As a recent article by Tim Flannery in the New York Review of Books explains:

Ever since Richard Leakey and Roger Lewin published The Sixth Extinction in 1995, we have known that humanity is extirpating species at a rate unmatched since the demise of the dinosaurs 65 million years ago. Hunting, deforestation, the introduction of nonnative organisms and diseases, and now climate change have increased the rate of species loss to the point that scientists fear for the functioning of entire ecosystems…

In this pioneering book, Leakey and Lewin take us by the hand through recent (in 1995) discoveries in ecology, palaeontology, palaeoanthropology and geology, to present a whole new worldview, a new way of seeing the natural world and our place in it.

1. Human evolution is a random accident

This is that we – human beings – are NOT the product of some ineluctable force driving evolution towards higher and more sophisticated species and, ultimately, towards Mind and Consciousness. We are emphatically not the pinnacle of the universe. The reverse: we are a cosmic accident. We now know that the long fossil record of life on earth has been marked by countless disasters, accidents, extinctions, most of which have no intrinsic or logical rationale, and that we are the incredibly fortuitous outcome of these massively random events.

2. There is no balance of nature

Older naturalists held that there was a Balance of Nature whereby the complete global system of life worked together to keep things – oxygen levels, complex ecosystems – in a careful balance which favoured the optimum thriving of life forms. But the closer we look at the record, the more obvious it becomes that there is no balance of nature. The more we learn, the more we realise that nature is in fact given to chaotic  and random fluctuations. It is also much more complex than we ever suspected.

3. Fluctuation and accident are the norm

Taken together, these two ideas suggest that flux and fluctuation are an intrinsic part of the history of life on earth.

Humans long for predictability, in relation to the world of nature around us and, most particularly, in relation to our own existence and our future. But it is obvious that, in the realm of evolutionary biology and ecology, ours is an unpredictable world and our place in it an accident of history; it is a place of many possibilities that are influenced by forces beyond our control and, in some cases at least, beyond our comprehension. (p.231)

4. Extinction events

The most dramatic embodiment of this fluctuation – and of the workings of chance – are ‘extinction events’. In the 540 million years since multicellular life suddenly arose in what scientists call ‘the Cambrian Explosion’, there have been no fewer than 15 ‘extinction events’. These are relatively short periods in which – for some reason – the fossil record shows that between 15% and 40% of all species went out of existence, never to return.

Among these 15 were five really big extinction events, ‘the Big Five’, in each of which over 60% of all species went extinct. And king of the Big Five is one real monster, the extinction event at the end of the Permian Era, 250 million years ago, when an estimated 95% of all terrestrial species on earth were wiped out!

The more we learn about the extinction events, the more obvious it becomes that we are the lucky survivors of the lucky survivors of the lucky survivors of a whole series of catastrophes, not through any intrinsic merit in our forebears (who, if you go back far enough, were worms) but from sheer dumb luck.

5. Darwin’s theory of evolution is over-ridden by extinction events

Darwin’s theory of evolution through natural selection is undoubtedly the mechanism by which new species come into being and by which all life forms are continually competing with all others. But in Leakey’s view, Darwin’s theory is only relevant in the relatively stable periods between these global catastrophes. These periods have lasted tens, sometimes hundreds of millions of years – but the history of life on earth is certainly not the slow, steady evolution of more and more sophisticated life forms, as portrayed in older evolutionary theory.

Instead Darwin’s process has been overshadowed time and again – in terms of impact of the history of life on earth – by extinction, catastrophe and random events. In other words, by the accidents and arbitrariness of History.

6. Humans are the most destructive species on the planet

This new emphasis on the importance of destruction, of the really breath-taking mass extinction of life forms, in the long story of life on earth, dovetails with other, recent discoveries about man’s role in nature. For a variety of sources now suggest that Homo sapiens is and always has been, immensely destructive of the ecosystems around him.

