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The Neuroscience of Intelligence

Cambridge University Press

Table of Contents
Chapter 1. What We Know About Intelligence From the Weight of Studies
Chapter 2. Nature More than Nurture: The Impact of Genetics on Intelligence
Chapter 3. Peeking Inside the Living Brain: Neuroimaging Is a Game Changer For Intelligence Research
Chapter 4. 50 Shades of Gray Matter: A Brain Image of Intelligence Is Worth a Thousand Words
Chapter 5. The Holy Grail: Can Neuroscience Boost Intelligence?
Chapter 6. As Neuroscience Advances, What’s Next for Intelligence Research?


Preface
Why are some people smarter than others? This book is about what neuroscience tells us about intelligence and the brain. Everyone has a notion about defining intelligence and an opinion about how differences among individuals may contribute to academic success and life achievement. Conflicting and controversial ideas are common about how intelligence develops. You may be surprised to learn that the scientific findings about all these topics are more definitive than you think. The weight of evidence from neuroscience research is rapidly correcting outdated and erroneous beliefs.

I wrote this book for students of psychology and neuroscience, educators, public policy makers, and for anyone else interested in why intelligence matters. On one hand this account is an introduction to the field that presupposes no special background; on the other hand it is more in depth than popularized accounts in the mass/social media. My emphasis is on explaining the science of intelligence in understandable language. The viewpoint that suffuses every chapter is that intelligence is 100% a biological phenomenon, genetic or not; influenced by environment or not, and that the relevant biology takes place in the brain. That is why there is a neuroscience of intelligence to write about. 

This book is not neutral but I believe it is fair. My writing is based on over 40 years of experience doing research on intelligence using mental ability testing and neuroimaging technology. My judgments about the research to include are based on the existing weight of evidence. If the weight of evidence changes for any of the topics covered, I will change my mind and so should you. No doubt, the way I judge the weight of evidence will not please everyone but that is exactly why a book like this elicits conversation, potentially opens minds, and with luck, fosters a new insight or two.

Be advised, if you already believe that intelligence is due all or mostly to the environment, new neuroscience facts might be difficult to accept. Denial is a common response when new information conflicts with prior beliefs. The older you are, the more impervious your beliefs may be. Santiago Ramon Cajal (1852-1934), the father of neuroscience, once wrote, “Nothing inspires more reverence and awe in me than an old man who knows how to change his mind” (Cajal, 1924). Students have no excuse.

The challenge of neuroscience is to identify the brain processes necessary for intelligence and discover how they develop. Why is this important? The ultimate purpose of all intelligence research is to enhance intelligence. Finding ways to maximize a person’s use of their intelligence is one goal of education. It is not yet clear from the weight of evidence how neuroscience can help teachers or parents do this. Finding ways to increase intelligence by manipulating brain mechanisms is quite another matter and one where neuroscience has considerable potential. Surely, most people would agree that increasing intelligence is a positive goal for helping people in the lower than normal range who often cannot learn basic self-care routines or employment skills. What then is the argument against enhancing intelligence so students can learn more, or adults can enjoy increased probability of greater achievement? If you have a negative reaction to this bold statement of purpose, my hope is that by the end of this book you reconsider. 

Three laws govern this book: 1) No story about the brain is simple; 2) No one study is definitive; 3) It takes many years to sort out conflicting and inconsistent findings and establish a compelling weight of evidence. With these in mind, Chapter 1 aims to correct popular misinformation and summarizes how intelligence is defined and measured for scientific research. Some of the validity data will surprise you. For example, childhood IQ scores predict adult mortality. Chapter 2 reviews the overwhelming evidence that there are major genetic effects on intelligence and its development. Conclusive studies from quantitative and molecular genetics leave no doubt about this. Since genes always work through biological mechanisms, there must be a neurobiological basis for intelligence, even when there are environmental influences on those mechanisms. Genes do not work in a vacuum; they are expressed and function in an environment. This interaction is a theme of  “epigenetics” and we will discuss its role in intelligence research.

