The identical “Jim twins” were raised in separate families and met for the first time at age thirty-nine, only to discover that they both suffered tension headaches, bit their fingernails, smoked Salems, enjoyed woodworking, and vacationed on the same Florida beach. This example of the potential power of genetics captured widespread media attention in 1979 and inspired the Minnesota Study of Twins Reared Apart. This landmark investigation into the nature-nurture debate shook the scientific community by demonstrating, across a number of traits, that twins reared separately are as alike as those raised together.
As a postdoctoral fellow and then as assistant director of the Minnesota Study, Nancy L. Segal provides an eagerly anticipated overview of its scientific contributions and their effect on public consciousness. The study’s evidence of genetic influence on individual differences in traits such as personality (50%) and intelligence (70%) overturned conventional ideas about parenting and teaching. Treating children differently and nurturing their inherent talents suddenly seemed to be a fairer approach than treating them all the same. Findings of genetic influence on physiological characteristics such as cardiac and immunologic function have led to more targeted approaches to disease prevention and treatment. And indications of a stronger genetic influence on male than female homosexuality have furthered debate regarding sexual orientation.
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About the Author
Nancy L. Segal is Distinguished Professor in Humanities and Social Sciences and Director of the Twin Studies Center at California State University, Fullerton.
Table of Contents
1 The Jim Twins (February-March 1979) 17
2 15,000 Questions x 137 Pairs 32
3 Early Findings (1979-1983) 65
4 Sexual Orientation, Cognition, and Medical Traits (1984-1987) 82
5 Pivotal Papers: Personality and IQ (1988 and 1990) 97
6 Job Satisfaction, Cardiac Characteristics, and More (1989-1990) 116
7 Psychopathology and Religiosity (1990) 136
8 Dental Traits, Allergies, and Vocational Interests (1991-1992) 153
9 Creativity, Work Values, and Evolution (1992-1993) 171
10 Family Environments, Happiness, Sensation Seeking, and the MMPI (1994-1997) 195
11 "Larks" and "Owls," Ego Development, and Authoritarianism (1998-2002) 221
12 Twin Relationships, Social Attitudes, and Mental Abilities (2003-2005) 246
13 Sexual Development, Fluctuating Asymmetry, Body Size, and the Structure of Intelligence (2006 and Beyond) 266
14 Questions, Answers, and Twin Studies of the Future 298
Appendix A Funding Sources 329
Appendix B Glossary 333
What People are Saying About This
This is a vivid and intimate account of one of the classic episodes in the history of the nature-nurture debate by somebody who was part of the team. The Minnesota Study of Twins Reared Apart made a vital difference, against often entrenched conventional wisdom, to the understanding of human development. Nancy Segal tells the story of the project superbly.
Matt Ridley, author of Nature via Nurture
Twins separated at birth are endlessly fascinating and give us some of the best scientific insights into nature and nurture. No one can tell this story better than Nancy Segal, gifted scientist, raconteur, and wise observer of human nature.
Steven Pinker, author of The Better Angels of Our Nature
Born Together-Reared Apart is a tour-de-force in the history of psychology and twin research. Describing the twins' life stories as well as the scientific data, Segal gives us a compelling account of the findings, implications, and controversies from the world famous Minnesota Study of Twins Reared Apart. The book will be essential for anyone seriously interested in the genetic and environmental influences affecting human health and behavior.
Robert Plomin, Professor of Behavioral Genetics, King's College London