If I might add to that:
Most "Special Forces" and by that I mean SAS, SBS, DELTA, SEAL, RANGERS et al only take individuals who are of above average intelligence.

In other words: No idiots.

All of the above run selection courses that are specifically designed to test for mental toughness and agility.

Physical toughness is desired certainly, but anyone in reasonable physical condition can be trained up to the desired level.

How does that translate in to our terms of reference?

Simple: Survival is 40% preparation and training (to give you the basic skills and tools), 60% determination to survive.

It has been demonstrated time and again that people who want to survive, will survive. People who don't have that determination won't, no matter how lavishly you equip them.