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When the Central Intelligence Agency (CIA) failed to prevent the September 11, 2001 attacks, many asked whether more could have been done. But the true reason why the agency was blind to the signs may be a diversity problem, writes Matthew Syed.

The failure of the CIA to spot the warning signs of the 9/11 plot has become one of the most hotly contested issues in the history of intelligence. There have been commissions, reviews, internal investigations and more.

On the one side are those who say that the CIA missed obvious warning signs. On the other are those who argue that it is notoriously difficult to identity threats in advance, and that the CIA did everything they reasonably could.

But what if both sides are wrong? What if the true reason why the CIA failed to detect the plot is more subtle than either side has realised? And what if this problem extends beyond intelligence and silently afflicts thousands of organisations, governments and teams today?

While many of the inquiries focused on particular judgements in the frenetic build-up to 9/11, few took a step back to examine the internal structure of the CIA itself and, in particular, its hiring policies. At one level, these were state of the art. Potential analysts were put through a battery of psychological, medical and other exams. And there is no doubt they hired exceptional people.

"The two major exams were a SAT-style test to probe a candidate's intelligence and a psychological profile to examine their mental state," says a CIA veteran. "The tests filtered out anyone who was not stellar on both tests. In the year I applied, they accepted one candidate for every 20,000 applicants. When the CIA talked about hiring the best, they were bang on the money ."

And yet most of these recruits also happened to look very similar - white, male, Anglo-Saxon, Protestant Americans.

This is a common phenomenon in recruiting, sometimes called "homophily": people tend to hire people who think (and often look) like themselves. It is validating to be surrounded by people who share one's perspectives and beliefs. Indeed, brain scans suggest that when others reflect our own thoughts back to us, it stimulates the pleasure centres of our brains.

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In their study of the CIA, the intelligence experts Milo Jones and Phillipe Silberzahn write: "The first consistent attribute of the CIA's identity and culture from 1947 to 2001 is homogeneity of its personnel in terms of race, sex, ethnicity and class background (relative both to the rest of America and to the world as a whole)."

An inspector general's study on recruitment found that in 1964, one branch of the CIA, the Office of National Estimates, "had no black, Jewish, or women professionals, and only a few Catholics".

By 1967, the report said, there were fewer than 20 African Americans out of some 12,000 non-clerical CIA employees, and the agency maintained the practice of not hiring minorities from the 1960s through the 1980s. And until 1975, the US intelligence community "openly barred the employment of homosexuals".

Talking of his experience of the CIA in the 1980s, one insider wrote that the recruitment process "led to new officers who looked very much like the people who recruited them - white, mostly Anglo-Saxon; middle and upper class; liberal arts college graduates". There were few women and "few ethnics, even of recent European background".

"In other words, not even as much diversity as there was among those who had helped create the CIA."

Diversity was squeezed further after the end of the Cold War. A former operations officer said that the CIA had a "white-as-rice culture".

In the months leading up to 9/11, the International Journal of Intelligence and Counterintelligence commented: "From its inception, the Intelligence Community [has been] staffed by the white male Protestant elite, not just because that was the class in power, but because that elite saw itself as the guarantor and protector of American values and ethics."

Image copyright DANIEL SLIM/AFP/Getty Images Image caption The headquarters of the CIA in Langley, Virginia

Why did this homogeneity matter? If you are hiring a relay team, don't you just want the fastest runners? Why would it matter if they are the same colour, gender, social class, etc?

Yet this logic, while sound for simple tasks like running, flips for complex tasks like intelligence. Why? Because when a problem is complex, no one person has all the answers. We all have blind spots, gaps in our understanding.

This means, in turn, that if you bring a group of people who share similar perspectives and backgrounds, they are liable to share the same blind spots. And this means that far from challenging and addressing these blind spots, they are likely to be reinforced.

Perspective blindness refers to the fact that we are often blind to our own blind spots. Our modes of thought are so habitual that we scarcely notice how they filter our perception of reality.

The journalist Reni Eddo-Lodge describes a period when she had to cycle to work: "An uncomfortable truth dawned upon me as I lugged my bike up and down flights of stairs: the majority of public transport was not easily accessible… Before I'd had my own wheels to carry, I'd never noticed this problem. I'd been oblivious to the fact that this lack of accessibility was affecting hundreds of people."

This example doesn't necessarily imply that all stations should be equipped with ramps or lifts. But it does show that we can only perform a meaningful analysis if the costs and benefits are perceived. This hinges on diversity of perspective. People who can help us to see our own blind spots, and who we can help to see theirs.

Osama Bin Laden made his declaration of war on the United States from a cave in Tora Bora in February 1996. Images revealed a man with beard reaching down to his chest. He was wearing cloth beneath combat fatigues.

Today, given what we now know about the horror he unleashed, the declaration looks menacing. But an insider in the foremost US intelligence agency said the CIA "could not believe that this tall Saudi with a beard, squatting around a campfire, could be a threat to the United States of America".

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To a critical mass of analysts, then, Bin Laden looked primitive and of no serious danger. Richard Holbrooke, a senior official under President Clinton, put it this way: "How can a man in a cave out-communicate the world's leading communications society?"

