"While there are some domains where knowing _is_ half the battle, there are many more where it is not. Recent work in cognitive science has demonstrated that knowing is a shockingly tiny fraction of the battle for most real-world decisions.
You may know that $19.99 is essentially the same as $20, but the first still feels like a significantly better deal...you may know that a piece of chocolate fudge shaped like dogshit will taste delicious, but you might still be hesitant to eat it.
...
"You might consider this old news. After all, thinkers for 2500 years have been pointing out that much of human action isn't under rational control. Don't we _know_ by now that the 'G.I. Joe fallacy...is a fallacy?
Yeah, we _know_, but..."
-Laurie R. Santos & Tamar Gendler, Cognitive Science Dept., Yale University