Story highlights FBI, CIA criticized for not keeping better track of Tamerlan Tsarnaev before attack

Bruce Schneier: Connecting the dots seems easy in hindsight, but in real life, it's not

He says there are an enormous number of potential bad guys

Schneier: If a terrorist plot succeeds, it doesn't mean law enforcement systems failed

The FBI and the CIA are being criticized for not keeping better track of Tamerlan Tsarnaev in the months before the Boston Marathon bombings. How could they have ignored such a dangerous person? How do we reform the intelligence community to ensure this kind of failure doesn't happen again?

It's an old song by now, one we heard after the 9/11 attacks in 2001 and after the Underwear Bomber's failed attack in 2009. The problem is that connecting the dots is a bad metaphor, and focusing on it makes us more likely to implement useless reforms.

Connecting the dots in a coloring book is easy and fun. They're right there on the page, and they're all numbered. All you have to do is move your pencil from one dot to the next, and when you're done, you've drawn a sailboat. Or a tiger. It's so simple that 5-year-olds can do it.

But in real life, the dots can only be numbered after the fact. With the benefit of hindsight, it's easy to draw lines from a Russian request for information to a foreign visit to some other piece of information that might have been collected.

Bruce Schneier

In hindsight, we know who the bad guys are. Before the fact, there are an enormous number of potential bad guys.

How many? We don't know. But we know that the no-fly list had 21,000 people on it last year. The Terrorist Identities Datamart Environment, also known as the watch list, has 700,000 names on it.

We have no idea how many potential "dots" the FBI, CIA, NSA and other agencies collect, but it's easily in the millions. It's easy to work backwards through the data and see all the obvious warning signs. But before a terrorist attack, when there are millions of dots -- some important but the vast majority unimportant -- uncovering plots is a lot harder.

Rather than thinking of intelligence as a simple connect-the-dots picture, think of it as a million unnumbered pictures superimposed on top of each other. Or a random-dot stereogram. Is it a sailboat, a puppy, two guys with pressure-cooker bombs or just an unintelligible mess of dots? You try to figure it out.

It's not a matter of not enough data, either.

Piling more data onto the mix makes it harder, not easier. The best way to think of it is a needle-in-a-haystack problem; the last thing you want to do is increase the amount of hay you have to search through.

The television show "Person of Interest" is fiction, not fact.

There's a name for this sort of logical fallacy: hindsight bias.

First explained by psychologists Daniel Kahneman and Amos Tversky, it's surprisingly common. Since what actually happened is so obvious once it happens, we overestimate how obvious it was before it happened.

We actually misremember what we once thought, believing that we knew all along that what happened would happen. It's a surprisingly strong tendency, one that has been observed in countless laboratory experiments and real-world examples of behavior. And it's what all the post-Boston-Marathon bombing dot-connectors are doing.

Before we start blaming agencies for failing to stop the Boston bombers, and before we push "intelligence reforms" that will shred civil liberties without making us any safer, we need to stop seeing the past as a bunch of obvious dots that need connecting.

Kahneman, a Nobel prize winner, wisely noted: "Actions that seemed prudent in foresight can look irresponsibly negligent in hindsight." Kahneman calls it "the illusion of understanding," explaining that the past is only so understandable because we have cast it as simple inevitable stories and leave out the rest.

Nassim Taleb, an expert on risk engineering, calls this tendency the "narrative fallacy." We humans are natural storytellers, and the world of stories is much more tidy, predictable and coherent than the real world.

Millions of people behave strangely enough to warrant the FBI's notice, and almost all of them are harmless. It is simply not possible to find every plot beforehand, especially when the perpetrators act alone and on impulse.

We have to accept that there always will be a risk of terrorism, and that when the occasional plot succeeds, it's not necessarily because our law enforcement systems have failed.