1. For a long time Europeans have thought that when European explorers and colonisers encountered native peoples in places like America, the Pacific islands, Australia and so on, those peoples were living in a blessed ‘harmony’ with nature. Only in recent decades have scientists realised that the supposedly ‘pristine’ environments of all these places had in fact been severely damaged by the arrival of those peoples. One of the most dramatic examples is Hawaii, which looks like a tropical paradise to tourists, but where we have now discovered evidence that the hunter-gatherers who arrived there 1,500 years ago proceeded to burn down much of the rainforest and wipe out most of the larger species, including a majority of the bright songbirds.
2. This pattern has been replicated wherever humans appeared, most notably in the Americas, where the arrival of the first hunter-gatherers around 12,000 years ago across the then-existing land-bridge from Asia, and their slow spread southwards, coincides exactly with the extermination of the continent’s megafauna i.e. all its large mammals. There is debate about whether other factors were involved as well but the case of New Zealand presents the case with brutal clarity. The Maori only arrived there 1,000 years ago, and promptly cleared much of the rainforest and hunted all the species of flightless birds and large mammals to extinction. So the native peoples which European explorers encountered in the 16th, 17th and 18th centuries were far from living in harmony with nature; they were living amid the ecological devastation their ancestors had wreaked wherever they – wherever humans – went.
3. In the 1990s (this book was published in 1995) everybody knew that tropical rainforests around the world, especially in the vast Amazon basin, were being destroyed at an unprecedented rate (an acre a minute is one calculation), each acre home to millions of species. Since then the news about endangered species or about the rainforests has been overshadowed by the growing sense of crisis about man-made global warming. This has distracted attention away from the story on the ground, which is the alarming rate at which we are continuing to exterminate species throughout the world by the incessant demands of an ever-growing population. When Leakey wrote this book there were 6 billion people in the world. Now that number is 7.5 billion and climbing. The pressure to destroy natural habitat to convert it to farm or grazing land, along with the relentless polluting of the seas, the rivers and the air, can only escalate.

7. Humans are responsible for the sixth mass extinction event in global history

Having given a thorough account of modern understanding of the 20 or so extinction events which punctuate the fossil record – and especially of the Big Five in which 60%-plus of species went extinct – Leakey’s last chapters introduce us to the final conclusion of their long survey – the idea that we, Homo sapiens, are now having such a destructive impact on the natural world that many if not most environmentalists think we are living through the Sixth Mass Extinction of life on earth.

This is an event so momentous that many geologists and evolutionary scientists think it deserves to be defined as a distinct geological era – the Anthropocene Era – the era in which we human beings are irreversibly destroying the vast majority of life forms on the planet we share with them, on a scale only comparable with the devastation caused by the Big Five extinction events.

We are destroying the world.

Leakey and Lewin

Richard Leakey (b.1944) is a paleoanthropologist and ecologist, born and bred in Kenya, where he made significant discoveries of fossils of early humans, before going on to run the country’s national museums and then become its overall Director of the Wildlife Services.

Roger Lewin (b.1944) is a British prize-winning science writer, a staff member of New Scientist for nine years before going to America to become News Editor for Science. He’s written about 20 books, including three in collaboration with Leakey.

The first two of their collaborations are about Leakey’s work into the origins of the human species, in and around Lake Turkana in the north of Kenya, part of the enormous Rift Valley. Due to the fact that hominids need water, and the mud around rivers and lakes preserves footprints and the bones of dead animals better than the harsh savannah or bare rock, Lake Turkana has been a goldmine for fossil hunters looking for relics of our earliest ancestors. In his early explorations, Leakey’s team discovered Turkana Boy, the most complete early human skeleton ever found, believed to be between 1.5 and 1.6 million years old.

In the late 1980s Leakey’s interest shifted away from paleoanthropology towards wildlife conservation and ecology. This book – itself now quite dated – combines his two areas of expertise to give a thorough and quite academic history of the evolution of life on earth and to situate the evolution of hominids and Homo sapiens within it, before going on to present its Big Issue.

Key dates

• Age of the universe – 13.772 billion years
• Age of the solar system – 5 billion years
• Age of planet earth – 4.6 billion years
• Simplest life forms – prokaryotes, single-celled organisms which lack a nucleus -3.5 billion years ago
• Eukaryotic organisms, whose cells contain a nucleus – 1.8 billion years ago
• 530 million year ago – the Cambrian Explosion, when suddenly a huge diversity of multi-celled life forms and body shapes and sizes emerges in the fossil record
• The Cretaceous Period, the last and longest segment of the Mesozoic Era, lasted approximately 79 million years, from the minor extinction event that closed the Jurassic Period about 145.5 million years ago to the Cretaceous-Paleogene (K-Pg) extinction event dated at 65.5 million years ago. Period when ‘dinosaurs ruled the earth’.
• 7 million years ago, approximate parting of the line which led to humans from the lines which led to the great apes
• 150,000 years ago, evolution of the new species, Homo sapiens
• 13,000 years ago – end of the last Ice Age triggers the invention of agriculture in the Fertile Crescent, which slowly spreads around the globe and with it the arrival of what we like to call human ‘civilisation’

