Behavioral Genetics: An introduction to how genes and environments interact through development to shape differences in mood, personality, and intelligence

This book is an introduction for non-scientists to the science of behavioral genetics and its broader ethical and social implications. Among the topics covered are how scientists explore the influence of genes and environment on behavior and how such research may challenge our understanding of human nature, personal responsibility, and equality.

It is a product of a project on behavioral genetics conducted by the American Association for the Advancement of Science (AAAS) and The Hastings Center.

INTRODUCTION
Why do humans range so widely in their susceptibility to mental illness, in their willingness to take risks, and in their performance on intelligence tests? One answer to this question comes from scientists in the field of behavioral genetics.

But what does it really mean to say that there is a link between genes and behavior? Does it mean that there is a gene that makes some of us blush when embarrassed; that there is one gene that makes you prefer classical music and another gene that makes you dislike it; that there is a bunch of genes that each provides for different levels of skill in playing poker? The answer to all these questions is no. Does it mean behavior passes down from generation to generation, i.e., is inherited, just like baldness and eye color? Again, the answer is no.

So when next you see an article that proclaims, “Gene for [insert a human behavior here] discovered,” read it with a critical eye. Or when you next hear someone say, “He inherited his [insert a human behavior here] from his father,” receive that with skepticism, too. ...

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TABLE OF CONTENTS
Preface. . . . . . . . . . . . . . . . . . . . . VI–VII
Introduction . . . .. . . . . . . . . . . VII–X
Acknowledgements. . . .. . . . . . . . . . . . . XI
1. What is behavioral genetics? . . . . . . .. . . . . . . . . . . . . . . . . . . . . . 1
Margaret, an ambitious mother . . . . . . . . . . . . .. . . . . . . . . . . . . 1
Defining behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2–3
Forms of behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . 3–4
Behavioral genetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  . . . . . . . . . . 4–5
Margaret’s ambition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  . . . 5–6
Science in society . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2. How do genes work within their environments? . . . . . . .. . . . . . . . . . 9
Hoda, a perplexed nurse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
The human genome . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Function of the human genome . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10–13
Variety within the human genome. . . . . . . .. . . . . . . . . . . . . . . . . . . . . 13–14
Similarity across genomes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14–15
Imagining the genome . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . 16–17
Behavior and the genome . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17–18
Hoda’s perplexity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . 18–20
Some caveats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20–21
3. How do environments impinge upon genes? . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . 25
Skip, a regretful man. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Environment illustrated. . . . . . . . . . . . . . . . . . . . . . .  . . . . . . . . . . . 26–28
Gene/environment interactions . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . 28–29
Developmental noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Gene/environment correlations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Shared and nonshared environments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31–33
Heritability (and environmentability) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33–34
Skip’s regrets. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34–35
Developmental pathways . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
4. How is genetic research on behavior conducted? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Anja, an identical twin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Animal studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40–41
Family studies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
Twin studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42–44
Adoption studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44–45
Combined studies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
Linkage analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45–47
Association studies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
Microarray analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48–49
Knockout studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49–50
Anja’s question. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50–51
Concerns about non-molecular research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51–53
Concerns about molecular research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54–55
Overcoming the research concerns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55–56
5. How do mental disorders emerge from the mix of genes and environments? . . . . . . 59
Lamar, a man with bad news . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
Genotype/phenotype complexity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60–62
More genotype/phenotype complexity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62–63
Polygenic disorders: complexity multiplied . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63–64
Schizophrenia, a polygenic disorder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64–65
Bipolar disorder, also polygenic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66–67
Research challenges. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67–68
Lamar’s dilemma. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68–70
Normal and abnormal traits. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
6. How is the ability to control impulses affected by genes and environments? . . . . . . . 75
Trevor, in trouble with the law . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
Impulsive behavior and ADHD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76–78
Is ADHD a disorder or a trait? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78–79
Novelty-seeking: a positive impulsive trait . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
Antisocial personality: a negative impulsive trait. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80–82
Criminality: a legal description, not a trait . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82–83
Research into criminality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83–85
The myth of “genes for criminality” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85–87
Trevor’s defense. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87–89
Potential research consequences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89–90
Treatment concerns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90–92
Other research concerns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92–93
7. How is intellect molded by genes and environments? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
Mr. Huang, a puzzled patriarch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
Defining intelligence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
History of intelligence testing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98–101
Measuring g. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  . . . . . . . . . . . . . 101–103
Quantitative research into intelligence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103–105
Molecular research into intelligence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105–107
Predicting individual intelligence . . . . . . . . . . . . . . . . .  . . . . . . . . . . . . . . 107–109
Mr. Huang’s speculations. . . . . . . . . . . . . . . . . . . . . . . . .  . . . . . . . . . . . . 109–110
Accounting for disparities in population IQs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110–114
Eugenic concerns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114–116
Glossary/Index. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119–130
Project Participants . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . 131

PREFACE
Behavioral geneticists aim at no less than showing us how genes help to explain why we behave the way we do. One big reason they do their work is that understanding why we behave the way we do is inherently interesting. The second big reason is that they hope their work eventually will lead to curing behavioral disorders as different as alcoholism and schizophrenia. Today they are far from understanding how genes influence those behaviors, but that is their goal.

One might think that such a fascinating field would by now have spawned many brief introductions for lay readers. But that hasn’t happened. Part of the explanation for this gap is that the science is complicated. Part of the explanation is that the subject matter of behavioral genetics—mental disease, personality, intelligence—is controversial.

Moreover, in the past, research in behavioral genetics has been used to support hateful prejudices, and so perhaps this has led many otherwise-interested writers to steer clear of the topic. ...

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