When we estimate the likelihood of being eaten by a grumple bug, we substitute for other statistical methods a subjective measurement of how easy it is to imagine being eaten by a grumple bug. If I live in a small nomadic tribe where no one has ever recounted the tale of how his great aunt was eaten by a grumple bug while out hunting--indeed, a tribe where no one has ever even heard of a grumple bug--I'll understandably estimate the likelihood of being eaten by a grumple bug to be low. As a result, I'll spend very little time worrying about grumple bugs, and will instead devote my resources to watching for dangers I hear about all the time, like tigers, rival tribes, and poisonous mushrooms that take your soul to the afterlife even before your body bites the dust. Grumple bugs aren't very available in memory compared to tigers.
If, however, the tale of the time great aunt Cathy was eaten by a tiger while out on a hunt is told over and over again around the camp fire, I may begin to spend more of my time watching out for tigers than avoiding heat exhaustion. Never mind that heat exhaustion is actually much more likely and equally deadly. When that happens, the availability heuristic dons its other masque: the availability bias.
Ease of Imagery
Availability, as both a heuristic and a bias, apparently comes down to ease of imagery. By "imagery", I mean something broader than "how easy it is to conjure up a visual representation". When you imagine a tiger, you probably don't just see a still photograph or painting of a tiger in your mind. Imagination can be fully immersive; imaginary tigers are big and orange with black stripes, but they also growl, slink stealthily while stalking prey, drip wet blood from their fangs, and smell of musk and raw meat.
Several things contribute to ease of imagery. One is actual frequency in the local environment, which might or might not match global frequency. Maybe grumple bugs are a thing a couple hundred miles south, and I'll be caught unaware if the tribe heads that direction. Another is repetition. It's useful to rely on ease of imagery when I'm unlikely to hear about grumple bugs very often in a place where there are no grumple bugs; on the other hand, I'll probably hear about tigers more frequently than the occurrence of tigers in the local environment warrants, because tiger stories are way more gripping than heat exhaustion stories. They have conflict, protagonists, antagonists, narrative arcs, and often social drama. That's the formula for deep significance to a human brain. "Tom died 'cause he got too hot" doesn't measure up. As with tigers on the ancestral savanna, so too with cougars in the modern American Midwest, alligators in the sewers of New York, and kidney theft.
Tigers seem more at home in the imagination than does heat exhaustion, don't they? I can come up with an equally detailed description of heat exhaustion if I try, but it takes more work. There's something more going on with ease of imagery than frequency of exposure and narrative structure. Since I'm cheating with the picture of the tiger in the top right, imagine instead a human-sized duck holding an umbrella while playing a kazoo.
Got it? Ok, now imagine the availability heuristic. Very different sort of experience, right?
Here's what's up with the wacky duck. The duck is simple, concrete, vivid (for multiple sensory modalities), and emotionally engaging (humorous, specifically, and also surprising due to its strangeness). The availability heuristic, by contrast, is complicated, abstract, murky, and boring (at least at first blush).
To recap, the core of availability is ease of imagery, which is a combination of frequency of exposure (repetition), meaningful narrative context, concreteness, vividness, and emotional impact.
It's not true, of course, that drugged travelers sometimes wake up in bathtubs full of ice to discover that their interanal organs have been stolen for sale on the black market. But it's a concrete, vivid, meaningful story oft repeated for emotional impact, so people accept it as fact before System Two ever gets a chance to go, "Wait a minute, you want me to believe what?"
- intro to availability by Eliezer Yudkowsky
- study of ease of imagery, purposful imagining, and availability
- study of emotion and availability
- wonderful book about why some ideas survive and others die
- the next post in this sequence, which I'll link to as soon as it exists and will be about engineering memory for availability