Chapters 3 and 4 delve into neuroimaging and how these revolutionary technologies visualize intelligence in the brain, and indicate the neurobiological mechanisms involved. New twin studies of intelligence, for example, combine neuroimaging and DNA analyses. Key results show common genes for brain structure and intelligence. Chapter 5 focuses on enhancement. It begins with critiques of three widely publicized but incorrect claims about increasing IQ and ends with electrical brain stimulation. So far, there is no proven way to enhance intelligence but I explain why there is a strong possibility that manipulation of some genes and their biological processes may achieve dramatic increases. Imagine a moonshot-like national research effort to reach this goal; guess which nation apparently is making this commitment (it is not the United States). 

Chapter 6 introduces several astonishing neuroscience methods for studying synapses, neurons, circuits, and networks that move intelligence research even deeper into the brain. Soon we might measure intelligence based on brain speed, and build intelligent machines based on how the brain actually works. Large collaborative efforts around the world are hunting intelligence genes, creating virtual brains, and mapping brain fingerprints unique to individuals—fingerprints that predict intelligence. Overlapping neuro-circuits for intelligence, consciousness, and creativity are explored. Finally, I introduce the terms “neuro-poverty” and “neuro-SES” (social-economic-status) and explain why neuroscience advances in intelligence research may inform education policies.

Personally, I believe we are entering a Golden Age of intelligence research that goes far beyond nearly extinct controversies about whether intelligence can be defined or measured and whether genes are involved. My enthusiasm about this field is intended to permeate every chapter. If you are an educator, policy maker, parent, or student you need to know what 21st century neuroscience says about intelligence. If any of you are drawn to a career in psychology or neuroscience and pursue the challenges of intelligence research, then that is quite a bonus.

 

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Preface to the Second Edition 

A lot has happened since I submitted the final manuscript for the first edition of this book in 2015, but not much has changed. As I discovered writing this second edition in 2022, the weight of evidence for the key topics is even stronger now. This is due in no small way to a new generation of researchers who have access to more advanced technology and data analysis methods, and access to incredibly large databases that include DNA, neuroimaging, and cognitive testing. Intelligence research is subject to the same inevitable progress as all other areas of science driven mostly by observations and insights from methodological and technical advances. I am happy to report that the last seven years have sharpened our neuroscience understandings and the formulations of questions still to be answered. 

This edition includes new sections on predicting intelligence measures from DNA using polygenic scores (PGSs). The fact that such prediction is possible further debunks stubborn views that intelligence has little to do with genes and that intelligence cannot be assessed for scientific study. But, more importantly, these findings invigorate efforts to understand how gene expression influences intelligence. They open the door for molecular biology research that could identify salient mechanisms that underlie the cognitive processes necessary for intelligence. It is this kind of research that I believe could someday result in ways to dramatically enhance intelligence for individuals. I still believe that is the ultimate and noble goal of intelligence research. 

There also are new sections on neuroimaging and connectivity analyses to identify specific brain networks and circuits relevant to intelligence and to individual differences. This kind of research similarly has potential to identify ways to manipulate network information flow within and across brain areas to enhance intelligence. Related to these advances, many cognitive neuroscientists previously focused on learning and memory have broadened their interests to specially study intelligence and individual differences. It’s not just about psychometrics anymore, but meaningful collaborations still require psychometric sophistication.  

The chapters in this edition have been updated with new research findings and references. There were many to choose from and I could not include them all. Mostly, their findings are consistent with those discussed in the first edition. This may be the result of my unconscious cherry-picking or it may reflect the robustness of the underlying phenomena. Time will tell and I will be the first to change my mind if the weight of evidence changes. But, as always in science, new data typically make the explanatory picture more complex, not less. We see this in the stunning images from the James Webb telescope and their impact on cosmology theories. There is poetry and a bit of magic in these images. As neuroscience approaches go deeper and deeper into smaller and smaller brain structures and faster and faster functions, the complexity is both beautifully grand and nightmarishly challenging. That dynamic is the excitement of intelligence research that I hope to convey on every page. The poetry and magic are up to you.