It’s from the Cambrian Explosion – 530 million years ago – that everything most of us think of as life forms – fish and dinosaurs, plants and trees, then later we mammals – derive. Most of the epochs and periods we hear about – Jurassic, Triassic etc – occur during that 530 million period, most fossils of life forms derive from that period.

• Timeline of the evolutionary history of life

The sixth extinction

The central premise of the book, which gives it its title, is that, over the half-billion-year history of multi-celled life on earth, there have been a number of ‘moments’ in the geological record when a significant percentage of the flora and fauna of a certain era seem to have died out very suddenly (in geological terms) – known as ‘mass extinction events’. Having explained the background and possible reasons for them, the book then goes on to point out that we are living through a sixth mass extinction event, in which huge numbers of species are being driven to extinction. There is no doubt at all what is causing it: it is us – humans. Human beings are wiping out the earth’s ecosystems and wildlife.

The ‘Big Five’ mass extinctions

The Big Five are defined as extinction events in which at least 65% of species were obliterated. The end-Permian is the biggest, in which an estimated 95% of species on earth were wiped out.

• at the end of the Ordovician Period – 440 million years ago
• the Late Devonian 365 million years ago
• the end-Permian 225 million years ago
• the end-Triassic 210 million years ago
• the end-Cretaceous 65 million years ago

There is huge debate about the possible causes of these great ‘dying outs’. Climate change? The conglomeration of all the continents through continental drift into one mega-continent? The most dramatic suggestion, first mooted in the 1970s by a team led by Luis Alvarez, is that it was asteroids. They found thin layers of iridium at the archaeological line marking the end of the Cretaceous period, an element which is extremely rare on earth but is found in asteroids. This discovery has been replicated at other end-Cretaceous sites, and then a candidate for the giant crater caused by a monster asteroid was discovered on the coast of Mexico. The idea is simple: monster asteroid hits earth with the power of a million hydrogen bombs, throws up vast amounts of dirt and dust into the air which blocks out the sun, as well as triggering widespread volcanic activity. Result: mass extinctions of life.

There’s a lot of evidence for it, but archaeologists and biologists are an argumentative lot, as this book amply demonstrates, and other scientists have piled in to claim that asteroids might have put only the finishing touches to what other causes – climate change, sea level rises, environmental or atmospheric fluctuations and so on – had started. Others – David Raup and Jack Sepkoski – have pointed out that there have been over twenty extinction events over that half billion year span, of which the Big Five are only the most notable (p.56), and which occur at roughly 26 million year intervals. Only the recurrent arrival of a shower of asteroids could explain this regularity, although more recently doubt has been cast on the evidence for this neat pattern. But there’s no doubting, now, that externally-prompted mass extinctions have been a recurrent feature of terrestrial evolution.

Which gives rise to an immense debate about the deep meaning of the theory of evolution, which can be summarised in the phrase ‘bad genes or bad luck’. Is there an inevitability in the way life has evolved? If we ran the tape of evolution again, would life forms turn out much as we see them around us? Is there a kind of deep logic to the way things would have evolved, to fit the available niches?

Or has the evolution of life on earth been subject to mind-boggling accidents and contingency? Could things easily have turned out wildly differently? Was it the merest luck which led to the various mass extinctions, to the death of the dinosaurs 65 million years ago, to the rise of the mammals and then, right at the end of this string of improbable accidents – to us, reading these words?

These and many related questions are tackled – with the help of quite technical diagrams and explanations – in the first half of the book. It takes a few rereadings to get the timelines clear in your head, and then more rereading to understand what the numerous debates are about.

