Table Of Contenti
Gay, Straight, and the Reason Why
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Gay, Straight, and
the Reason Why
The Science of Sexual Orientation
SECOND EDITION
SIMON LeVAY
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Library of Congress Cataloging- in- Publication Data
Names: LeVay, Simon, author.
Title: Gay, straight, and the reason why : the science of sexual orientation / Simon LeVay.
Description: Second edition. | Oxford ; New York : Oxford University Press, [2017] |
Includes bibliographical references and index.
Identifiers: LCCN 2016006377 (print) | LCCN 2016014053 (ebook) |
ISBN 9780190297374 (pbk. : alk. paper) | ISBN 9780190297381 (UPDF) |
ISBN 9780190297398 (EPUB)
Subjects: LCSH: Sexual orientation. | Sex (Psychology) | Sex (Biology)
Classification: LCC BF692 .L476 2017 (print) | LCC BF692 (ebook) | DDC 155.3/4—dc23
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CONTENTS
Preface to the Second Edition vii
Introduction ix
1. What Is Sexual Orientation?
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2. Why We Need Biology
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3. The Outline of a Theory
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4. Childhood
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5. Characteristics of Gay and Straight Adults
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6. The Role of Sex Hormones
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7. The Role of Genes
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8. The Brain
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9. The Body
123
10. The Older- Brother Effect
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11. Beyond Gay and Straight
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12. Conclusions
163
Notes 179
Glossary 193
Bibliography 199
Author Index 235
Subject Index 237
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PREFACE TO THE SECOND EDITION
The five years that have passed since the publication of the first edition of this
book have seen considerable progress in our understanding of how sexual ori-
entation develops. Molecular geneticists have come closer to identifying “gay
genes.” Brain scientists have probed deeper into the neural wiring that under-
lies sexual attraction. Psychologists have given us a clearer view of the cogni-
tive and personality traits that distinguish gay people from straight people.
They have also helped us better understand how a trait as counterintuitive
as homosexuality can persist over the generations and why it exists in most
human cultures. To cover these advances I have added mention of over 170
research papers that have appeared since I wrote the first edition.
In general, the recent studies strengthen the idea that gay people differ
from straight people in more than the direction of their sexual feelings. There
is great diversity among lesbians and gay men, to be sure, but in general ho-
mosexuality is part of a collection of gender- atypical traits, just as heterosex-
uality is part of a collection of gender- typical traits. These different “packages”
arise because the sexual differentiation of the brain goes forward differently
in individuals destined to become gay adults as compared with their same- sex
heterosexual peers. Differences in genes, sex hormones, and their interactions
with the developing brain are what cause this divergence.
Some of the recent findings are pointing in interesting new directions,
however. There is increasing evidence, for example, that sexual orientation
is affected by epigenetic processes. These are processes that involve chemical
alterations to the genome but not to the DNA base sequence itself— the “let-
ters” of the genetic code. Another area of active inquiry is the “older- brother
effect”— the observation that boys with older brothers have an increased likeli-
hood of becoming gay men. Canadian researchers believe that they are homing
in on the biological basis for this effect.
Perhaps most significantly, it’s becoming obvious that there is much more to
sexual attraction than “gay” and “straight.” Bisexuality, asexuality, attraction
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to different age groups,and the various kinds of gay sexuality indicated by the
colloquial terms “butch,” “femme,” “top,” and “bottom”— all these have begun
to attract the attention of biologically oriented researchers. So has transexual-
ity, which has some features in common with homosexuality. I have added a
new chapter, titled “Beyond Gay and Straight” (Chapter 11), that is devoted to
these other important aspects of sexuality and gender.
The University of Lethbridge, Alberta, hosts a conference every few years
under the title “The Puzzle of Sexual Orientation.” The 2015 conference, organ-
ized by Paul Vasey, Kelly Suschinksy, and Jean- Baptiste Leca, attracted many
of the leading researchers in the field. In this book I mention some of the re-
search findings presented at the conference. It is expected that most of these
presentations will be published in a special edition of Archives of Sexual Behavior
sometime in 2016.
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INTRODUCTION
In August of 1991, when I was a neuroscientist working at the Salk Institute
for Biological Studies in San Diego, I published a short research paper in the
journal Science. The article was titled “A Difference in Hypothalamic Structure
Between Heterosexual and Homosexual Men.” It attracted a great deal of in-
terest from the media, the general public, and the scientific community, and
it helped trigger a wave of new research into an age- old question: What makes
people straight or gay?
The hypothalamus* is a small region at the base of the brain that helps reg-
ulate several of our instinctual drives, including our sex drive. In my study,
I took specimens of the hypothalamus from men and women who had died and
were undergoing autopsy. About half of the men were gay. I focused on a region
at the front of the hypothalamus that is known to be involved in regulating the
sexual behaviors typically shown by males. Within this region lies a rice- grain-
sized collection of nerve cells named INAH3, which is usually larger in men
than in women. I confirmed this basic sex difference. In addition, however,
I found that INAH3 was significantly smaller, on average, in the gay men than
in the straight men. In fact, there was no difference in size between INAH3 in
the gay men and the women in my sample. I interpreted this finding as a clue
that biological processes of brain development may influence a man’s sexual
orientation.
I was certainly not the first person to have thought about sexual orientation
from a biological perspective. A hundred years ago a German physician and
sex researcher, Magnus Hirschfeld, proposed that brain development followed
different paths in fetuses destined to become gay adults and those destined
to become straight. Just a year before I published my study, a Dutch group re-
ported that another cluster of cells in the hypothalamus, the suprachiasmatic
* Most technical terms are italicized at first mention and are defined in the Glossary.
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