Jim Collins and Jerry Porras coined the term BHAG (“big, hairy, audacious goal”) in their article, Building Your Company’s Vision, back in 1996. Since that time it’s become so much a part of the lingua franca of business that you practically can’t call yourself a leader if you haven’t set some BHAGs for your company, your team, or yourself.

It’s fascinating, though, to see just how many BHAGs are entombed in 2” D-ring binders collecting dust on people’s bookshelves, with pretty much zero chance of actually being implemented. There are all kinds of reasons—you don’t have the time or money or people, for example, or first you have to take care of your boss’s stupid pet project, or you’re trapped in too many meetings—but regardless of the excuse, those BHAGs are joining flying pigs in the list of things you won’t see in this life.

Now Shawn Achor, author of The Happiness Advantage: The Seven Principles of Positive Psychology that Fuel Success and Performance at Work, explains why in an article in CIO:

Goals that are too big paralyze you. They literally shut off your brain, says Achor.

Here’s what happens to your brain when faced with a daunting goal or project: The amygdala, the part of the brain that responds to fear and threats, hijacks the “thinker” part of the brain, the prefrontal cortex, says Achor. The amygdala steals resources from the prefrontal cortex, the creative part of the brain that makes decisions and sees possibilities.

“We watch this on a brain scan,” he says. “The more the amygdala lights up, the less the prefrontal cortex does.”

Breaking a big goal into smaller, more achievable goals prevents the fear part of your brain from hijacking your thinking cap and gives you victories.

Don’t get me wrong: I don’t think your lizard brain (in Seth Godin’s term) is the only reason that so many organizations fail to achieve their BHAGs. Corporate inertia has a thousand fathers—reading any Dilbert is proof of that. But the daunting prospect of a BHAG, combined with a lack of clarity of how, precisely, to get from here to there, often plays a role in paralysis at the individual level.

In companies that struggle to realize their BHAGs, it’s frequently because no one has taken the time to map out precisely what small steps are needed to reach them. When I worked at Asics years ago, we set ourselves a goal to dethrone Nike as the number one brand among running enthusiasts. (To put this goal in perspective: Nike was a $4 billion company at the time. Asics was $180 million.) Pretty ambitious stuff for us.

We laid out a careful roadmap to reach this goal: recasting our running product line by eliminating lower-end shoes and building our first legitimate high-end shoe; providing special sales and customer service support to specialty running stores; creating special sales programs; focusing our advertising on the core running enthusiast; and having the product marketing and development teams spend more time visiting specialty running retailers during the product development stage. No step by itself would have done the job, but the steady accretion of these moves eventually toppled Nike among these customers.

We didn’t talk about BHAGs then. (That was before Collins’ article, for one thing.) But we did achieve one, by rigorously implementing a series of small steps. And because we were dealing with small steps, we didn’t have to worry about illuminated amygdalae, or struggle to clarify the vacuous ambiguities that too often paralyze good people.