There is no doubt that knots weaken lines. The best knot still weakens a line by 40%. Some of the worse knots weaken line by 60%. A double fisherman is a good knot that is closer to the 40% figure. I've read many scientific studies related to this. It's a fact.

Again though the failure point is usually a wear point in the line. Because after a line hits the real world you can throw all those scientific studies out the window as the real world is a very rough place.

Mike Hill