Born Together : Reared Apart : The Landmark Minnesota Twin Study

417 Pages·2012·1.77 MB·English

Born Together — R eared Apart

B O R N TO G E T H E R —
R E A R E D A PA RT
The Landmark Minnesota Twin Study
NANCY L. SEGAL
Harvard University Press
Cambridge, Massachusetts, and London, En gland 2012
In memory of my parents, Al and Esther Segal,
and
To twins— reared apart, reared together,
and yet to meet.
Copyright © 2012 by the President and Fellows of Harvard College
All rights reserved
Printed in the United States of America
Library of Congress Cataloging- in- Publication Data
Segal, Nancy L., 1951–
Born together— reared apart : the landmark Minnesota twin study /
Nancy L. Segal.
p. cm.
Includes bibliographical references and index.
ISBN 978- 0- 674- 05546- 9 (alk. paper)
1. Twins— Psychology—Research. 2. Nature and nurture— Research. I. Title.
BF723.T9S436 2012
306.875—dc23 2011041636
Contents
Introduction 1
1 The Jim Twins (February– March 1979) 17
2 15,000 Questions × 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
vi CONTENTS
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
Notes 339
Ac know ledgm ents 397
Index 401
Introduction
Genes and glands are obviously important, but social learning
also has a dramatic role. Imagine the enormous differences that
would be found in the personalities of twins with identical
ge ne tic endowments if they were raised apart in two different
families— or, even more striking, in two totally different cultures.
—Walter Mischel
C
olumbia University professor Walter Mischel has been a
prominent fi gure in personality assessment and the study of emo-
tional control. He is also the 2011 recipient of the prestigious
Grawemeyer Award for contributions to psychology. The passage quoted
above, from his well-k nown undergraduate textbook, largely refl ected
1970s’ and early 1980s’ views on factors affecting behavioral develop-
ment, but it would not appear in textbooks today.1 It has been replaced
by statements such as, “Currently, most kinship fi ndings support a moder-
ate role for heredity. . . . Heritability research also reveals that ge ne tic fac-
tors are important in personality.”2 This shift in perspective, from general
neglect or even contempt to increased acc ep tance of ge ne tic contributions
to behavior, was not caused by the Minnesota Study of Twins Reared
Apart (MISTRA), but many claim that the study was instrumental in its
progression.
The classic twin method is a simple and elegant natural experiment for
examining ge ne tic and environmental infl uences on behavior. The idea
of using twins to study factors affecting human behavioral variation was
fi rst described by the British scholar Sir Frances Galton in 1875. Galton
recognized the usefulness of comparing twins who w ere “closely alike [in
childhood]” with those who were “exceedingly unlike.” He asserted that
“their history affords means of distinguishing between the effects of ten-
dencies received at birth, and of those that w ere imposed by the circum-
stances of their after lives; in other words, between the effects of nature
and nurture.”3 If what we now call monozygotic (MZ or identical) twins
show greater trait resemblance than dizygotic (DZ or nonidentical) twins,
2 INTRODUCTION
then ge ne tic infl uence is likely. The biological bases of twinning had not
been established in Galton’s time, but he correctly surmised that “look-
alike” twins shared 100% of their genes and “less- alike” twins shared
fewer.
Richard Rende and his colleagues do not credit Galton with being
“Father of the Twin Method” because “he did not propose the compari-
son between identical and fraternal twin resemblance which is the essence
of the twin method.”4 They give this distinction to Curtis Merriman and
Hermann Siemens, whose 1924 writings described a more specifi c twin-
based approach with more familiar scientifi c terms. Director of the Min-
nesota Study of Twins Reared Apart, Thomas J. Bouchard Jr., and human
ge ne ticist, Peter Propping, called Rende’s assertion a “disingenuous argu-
ment” based on ranking quantitative arguments above conceptual ones.
I would agree.5
The scientifi c study of twins did not become truly useful until the
fundamental distinction between twin types and the methods for distin-
guishing between them w ere established.6 These accomplishments w ere
made by a series of investigators between 1919 and 1925. In 1922, Leslie
