Author Guide · Psychology & Twin Research
Books Written by
In 2013 a young woman in Bogotá mistook a stranger for her colleague Jorge. The stranger was William, Jorge’s identical twin, sent home from a Colombian hospital 25 years earlier with the wrong brother. Cases like that are Nancy L. Segal’s material. Born in Boston in 1951, a fraternal twin herself, she worked on the Minnesota Study of Twins Reared Apart from 1985 to 1991 and then founded the Twin Studies Center at California State University, Fullerton. Her books leave the journal register behind and tell real stories — twins switched at birth, separated by an adoption agency, photographed as survivors of Auschwitz — to show what twins reveal about nature and nurture. This guide covers her nine key books, in order, and where to begin.
By Lisanne Swart · 9 key books · 1999–2023 · Updated October 2026
ClickTap a spine to jump to that book below.
Quick answer
Nancy L. Segal’s best-known books are Born Together—Reared Apart (2012), her history of the Minnesota twin study, which won the APA’s William James Book Award, Entwined Lives (1999), Accidental Brothers (2018) and Deliberately Divided (2021). Start with Entwined Lives, her first book for general readers, which explains what twin research can and cannot show.
Where to Start with Nancy L. Segal
If you want the basics of twin research
→ Entwined Lives (1999) — her first book for general readers.
If you want her defining book
→ Born Together—Reared Apart (2012) — the inside history of the Minnesota twin study.
If you want a true story
→ Accidental Brothers (2018) — two pairs of Colombian twins switched at birth.
If you want the ethics of twin research
→ Deliberately Divided (2021) — the study behind Three Identical Strangers.
9 Nancy L. Segal Books, in Order
Entwined Lives
Her first book for a general audience, published by Dutton. It explains the types of twins and what twin studies have found about intelligence, personality and other traits, then turns to how twins relate to each other: bonding, cooperation, competition and grief when a twin dies. Fraternal, identical and conjoined twins are all covered.
Indivisible by Two
Published by Harvard University Press, it tells the stories of ten sets of twins, one set of triplets and one set of quadruplets in detail. Among them are the “fireman twins”, separated brothers who both became firefighters, and identical triplet brothers, one of whom is gay. Segal uses each case to test what genes and environment explain, and touches on the questions raised by human cloning.
Someone Else’s Twin
Identical twins Begoña and Delia were born in the Canary Islands, where a hospital mix-up sent Delia home in place of another baby, who was raised as Begoña’s fraternal twin. The truth came out when the women were 28, after a shop assistant mistook one for the other. Segal follows the discovery and what the accidental experiment says about nature and nurture. Published by Prometheus Books.
Born Together—Reared Apart
The history of the Minnesota Study of Twins Reared Apart, the University of Minnesota project that tested twins separated early in life and reunited as adults, and on which Segal worked from 1985 to 1991. She covers its origins, methods, best-known pairs and the criticism the study attracted. Published by Harvard University Press, it won the American Psychological Association’s William James Book Award in 2013.
Twin Mythconceptions
Published by Academic Press, it takes common beliefs about twins one at a time and checks each against the research. It is the most reference-like of her books, written for parents, twins and readers who want to know what has actually been established.
Accidental Brothers
Written with the psychologist Yesika S. Montoya. Two pairs of identical twins were mixed up in a Colombian hospital: Jorge grew up with Carlos in Bogotá, William with Wilber in the rural village of La Paz, each family believing it had fraternal twins. The error came to light in 2013, when the brothers were about 25. Published by St. Martin’s Press.
Deliberately Divided
The story of a study in which the New York adoption agency Louise Wise Services placed twins and triplets in separate homes, and the psychiatrist Peter Neubauer followed them for years without telling them they had a sibling: four pairs of twins and one set of triplets, the case behind the film Three Identical Strangers. Segal interviewed several of those separated and ethicists about the study. Published by Rowman & Littlefield.
The Twin Children of the Holocaust
An annotated collection of photographs Segal took of twins who survived Josef Mengele’s experiments at Auschwitz-Birkenau, around the 40th anniversary of the camp’s liberation in 1985 and at the public hearing on Mengele’s crimes at Yad Vashem that followed. Ten sections, each with an introductory essay, also cover trips to Kraków and Warsaw and survivors’ testimony. Published by Academic Studies Press.
Gay Fathers, Twin Sons
Andrew Banks and Elad Dvash, married in Canada, had twin sons from one egg donor and one surrogate but different biological fathers. When the US consulate in Toronto granted citizenship only to Aiden, Andrew’s son, and not to Ethan, the family sued, and the case drew attention worldwide. Published by Rowman & Littlefield in August 2023.
In What Order to Read Nancy L. Segal
The books do not continue from one another, so you can read by subject. Entwined Lives is still the best start because it explains the method — why comparing identical and fraternal twins says anything about genes — that the later case books take for granted.
After that, the books fall into two groups: research books (Born Together—Reared Apart, Twin Mythconceptions) and single true stories (Someone Else’s Twin, Accidental Brothers, Deliberately Divided, Gay Fathers, Twin Sons). The Twin Children of the Holocaust is a book of photographs and testimony, best read on its own terms.
A reading path
1. Entwined Lives (1999)
2. Accidental Brothers (2018)
3. Born Together—Reared Apart (2012)
4. Deliberately Divided (2021)
5. The Twin Children of the Holocaust (2023)
Everything Else Nancy L. Segal Published
Beyond the 9 key books: 1 more, grouped by kind.
Open a group with the + to see every book in it.
