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  Intelligence: A Very Short Introduction

  ‘I can, and shall, recommend this engaging book to anyone, student or layperson … a reasoned and reasonable view of this interesting and important topic.’

  Professor N. J. Mackintosh, Cambridge University

  ‘This book, written by one of the world’s leading researchers on intelligence, provides an ideal introduction to a controversial topic. Deary … tells us in an entertaining and clear way what was done, what was found, and what it does and does not mean. … If you want to know how we know what we know about intelligence read this book.’

  Nat Brody, Wesleyan University

  ‘Professor Deary’s short introductory book about human intelligence is like no other account available. He addresses the big issues that the experts continue to debate … all in an easy-to-digest, balanced style that meets his aim to put the reader in touch with the scientific research into this challenging field. This book is first class.’

  Ted Nettelbeck, Adelaide University

  ‘succinct and highly readable … an excellent overview of what is and is not known about human intelligence … A gem of a book that will be of interest to a wide audience.’

  Tony Vernon, University of Western Ontario

  * * *

  VERY SHORT INTRODUCTIONS are for anyone wanting a stimulating and accessible way in to a new subject. They are written by experts, and have been published in more than 25 languages worldwide.

  The series began in 1995, and now represents a wide variety of topics in history, philosophy, religion, science, and the humanities. Over the next few years it will grow to a library of around 200 volumes – a Very Short Introduction to everything from ancient Egypt and Indian philosophy to conceptual art and cosmology.

  * * *

  Very Short Introductions available now:

  ANCIENT PHILOSOPHY Julia Annas

  THE ANGLO-SAXON AGE John Blair

  ANIMAL RIGHTS David DeGrazia

  ARCHAEOLOGY Paul Bahn

  ARCHITECTURE Andrew Ballantyne

  ARISTOTLE Jonathan Barnes

  ART HISTORY Dana Arnold

  ART THEORY Cynthia Freeland

  THE HISTORY OF ASTRONOMY Michael Hoskin

  ATHEISM Julian Baggini

  AUGUSTINE Henry Chadwick

  BARTHES Jonathan Culler

  THE BIBLE John Riches

  BRITISH POLITICS Anthony Wright

  BUDDHA Michael Carrithers

  BUDDHISM Damien Keown

  CAPITALISM James Fulcher

  THE CELTS Barry Cunliffe

  CHOICE THEORY Michael Allingham

  CHRISTIAN ART Beth Williamson

  CLASSICS Mary Beard and John Henderson

  CLAUSEWITZ Michael Howard

  THE COLD WAR Robert McMahon

  CONTINENTAL PHILOSOPHY Simon Critchley

  COSMOLOGY Peter Coles

  CRYPTOGRAPHY Fred Piper and Sean Murphy

  DADA AND SURREALISM David Hopkins

  DARWIN Jonathan Howard

  DEMOCRACY Bernard Crick

  DESCARTES Tom Sorell

  DRUGS Leslie Iversen

  THE EARTH Martin Redfern

  EGYPTIAN MYTHOLOGY Geraldine Pinch

  EIGHTEENTH-CENTURY BRITAIN Paul Langford

  THE ELEMENTS Philip Ball

  EMOTION Dylan Evans

  EMPIRE Stephen Howe

  ENGELS Terrell Carver

  ETHICS Simon Blackburn

  THE EUROPEAN UNION John Pinder

  EVOLUTION Brian and Deborah Charlesworth

  FASCISM Kevin Passmore

  THE FRENCH REVOLUTION William Doyle

  FREUD Anthony Storr

  GALILEO Stillman Drake

  GANDHI Bhikhu Parekh

  GLOBALIZATION Manfred Steger

  HEGEL Peter Singer

  HEIDEGGER Michael Inwood

  HINDUISM Kim Knott

  HISTORY John H. Arnold

  HOBBES Richard Tuck

  HUME A. J. Ayer

  IDEOLOGY Michael Freeden

  INDIAN PHILOSOPHY Sue Hamilton

  INTELLIGENCE Ian J. Deary

  ISLAM Malise Ruthven

  JUDAISM Norman Solomon

  JUNG Anthony Stevens

  KANT Roger Scruton

  KIERKEGAARD Patrick Gardiner

  THE KORAN Michael Cook

  LINGUISTICS Peter Matthews

  LITERARY THEORY Jonathan Culler

  LOCKE John Dunn

  LOGIC Graham Priest

  MACHIAVELLI Quentin Skinner

  MARX Peter Singer

  MATHEMATICS Timothy Gowers

  MEDIEVAL BRITAIN John Gillingham and Ralph A. Griffiths

  MODERN IRELAND Senia Pašeta

  MOLECULES Philip Ball

  MUSIC Nicholas Cook

  NIETZSCHE Michael Tanner

  NINETEENTH-CENTURY BRITAIN Christopher Harvie and H. C. G. Matthew

  NORTHERN IRELAND Marc Mulholland

  PAUL E. P. Sanders

  PHILOSOPHY Edward Craig

  PHILOSOPHY OF SCIENCE Samir Okasha

  PLATO Julia Annas

  POLITICS Kenneth Minogue

  POLITICAL PHILOSOPHY David Miller

  POSTCOLONIALISM Robert Young

  POSTMODERNISM Christopher Butler

  POSTSTRUCTURALISM Catherine Belsey

  PREHISTORY Chris Gosden

  PRESOCRATIC PHILOSOPHY Catherine Osborne

  PSYCHOLOGY Gillian Butler and Freda McManus

  QUANTUM THEORY John Polkinghorne

  ROMAN BRITAIN Peter Salway

  ROUSSEAU Robert Wokler

  RUSSELL A. C. Grayling

  RUSSIAN LITERATURE Catriona Kelly

  THE RUSSIAN REVOLUTION S. A. Smith

  SCHIZOPHRENIA Chris Frith and Eve Johnstone

  SCHOPENHAUER Christopher Janaway

  SHAKESPEARE Germaine Greer

  SOCIAL AND CULTURAL ANTHROPOLOGY John Monaghan and Peter Just

  SOCIOLOGY Steve Bruce

  SOCRATES C. C. W. Taylor

  SPINOZA Roger Scruton

  STUART BRITAIN John Morrill

  TERRORISM Charles Townshend

  THEOLOGY David F. Ford

  THE TUDORS John Guy

  TWENTIETH-CENTURY BRITAIN Kenneth O. Morgan

  WITTGENSTEIN A. C. Grayling

  WORLD MUSIC Philip Bohlman

  Available soon:

  AFRICAN HISTORY John Parker and Richard Rathbone

  ANCIENT EGYPT Ian Shaw

  THE BRAIN Michael O’Shea

  BUDDHIST ETHICS Damien Keown

  CHAOS Leonard Smith

  CHRISTIANITY Linda Woodhead

  CITIZENSHIP Richard Bellamy

  CLASSICAL ARCHITECTURE Robert Tavernor

  CLONING Arlene Judith Klotzko

  CONTEMPORARY ART Julian Stallabrass

  THE CRUSADES Christopher Tyerman

  DERRIDA Simon Glendinning

  DESIGN John Heskett

  DINOSAURS David Norman

  DREAMING J. Allan Hobson

  ECONOMICS Partha Dasgupta

  THE END OF THE WORLD Bill McGuire

  EXISTENTIALISM Thomas Flynn

  THE FIRST WORLD WAR Michael Howard

  FREE WILL Thomas Pink

  FUNDAMENTALISM Malise Ruthven

  HABERMAS Gordon Finlayson

  HIEROGLYPHS Penelope Wilson

  HIROSHIMA B. R. Tomlinson

  HUMAN EVOLUTION Bernard Wood

  INTERNATIONAL RELATIONS Paul Wilkinson

  JAZZ Brian Morton

  MANDELA Tom Lodge

  MEDICAL ETHICS Tony Hope

  THE MIND Martin Davies

  MYTH Robert Segal

&
nbsp; NATIONALISM Steven Grosby

  PERCEPTION Richard Gregory

  PHILOSOPHY OF RELIGION Jack Copeland and Diane Proudfoot

  PHOTOGRAPHY Steve Edwards

  THE RAJ Denis Judd

  THE RENAISSANCE Jerry Brotton

  RENAISSANCE ART Geraldine Johnson

  SARTRE Christina Howells

  THE SPANISH CIVIL WAR Helen Graham

  TRAGEDY Adrian Poole

  THE TWENTIETH CENTURY Martin Conway

  For more information visit our web site

  www.oup.co.uk/vsi

  Ian J. Deary

  Intelligence

  A Very Short Introduction

  Great Clarendon Street, Oxford OX2 6DP

  Oxford University Press is a department of the University of Oxford.

  It furthers the University’s objective of excellence in research, scholarship,

  and education by publishing worldwide in

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  Oxford is a registered trade mark of Oxford University Press

  in the UK and in certain other countries

  Published in the United States

  by Oxford University Press Inc., New York

  © Ian J. Deary 2001

  The moral rights of the author have been asserted

  Database right Oxford University Press (maker)

  First published as an Oxford University Press paperback 2001

  All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, without the prior permission in writing of Oxford University Press, or as expressly permitted by law, or under terms agreed with the appropriate reprographics rights organizations. Enquiries concerning reproduction outside the scope of the above should be sent to the Rights Department,

  Oxford University Press, at the address above

  You must not circulate this book in any other binding or cover

  and you must impose this same condition on any acquirer

  British Library Cataloguing in Publication Data

  Data available

  Library of Congress Cataloging in Publication Data

  Data available

  ISBN 10: 0-19-289321-1

  ISBN 13: 978-0-19-289321-5

  7 9 10 8 6

  Typeset by RefineCatch Ltd, Bungay, Suffolk

  Printed in Great Britain by

  TJ International Ltd., Padstow, Cornwall

  Contents

  List of illustrations

  Preface and acknowledgements

  A word about correlation

  1 To see ‘g’ or not to see ‘g’ …

  How many types of intelligence are there?

  2 Ageing and intelligence: senility or sagacity?

  What happens to mental abilities as we grow older?

  3 Brainy?

  Why are some people cleverer than others?

  4 ‘They f—— you up, your mum and dad’ (Larkin)

  Are intelligence differences a result of genes or environments or both?

  5 The (b)right man for the job

  Does intelligence matter?

  6 The lands of the rising IQ

  Is intelligence increasing generation after generation?