For example, uniformitarianism is the idea that evolution takes place gradually and slowly over vast periods of time. Darwin had to arrive at his theory by battling essentially religious ideas that species were suddenly created by a Creator God, so he and his followers were vehement uniformitarianists. However, from the birth of geology as a science in the early 19th century, geologists recognised sudden abrupt changes in the record – catastrophic changes in the fossil record which the extinction events seem to support. Broadly this view of evolution is called catastrophism. The American paleontologist and evolutionary biologist Stephen Jay Gould developed his own version of catastrophism, which he called punctuated equilibrium – long periods of stasis interrupted by abrupt changes in earth’s biota, or life systems. Modern thinking about the importance of mass extinction events has led to what some call ‘neo-catastrophism’ i.e. Darwin’s laws work most of the time, except when some external force steps in to overshadow them – be it drastic climate change, asteroids, volcanic activity, sea level changes or whatever.

Man the destroyer

We are but one of millions of species here on earth, products of half a billion years of life’s flow, lucky survivors of at least twenty biotic crises, including the catastrophic Big Five. (p.71)

But these and various other theories and debates about the detail of historical evolution are really just the background, the introduction to the meat of the book, which is a lament for man’s destruction of the natural world. Leakey uses numerous examples to show how modern science has revealed just how much life there is, all around us.

He reports Danish scientists who investigated one square metre of tropical rainforest and discovered 46,000 earthworms, 12 million roundworms and 46,000 insects. Just one gram of this soil contained more than a million bacteria, 100,000 yeast cells and 50,000 fragments of fungi (p.136).

The rape and destruction of the earth which we are causing is mind-blowing. It is estimated that ‘we are losing upwards of 80,000 acres of tropical rainforest daily, and significantly degrading another 80,000 acres every day on top of that. Along with this loss and degradation, we are losing some 135 plant, animal and insect species every day – or some 50,000 species a year.’ (Scientific American)

But all that’s new is the scale: man has always been a destroyer. Between 12,000 and 10,000 years ago 50 or so large mammal species went extinct in North America. The extinctions coincided with the arrival of the first peoples from Asia (across the land bridge across what is now the Bering Straits) and their slow fanning out across the continent. Although some paleontologists prefer climate change or disease as the cause, many think these first human settlers of the Americas hunted its large mammals to extinction. This theory is called the Overkill hypothesis. The case is even clearer in New Zealand, which Maori colonised about 1,000 years ago and where they hunted the large flightless birds to extinction, while the rats they’d brought from Australia wiped out whole systems of ground-roosting birds and other fauna.

‘The notion of man-the-exterminator is secure in New Zealand.’ (p.186)

Same on Hawaii (pp.188-190).

Numbers

How many species are there on earth? Nobody knows. Leakey quotes an early estimate from the 1960s of 3 million. Terry Irwin, in 1982, estimated there may be 30 million species of insects alone in the rainforest canopy. Elsewhere, Leakey quotes estimates of the total number of species as 50 million, and then references Robert May’s speculation from the 1990s that there may be as many as 100 million species. These appear to be the end points, which is why later articles refer to ‘anywhere between 3 and 100 million’.

More recently, a 2011 estimate using a new methodology gives the total number of species in the world as 8.7 million – 6.45 million on land, 2.2 million in the sea. According to this calculation, 86% of all species on land and 91% of those in the seas have yet to be discovered, described and catalogued. But googling the subject, though, one comes across a bang up-to-date estimate from 2016 which says there might be as many as 1 trillion species on earth!

In other words, despite E.O. Wilson’s calls for governments to invest more in finding out how many species we share the world with – a plea from the 1990s quoted in this book (p.123) – we still haven’t a clue how many there are. What we can be confident about is that we are wiping out most of the species we share the earth with before we ever get to discover, identify, record or analyse any of them.

In fact, we don’t even know precisely how many species have been identified and catalogued. It appears to be about 1.25 million species – roughly 1 million on land and 250,000 in the oceans – but as many as 700,000 more are thought to have been described by local scientists which have yet to reach the central databases and so be included in global counts…

Value in diversity

How do we preserve nature? Well, in our demystified, instrumental, capitalist world, we have to give it a value, the only thing most people understand. Leakey identifies three types of value:

1. Tangible benefits we can extract from the environment, such as food, raw materials, medicine.
2. Maintenance of the environment: we need the full web of life to continue its circulation of gases, chemicals and moisture in order to make the world inhabitable by humans.
3. Psychological health: most people with the money, prefer to live in the country, people like to visit and roam in the country, patients in hospital with a bed by a window in a green space have better recovery rates than patients in a windowless room. The presence of greenery and nature keeps us psychologically healthy – and that greenery doesn’t exist in the abstract – it is made up of incalculably complex webs of organisms. E.O. Wilson has named this sense ‘biophilia’.

