built around the case histories of twins reared apart and reunited as adults. She uses these stories to walk through what twin studies have actually established about personality, intelligence, and development — and to correct a lot of pop-science overstatement about twins along the way.
The right entry point precisely because it’s the most foundational — it lays out the logic of twin research before the later books get into the more unusual and ethically fraught cases.
Psychology
Indivisible by Two: Lives of Extraordinary Twins
A survey of unusual twin pairings — conjoined twins, twins raised in different countries, twins with significant disability, twins who became public figures largely because they were twins. Segal uses each case to push at the edges of what twin studies can and can’t explain.
Where Entwined Lives sets up the field, this one shows how far the questions actually go once you leave the standard research pool behind.
True Story
Someone Else’s Twin: The True Story of Babies Switched at Birth
An account of a 1970s hospital switch in the Canary Islands that sent one twin of an identical pair home with the wrong family, and the legal and emotional reckoning decades later when the mistake surfaced. Segal treats it as both a human tragedy and an unintended natural experiment.
The most narrative of her books, and a good choice if you want the human stakes of twin research made concrete before returning to the more academic titles.
Landmark Study
Born Together—Reared Apart: The Landmark Minnesota Twin Study
Segal’s account of the Minnesota Study of Twins Reared Apart, the research program that produced most of what the public thinks it knows about identical twins separated at birth. She covers its origins, methods, most famous cases, and the controversies the study attracted. It won the American Psychological Association’s William James Book Award.
Her most substantial and most honored book — the one to read if you only add one Segal title beyond Entwined Lives.
Psychology
Twin Mythconceptions: False Beliefs, Fables, and Facts About Twins
A myth-by-myth debunking of popular beliefs about twins — telepathy, identical personalities, one twin always being “the evil one” — checked against what the actual research shows.
Useful as a corrective if you’ve read a lot of twin-adjacent pop psychology and want to know what’s actually been established versus what’s folklore.
True Story
Accidental Brothers: The Story of Twins Exchanged at Birth and the Power of Nature and Nurture
The case of two pairs of Colombian identical male twins, accidentally cross-switched at birth so that each pair was raised split between two families. Segal traces what happened when the mix-up was discovered as adults, and what the case does and doesn’t prove about nature versus nurture.
A companion piece to Someone Else’s Twin, and one of the clearest illustrations in any of her books of how tangled the nature-nurture question gets in a real family.
Investigative
Deliberately Divided: Inside the Controversial Study of Twins and Triplets Adopted Apart
An investigation into the covert mid-twentieth-century study — the same one dramatized in the documentary Three Identical Strangers — in which twins and triplets were deliberately separated at adoption and studied without their knowledge or consent.
Her most pointed book, and the one with the clearest ethical stakes — a study that treated separated siblings as an experiment rather than people, examined by a researcher who has spent her career arguing that twin research doesn’t have to work that way.
Where to start with Nancy L. Segal
If you have never read her
→ Start with Entwined Lives. It lays out the field and the logic of twin research before you get into the more unusual case studies.
If you want the definitive book
→ Born Together—Reared Apart, her most substantial and most awarded work, covering the landmark Minnesota Twin Study directly.
If the ethics of separating twins for research is what interests you
→ Deliberately Divided. The sharpest and most uncomfortable of her books.
If you want human stories over research summary
→ Someone Else’s Twin or Accidental Brothers — both built around real, wrenching cases of babies switched or cross-switched at birth.