Brainerd Arey proposed the labels “MZ” and “DZ” for “one egg” and
“two egg” twins, respectively.7 MZ twins share all their genes, having
split from a single fertilized egg (zygote) within the fi rst two weeks after
conception. DZ twins share half their genes on average, by descent, hav-
ing originated from two separately fertilized eggs. In 1924, Siemens intro-
duced a similarity method for assigning pairs as one- egg or two- egg twins,
based on co-t win (twin pair member) comparison of hereditary traits.8
This method was widely used and is not unlike currently administered
physical resemblance questionnaires.
In 1956, gen e ticist Gordon Allen proposed changing the terms to
“monozygous” and “dizygous” because they capture the term “zygosis”
that refers to zygote formation. He also argued that these terms had one
less syllable, making them easier to write and say.9
Variations on the two types of twinning include MZ female twins who
sometimes differ in the expression of X-linked gen e tic conditions (due to
random inactivation of one X chromosome in each cell early in gestation),
and DZ twins who can be conceived by different fathers (when double-
ovulating women have multiple sexual partners close in time).10 These
twin pairs can help identify factors underlying co-t win differences in
behavior and physique.
The classic twin method is a powerful investigatory tool. Despite some
differences in the early development of twins and nontwins, Danish re-
searchers established that after the age of six years disease incidence and
INTRODUCTION 3
mortality are comparable for twins and singletons (nontwins), a fi nding
confi rmed for most behavioral and physical traits. Thus, most twin re-
search fi ndings apply to the general population.11
The twin method rests on the equal environments assumption, the
premise that the environmental factors affecting similarity in a given trait,
such as sociability, are the same for MZ and DZ twin pairs. Although
MZ twins generally share more activities than do DZ twins, this differ-
ence is problematic only if (1) MZ twins are required to share school
activities to a greater degree than DZ twins, and (2) sharing activities
affects sociability in a meaningful way. If sharing activities increases socia-
bility, then the environments of the two types of twins would not be
equivalent with respect to this behavior. The equal environments assump-
tion would have been compromised in this hypothetical example. H ere,
the environmental effect (the requirement that MZ twins share their ac-
tivities more often than DZ twins) is assumed to be an established school
policy, unrelated to the twins’ ge ne tic proclivities.
Some critics have questioned the usefulness of twin studies, arguing
that MZ twins’ more similar rearing environments underlie their more
similar behaviors relative to DZ twins. However, these challenges and
others related to the equal environments assumption have been tested
empirically and have been refuted in most cases—s imilar treatment of
twins by others is not associated with similar behavioral outcomes. In
their landmark study of twins taking the National Merit Scholarship
Qualifying Test, John Loehlin and Robert Nichols showed that whether
co- twins were dressed alike, slept in the same room, or had the same
teachers was unrelated to their behavioral resemblance,12 a fi nding repli-
cated by others.13 In addition, more physically similar MZ twins (as rated
by parents) have not been perceived as being more alike in personality
than less physically similar twins.14 The lack of any biological connection
between physical appearance and psychological traits argues against the
former causally infl uencing the latter.15 However, twin studies of handed-
ness may violate the equal environments assumption because non-g enetic
factors affecting handedness differ for MZ and DZ twins, as discussed in
Chapter 13.
Studying reared- apart MZ twins (MZA) and DZ twins (DZA) circum-
vents these challenges because the twins are raised in separate environ-
ments, often unaware of each other’s existence. The fi rst scientifi c treat-
ment of MZA twins was a 1922 case study by psychologist Paul Popenoe
of twins Bessie and Jessie.16 The timing of this report is interesting because
the idea of studying twins raised apart had been brought to Galton’s at-
tention forty-s even years earlier. In a letter to Galton (circa 1874–7 5, the