Food and drugs are the obvious ones. The world is dangerously dependent on monocultural varieties of a handful of food crops. If a pest devastated the world’s wheat or rice crops, billions would starve. Wild varieties contain genes we haven’t identified or analysed which would provide important genetic variations which could help develop new varieties, if the worst ever happened.

Similarly, important worldwide medicines have been sourced from wholly unexpected wild plants and flora. Aspirin and penicillin are the two obvious examples, which changed the world and saved hundreds of millions of lives. Who knows what cures for cancer or AIDS may be lurking undiscovered in some of the 250,000 species of plants? And in species we are merrily burning to extinction every day?

Theories and ideas

Even twenty years ago when this book was published, all educated people should have known about the destruction of the rainforests and endangered species. That aspect shouldn’t be news to anyone. I think the real revelation of this book is the extraordinary complexity and difficulty of ecological and biological and archaeological science – the range of areas and levels and expertises which are now brought to bear on the natural world, the complexity of computer models and the plethora of rival theories.

Leakey’s book is in many places quite dauntingly technical. Plenty of paragraphs contain numbered points or aspects or theories which we need to learn and bear in mind. For example, we learn about:

• Allopatric speciation – or geographic speciation is speciation that occurs when biological populations of the same species become vicariant, or isolated from each other to an extent that prevents or interferes with genetic interchange.
• Cambrian Explosion – ‘the relatively short evolutionary event, beginning around 541 million years ago in the Cambrian period, during which most major animal phyla appeared, as indicated by the fossil record’ (Wikipedia)
• Chaos theory as it applies to ecosystems i.e. modern understanding has undermined the notion of a ‘balance of nature’ to reveal that all systems, even without any external influence, tend to boom and bust and be subject to other ‘internal’ pressures which create fluctuations. I.e. every ecosystem, and nature as a whole, is much more chaotic and unstable than had been appreciated.
• The ‘protective network‘ of an ecosystem which places an ‘activation barrier‘ around it to prevent new species intruding (p.162-3).
• The rivet-popper hypothesis and the redundancy hypothesis of how ecosystems are degraded. Rivet-popper = each species in a system is like the rivets in a ship – you can remove one or two without noticing but the more you remove the more you weaken the system until it reaches collapse. Redundancy = most species are passengers on a system held together by a few lynchpin systems: you can remove most with no change; but remove the lynchpins and the system collapses. (pp.140-141)

The relative importance of ‘history’ and evolution

One of the big points Leakey makes is that in his time older ideas like ‘the balance of nature’ and even the primacy of natural selection, have been thrown in doubt. Nature now appears to be much more chaotic than previously suspected. And he explains recent work which suggests that the world we see around us is radically contingent. Archaeologists examined the fossil record of animals off the North Atlantic coast over the past 60 million years, a huge duration during which sea levels rose and fell six times. They discovered that mature ecosystems repopulated the dry land once it was reflooded – but each time it was repopulated by a different combination of species. I.e. the reappearance of life was ‘inevitable’ – but which specific species fill all the niches and grow into a tangled web of an ecosystem – it can be different each time. There is nothing intrinsic or inevitable about the flourishing of particular species or combinations of species in ecosystems. Shake the dice and you get a different set.

As Leakey puts it, History matters. Life there will be, but what forms of life and how they combine, even in the same environment, can vary hugely depending on chance factors. Thus Darwin’s theory of evolution by natural selection still gives the best account of how species evolve over time – but in seeking an explanation for the natural world as we see it, the theory of evolution is dwarfed by the random events of what we call ‘history’.

Insights into human history

Obviously Leakey’s main concern is with nature and ecology but for someone like me who knows more about history than I do about biology or ecology, the multiple insights a scholarly book like this gives the reader into the natural world can also shed tremendous light on the deeper meaning of human history.

I’ve just finished Alan Taylor’s epic account of the colonisation of America. In it he emphasises that it wasn’t just European humans who arrived in the New World, but that they brought from the Old World , in order of importance – their devastating new diseases, new plants and new livestock. These spread like wildfire across the virgin continent, the diseases wiping out up to 95% of the native inhabitants of the Americas, while new plants spread like weeds, and livestock drove American rivals extinct.

