Good question...

A 1954 study (Stonecipher, et al) found that of 20 private pilots, of
varying levels of experience, exactly 0 'survived' a simulated
encounter with IFR conditions. 19 did graveyard spirals, and 1 was
going to do a whip stall when the data-taker/safety pilot said "I've
got it."

My experience on BFR's may not be representative, as I am considered a
'tough; insturctor and that reputation precedes me to the point that
most of the people who call me are pretty good pilots to begin with and
probably perform better than the average. Given that, my experience is
such that I think that in excess of 70% of private pilots would 'buy
it' if they wandered into instrument conditions. The real numbers may
be higher.

One of the problems we have (but is not well discussed) is that in
training the stress level (even on a BFR) is a tiny fraction of what
the pilot will experience if he runs into these conditions solo.
Stress degrades performance in every area, so I think that what we see
on the BFR is something of a best case scenario. I try to make the
hood work a little more demanding than the average CFI, to simulate
some of the stress that may occur in the real situation, but I think I
am still an order of magnitutde or two below what a
non-instrument-rated private pilot experiences in a real (solo) IMC
encounter. A clue can be had by listening to the video presentation
"17 Ways to Fall Out of the Sky"; there is an audio tape in that video
of a real life VFR-into-IMC encounter involving a VFR C310 pilot trying
to get down through an overcast. Noteably, when he first gets into
trouble his voice goes up at least two octaves. (He did not make it.)

Unlike some CFIs, I aggressively seek out actual instrument condtions
in which to take up not only my instrument students but also my private
students. I figure if they have proven to themselves that they can
indeed hold it right side up in turbulence in IMC, then their stress
will be (a little) lower if it ever happens for real. I like to train
them well beyond the PTS in this area, but not all students are willing
to do that (those that aren't have to go somewhere else--I sleep really
well at night and want to keep it that way.)

I have unique methods for teaching flight by reference to instruments,
and I have at least one known 'save' of a student pilot who got into
IMC on his first solo, and flew there for 20 min successfully before
breaking out and landing--using the methods he heard me present in a
Wings Seminar. (Please don't try this at home...!)

I think most pilots are grossly unaware of how insidious spatial
disorientation can be for the non-IMC-experienced pilot. Most people
seem to think that the accident pilot is unaware of his improper flight
attitude, and would correct to level if he or she knew; I think this is
a dangerous idea. I have personally observed a student pilot in IMC as
she rolled into a left spiral while literally screaming at me for help
because 'it's rolling to the left and I can't stop it.' Her conscious
mind could read the instruments and correctly deduce our real attitude,
but it had lost the battle for control of her hands to her unconscious,
which was following the (erroneous) inner ear indications into the left
spiral. To her it felt as if the controls were jammed, and she could
not get it to roll to the right. (I had her let go and then I rolled
out with two fingers.)

I think a major portion of the spatial disorientation accidents are of
this ilk; i.e. fully aware of the nature of the problem but unable to
move their hands to recover. JFK, for example; needle slap indicated
he had applied maximum takeoff power before he hit the water... I
suspect he knew fully well what was going on but could not stop it.

This is the difference between an instrument rated pilot and an
instrument competent pilot; the competent instrument pilot has enough
IMC experience that he has been able to condition his unconscious to
let go and let the conscious mind take control of the hands.

I think most VFR-rate pilots are nowhere near this level of IMC
experience, and when the stress maxes out, they are toast. All the
more reason to avoid situations that get anywhere close to the edge.
Gene