Another said: "They simply couldn't square the idea of putting resources into finding out more about Bin Laden and al-Qaeda given that the guy lived in a cave. To them, he was the essence of backwardness."

Now, consider how someone more familiar with Islam would have perceived the same images.

Bin Laden wore cloth not because he was primitive in intellect or technology, but because he modelled himself on the prophet. He fasted on the days the prophet fasted. His poses and postures, which seemed so backward to a western audience, were those that Islamic tradition ascribes to the most holy of its prophets.

As Lawrence Wright put it in his Pulitzer Prize winning book about 9/11, Bin Laden orchestrated his operation by "calling up images that were deeply meaningful to many Muslims but practically invisible to those who were unfamiliar with the faith".

Jones writes: "The beard and campfire anecdote is evidence of a larger pattern in which non-Muslim Americans - even experienced consumers of intelligence - underestimated Al Qaeda for cultural reasons."

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As for the cave, this had even deeper symbolism. As almost any Muslim knows, Mohammad sought refuge in a cave after escaping his persecutors in Mecca. To a Muslim a cave is holy. Islamic art overflows with images of stalactites.

Bin Laden modelled his exile to Tora Bora as his own personal hijrah, and used the cave as propaganda. As one Muslim scholar put it: "Bin Laden was not primitive; he was strategic. He knew how to wield the imagery of the Koran to incite those who would later become martyrs in the attacks of 9/11."

Analysts were also misled by the fact that Bin Laden often issued pronouncements in poetry. To white, middle class analysts, this seemed eccentric, reinforcing the idea of a "primitive mullah in a cave". To Muslims, however, poetry has a different meaning. It is holy. The Taliban routinely express themselves in poetry.

The CIA were studying the pronouncements with a skewed frame of reference. As Jones and Silberzahn put it: "The poetry itself was not merely in the foreign language of Arabic; it derived from a conceptual universe light years from Langley".

More on Bin Laden and 9/11

By 2000, the "anti-modern, uneducated rabble" had swelled to an estimated 20,000, mostly college educated and with a bias towards engineering. Yazid Sufaat, who would go on to become one of al-Qaeda's anthrax researchers, had a degree in chemistry and laboratory science. Many were ready to die for their faith.

Meanwhile, senior CIA official Paul Pillar (white, middle-aged, Ivy league), was discounting the very possibility of a major act of terrorism. "It would be a mistake to redefine counterterrorism as a task of dealing with 'catastrophic', 'grand' or 'super' terrorism," he said, "when in fact these labels do not represent most of the terrorism the United States is likely to face".

Another flaw in the CIA's deliberations was their reluctance to believe that Bin Laden would initiate conflict with the US. Why start a war he couldn't possibly win? They hadn't made the conceptual leap that the victory for the jihadists was to be secured not on earth but in paradise.

The al-Qaeda code name for the plot was The Big Wedding. In the ideology of suicide bombers, the day of a martyr's death is also his wedding day where he will be greeted by virgins in heaven.

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The CIA could have allocated more resources to al-Qaeda. They could have attempted infiltration. But they were incapable of grasping the urgency. They did not allocate more resources, because they didn't perceive a threat.

They didn't seek to penetrate al-Qaeda because they were ignorant of the gaping hole in their analysis. The problem wasn't (just) the inability to connect the dots in the autumn of 2001, but a failure across the entire intelligence cycle.

The dearth of Muslims is merely one illustration of how homogeneity undermined the world's foremost intelligence agency. It provides an insight into how a more diverse group would have created a richer understanding not just into the threat posed by al-Qaeda, but dangers throughout the world. How different frames of reference, different perspectives, would have created a more comprehensive, nuanced, and powerful synthesis.

A startlingly high proportion of staff at the CIA had grown up in middle class families, endured little financial hardship, or the signs that might act as precursors to radicalisation, or any of a multitude of other experiences that might have added formative insights to the intelligence process.

Each would have been assets in a more diverse team. As a group, however, they were flawed.

This isn't just about the CIA, however. Look at many governments, law firms, army leadership teams, senior civil servants and even executives at some tech companies. We are unconsciously drawn to people who think like ourselves, but rarely notice the danger because we are unaware of our own blind spots.

John Cleese, the comedian, put it this way: "Everybody has theories. The dangerous people are those who are not aware of their own theories. That is, the theories on which they operate are largely unconscious."

Getting the right mix of diversity in human groups isn't easy.

There is a science to putting together the right minds, with perspectives that challenge, augment, diverge and cross-pollinate rather than parrot, corroborate and restrict. This is set to become a key source of competitive advantage for organisations, not to mention security agencies. This is how wholes become more than the sum of their parts.

The CIA has made strides towards meaningful diversity since 9/11, but the issue continues to dog the agency.

An internal report in 2015 was damning.

As John Brennan, then director, put it: "The study group took a hard look at our agency and reached an unequivocal conclusion: CIA simply must do more to develop the diverse and inclusive leadership environment that our values require and that our mission demands."

Matthew Syed is the author of Rebel Ideas: The Power of Diverse Thinking