Humans live in a world of uncer- tainty. A shadowy figure on the sidewalk ahead could be a friend or a mugger. By flooring
your car’s accelerator, you might beat the
train to the intersection, or maybe not.
Last week’s leftover kung pao chicken
could bring another night of gustatory
delight or gut agony.

People’s paltry senses can’t always
capture what’s real. Luckily, though, the
human brain is pretty good at playing
the odds. Thanks to the brain’s intuitive grasp of probabilities, it can handle
imperfect information with aplomb.

“Instead of trying to come up with an
answer to a question, the brain tries to
come up with a probability that a particular answer is correct,” says Alexandre
Pouget of the University of Rochester in
New York and the University of Geneva
in Switzerland. The range of possible
outcomes then guides the body’s actions.

A probability-based brain offers a
huge advantage in an uncertain world.
In mere seconds, the brain can solve (or
at least offer a good guess for) a problem
that would take a computer an eternity
to figure out — such as whether to greet
the approaching stranger with pepper
spray or a hug.

A growing number of studies are illuminating how this certitude-eschewing
approach works, and how powerful it can
be. Principles of probability, researchers
are finding, may guide basic visual abilities, such as estimating the tilt of lines or
finding targets hidden amid distractions.
Other behaviors, and even simple math,
may depend on similar number crunching, some scientists think.

And such advanced statistical reasoning does not require paying attention
in math class. New studies suggest that
1-year-olds are already tiny probabilistic
machines who, in many situations, assess
statistical input and perform optimally
with ease.

Studying the guesstimating brain
is a new enough endeavor that no one
yet knows how people developed such
computational abilities. Nor do scientists know the precise brain machinery