Edited volume
1
1 book · 1997
- 1997Uniting Psychology and Biologyintegrative perspectives on human development, co-edited with Glenn E. Weisfeld and Carol C. Weisfeld (APA Press)
The years · 1951–2023
Quick facts about Nancy L. Segal
- Known for
- She is known for bringing twin research to general readers through real cases of twins reared apart, switched at birth or separated on purpose.
- 1951
- Born a fraternal twin in Boston
- 1985
- Joins the Minnesota twin study
- 1991
- Founds the Twin Studies Center, CSU Fullerton
- 2023
- Two new books, on Auschwitz twins and a citizenship case
- Born
- 2 March 1951, Boston; grew up in Riverdale, the Bronx
- Twin
- A fraternal twin, with a sister
- Education
- BA Boston University (1973); MA (1974) and PhD (1982), University of Chicago
- Worked
- Minnesota Center for Twin and Adoption Research, 1985–1991; then California State University, Fullerton
- Founded
- The Twin Studies Center at CSU Fullerton, 1991
- Honours
- James Shields Award for Lifetime Contributions to Twin Research (2005); William James Book Award (2013)
The red mark is 2012: Born Together—Reared Apart, her history of the Minnesota twin study.
How Nancy L. Segal Writes: One Case at a Time
Segal writes as a working scientist. Her books are built on cases she studied herself: the Minnesota twins she worked with, the switched pairs in Spain and Colombia, the people separated by Louise Wise Services, whom she interviewed. The stories carry the argument, but each comes back to data — what identical twins share that fraternal twins do not, and what that says about genes and upbringing.
She is careful about what a single case can prove. A switched-at-birth pair is, in her framing, an accidental experiment: striking, but small. That is why Twin Mythconceptions exists, and why Deliberately Divided weighs the scientific value of the Neubauer study against what it cost the people in it, with ethicists’ views set beside the accounts of the people separated.
Her tone stays plain, and the people in her case books are named and quoted rather than reduced to numbers. For other writers on nature, nurture and the mind, see Authors like Nancy L. Segal.
Watch
Nancy L. Segal on twin research
Segal talks about her twin research on In Focus, a programme of Titan TV, the station of her own university, California State University, Fullerton, in April 2018.
Frequently asked questions about Nancy L. Segal’s books
How many books has Nancy L. Segal written?
Nine books as author, all on this page, from Entwined Lives (1999) to Gay Fathers, Twin Sons (2023), plus one edited volume, Uniting Psychology and Biology (1997). She has also written about 300 scientific articles and book chapters.
What are Nancy L. Segal’s books in order?
Entwined Lives (1999), Indivisible by Two (2005), Someone Else’s Twin (2011), Born Together—Reared Apart (2012), Twin Mythconceptions (2017), Accidental Brothers (2018), Deliberately Divided (2021), The Twin Children of the Holocaust (2023) and Gay Fathers, Twin Sons (2023).
What is the best Nancy L. Segal book to start with?
Entwined Lives: Twins and What They Tell Us About Human Behavior (1999). It is her first general-audience book and explains the method and findings of twin research that her later, more specific books build on.
What is Nancy L. Segal’s most famous book?
Born Together—Reared Apart (2012), her history of the Minnesota Study of Twins Reared Apart, which won the American Psychological Association’s William James Book Award in 2013. Deliberately Divided is the best known to readers who saw Three Identical Strangers.
Is Nancy L. Segal a twin herself?
Yes. She is a fraternal twin, born in Boston in 1951, and has written that she and her sister “have always looked and behaved very differently”.
What is the Minnesota Twin Study, and which of her books covers it?
The Minnesota Study of Twins Reared Apart, run at the University of Minnesota, tested twins who had been separated early in life and reunited as adults. Segal worked there from 1985 to 1991. Born Together—Reared Apart (2012) tells its history, including its methods and its critics.
What is Deliberately Divided about?
The study in which Louise Wise Services, a New York adoption agency, placed twins and triplets in separate homes while the psychiatrist Peter Neubauer followed them without telling them they had a sibling — four pairs of twins and one set of triplets. It is the same case as the 2018 documentary Three Identical Strangers. Segal interviewed several of the people separated, and ethicists.
What is Accidental Brothers about?
Two pairs of identical twins mixed up in a Colombian hospital and raised as two pairs of “fraternal twins”, one in Bogotá and one in the village of La Paz. The mistake came to light in 2013. Segal wrote it with the psychologist Yesika S. Montoya.
Where does Nancy L. Segal work?
She is a professor of developmental psychology at California State University, Fullerton, where she founded the Twin Studies Center in 1991. She is also an associate editor of the journal Twin Research and Human Genetics.
What are her most recent books?
Two came out in 2023: The Twin Children of the Holocaust, her photographs of twins who survived Mengele’s experiments at Auschwitz, and Gay Fathers, Twin Sons, about a family’s fight for US citizenship for both their twin sons.
Footnotes
- Nancy L. Segal — official biography
- Nancy Segal — Wikipedia
- Segal, Nancy 1951– — Encyclopedia.com
- Accidental Brothers review — Bostonia, 2018
- Deliberately Divided: A Review — Quillette, 2021
- The Twin Children of the Holocaust — New Books Network
Bibliography last verified 9 October 2026. Where sources give different first-publication years, this page uses the year in library catalogues or the publisher’s own record.
This list was not put together by a machine.
The notes are all mine,
and the books are all his.
Lisanne x
Written by Lisanne Swart, who has curated reading lists and author guides at lisanneswart.com since 2021.
See more on her personal bookshelf.
“My sister and I have always looked and behaved very differently.”
— Nancy L. Segal, on her fraternal twin sister (quoted in Contemporary Authors)
If this shelf interested you, the full bookshelf has more: reading lists by theme, author guides, and notes on what is actually worth your time.
Browse the full bookshelf Get book recommendations