Quite apart from its ostensible purpose as a warning and a plea, a book like Leakey’s can immeasurably increase our understanding of human history by giving us a deep sense of the mind-boggling complexity of the natural world which human beings have been blundering around, reshaping and destroying, burning and deforesting and planting and mixing up, for centuries – the process we refer to as ‘history’.

It makes us realise what was at stake back then, as well as now. It makes us realise the depth of the damage we have been doing, and for centuries.

Attitude

Obviously the planet is indifferent to individual human opinions, attitudes and stands. Clicking ‘like’ on facebook to a photo of a polar bear or the rainforest isn’t going to change anything. Only a wholesale and comprehensive change to all of our lifestyles, combined with drastic attempts to control and reduce human population, will have any real, practical impact on the problem.

On a personal level, this knowledge does suggest a truer, more accurate understanding of human nature (destructive) and our place in the natural world (destructive) which should have a chastening effect on everything we think and do. It transforms our understanding and it should transform our behaviour.

From time to time Leakey mentions, or quotes other ecologists criticising, humanity’s ‘narcissism’ or ‘arrogance’, each of us infested with thoughts and feelings and desires which a) are ultimately trivial b) obscure to ourselves our fundamental role as destroyers of the environment.

A correct attitude, the accurate honest attitude to the devastation we cause, would be one of modesty, shame and penance.

In a way, understanding these issues better should lead us to a kind of attitude and – ideally – lifestyle, characterised by simplicity and humility. All of us need to consume less, vastly less, than our arrogance and ignorance and selfishness prompt us to.

A proper understanding of our place in the world should lead to the virtues praised by monks and nuns of all religious orders: shun the world, shun consumption, shun exploitation, work in humility and honesty to supply our bare needs. Only then, maybe, despite all the evidence to the contrary, might there be a slight possibility of hope that we do not exterminate most life forms on the planet including, of course, many that we depend and rely on for our own existence.

In order to know ourselves as a species and to understand our place in the universe of things, we have to distance ourselves from our own experience, both in space and time. It is not easily done but it is essential if we are truly to see a larger reality. (p.6)

Related links

• The Sixth Extinction on Amazon
• The Holocene Extinction Wikipedia article
• Anthropocene Wikipedia article
• Richard Leakey Wikipedia article
• Roger Lewin Wikipedia article
• Biophilia hypothesis Wikipedia article
• An A to Z of evolution

Reviews of other science books

Chemistry

• The Periodic Kingdom: A Journey Into the Land of the Chemical Elements by Peter Atkins (1995)

Cosmology

• The Perfect Theory by Pedro G. Ferreira (2014)
• The Book of Universes by John D. Barrow (2011)
• The Origin Of The Universe: To the Edge of Space and Time by John D. Barrow (1994)
• The Last Three Minutes: Conjectures about the Ultimate Fate of the Universe by Paul Davies (1994)
• A Brief History of Time: From the Big Bang to Black Holes by Stephen Hawking (1988)
• The Black Cloud by Fred Hoyle (1957)

The Environment

• The Sixth Extinction: An Unnatural History by Elizabeth Kolbert (2014)
• The Sixth Extinction by Richard Leakey and Roger Lewin (1995)

Genetics and life

• Life At The Speed of Light: From the Double Helix to the Dawn of Digital Life by J. Craig Venter (2013)
• What Is Life? How Chemistry Becomes Biology by Addy Pross (2012)
• Seven Clues to the Origin of Life by A.G. Cairns-Smith (1985)
• The Double Helix by James Watson (1968)

Human evolution

• The Origin of Our Species by Chris Stringer (2011)
• Guns, Germs and Steel by Jared Diamond (1997)

Maths

• Alex’s Adventures in Numberland by Alex Bellos (2010)
• Nature’s Numbers: Discovering Order and Pattern in the Universe by Ian Stewart (1995)
• Innumeracy: Mathematical Illiteracy and Its Consequences by John Allen Paulos (1988)
• A Mathematician Reads the Newspaper: Making Sense of the Numbers in the Headlines by John Allen Paulos (1995)

Particle physics

• Atomic by Jim Baggott (2009)

Psychology

• Tips For Trying To Think Less Irrationally
• Irrationality: The Enemy Within by Stuart Sutherland (1992)
• Judgement Under Uncertainty: Heuristics and Biases by Amos Tversky and Daniel Kahneman (1974)