Excellent and fact filled writeup on Overcoming Bias about getting people past the Planning Fallacy.

Basically, it says humans don't predict... we hope. Only past measures and outside observation can get you anywhere close to accurate timelines. Which in the software project business is the definition of margin. It's why most software development companies live in a rolling state of fail... they optimistically estimate project completion and then run into trouble when the ones they make promises to get angry and won't keep paying for the real time required.