  7 Eleven Twelve (not-so-)angry men (and women)

  Psychologists actually agree about human intelligence differences

  Further reading

  Index

  List of illustrations

  1 WAIS-III structure

  2 Matrix reasoning test

  Courtesy of John Raven

  3 Digit symbol-coding-type test

  4 Carroll’s structure of intelligence

  5 Moray House Test scattergram 1932–98

  6 Scottish Mental Survey 1932 specimen test

  7 Scottish Mental Survey 1932 participants resit Moray House Test in 1998

  Courtesy of the Aberdeen Press and Journal

  8 Design of the Seattle Longitudinal Study

  9 Age changes in verbal and non-verbal ability

  10 Salthouse’s cognitive ageing theory

  11 Measuring brain volume

  12 The brain’s electrical response

  13 Inspection time

  14 Reaction time

  15 Identical twins reared together

  16 Non-identical twins reared together

  17 Identical twins reared apart

  18 Non-identical twins reared apart

  19 Photograph of identical twins reared apart

  Photo: Tom K. Wanstall, Firehouse magazine

  20 IQ similarity in identical twins reared apart

  21 The OctoTwin study report

  © 1997 American Association for the Advancement of Science

  22 Adoption

  23 OctoTwin study diagram: heritability of intelligence in old age

  24 Predicting job performance

  25 IQ gains in different nations

  26 IQ: knowns and unknowns

  © American Psychological Association

  Preface and acknowledgements

  People value their powers of thinking, and most of us are interested in why some people seem to drive a highly tuned Rolls Royce brain while others potter along with a merely serviceable Ford Fiesta. The fact that the broad powers of human intelligence show differences has been recognized since antiquity. Our language is full of words that signify the possession or lack of an efficient brain. Within the academic discipline of psychology there is a subsection of researchers and teachers called ‘differential psychologists’. They study the differences between people in intelligence and personality. In this short book, I want to describe what they have discovered about how and why people differ in their thinking powers.

  There are many books on human intelligence differences and it needed a good reason to add one to the pile. Beyond the tracts written by academics for their peers and students, two sorts of popular book predominate. On the one hand there are many test-your-IQ-type books that offer an introduction to the field of mental measurement. Depending on how you score on their tests, they will flatter or depress. They act as a sort of do-it-yourself fitness diagnosis for your brain. They are a mostly harmless diversion: probably it’s only rather bright people who buy them anyway, and end up rather pleased with the results. On the other hand, there are books which denounce IQ testing as a form of social evil, as a tool used by a social elite to keep the lower orders in their places. Neither of these types of book is satisfactory for understanding the key information about human mental abilities. The former is a quack diagnostic kit and the latter sells a political message that relegates research facts and emphasizes spin.

  And it is facts that drive this present book. It is an attempt to cut out the middle man and put you in touch with some actual research data in human intelligence. There is no such thing as a theory of human intelligence differences – not in the way that grown-up sciences like physics or chemistry have theories. We don’t know enough about the workings of the brain to say why some brains seem to be more efficient than others. However, there are some hard facts about human intelligence differences. Just as in other sciences, these hard facts constrain what we can say about the topic: we should not be claiming things that go against or ignore the best evidence in the field. And just as we should expect of a science, we also have to be frank in admitting the faults of each study, especially when the results seem to agree with our own prejudices. The best scientists are their own most severe critics.

  The plan of the book is to present a series of diagrams, each of which captures a solid finding about human intelligence differences. Here and there, the diagrams might look quite complicated. The promise is that you will understand them by reading the accompanying text. My efforts have been ai
med toward a clear, non-technical, but also uncompromisingly accurate, account of some of the important areas in human intelligence. The sources from which I drew the information are fully documented here, but no one study is without fault and no single study can settle an issue. My opinion, though, is that it is better to know some influential studies and their main results than merely to amass third-hand accounts which sell a point of view by selective reporting.

  I’ve selected 11 sets of research results, 11 datasets, that I think address central questions about human intelligence: not exactly ‘11 datasets that shook the world’, but all are influential in the field. Some of these are remarkable single sets of data that represent huge amounts of effort, luck, and/or ingenuity on the part of the investigator(s). Some of them are collections of studies on a topic that have taken decades to put together and synthesize. There are some descriptions of the work involved in conducting the studies, so that they are not just dry numerical accounts. The datasets address some of the most interesting questions about human intelligence: what forms does it take?; what happens to it as we grow old?; are its origins in our genetic code and the environment’s influences?; does it matter in real life?; why is it rising generation after generation?; and do psychologists themselves agree about intelligence?

  For each of the datasets I have chosen one or more illustrations that capture some important aspect of the results. Most of these illustrations originally appeared in the research articles reporting the data. Rather than reproduce these sometimes technical diagrams, they are redrawn in a more accessible form.

  Really the 11 datasets are just introductions to a field in which many of us are spending our research lives investigating one or more small patches. In order to assist interested readers in following up some specific topics, there are suggestions at the end of each chapter for how you might develop your interest in the given areas and do some further reading. There is also a section at the end of the book offering general ideas on further resources.