I was in a psychology class today and my professor made this claim and I was curious as to how factual it was from a historical standpoint.
That claim is based on the books "On Killing: The Psychological Cost of Learning to Kill in War and Society", by David Grossman; and "Men against Fire", by SLA Marshall.
I'll get my bias out here - I think this idea is crap, and the basic reason is that extraordinary claims require extraordinary evidence, and there is no evidence to support that claim.
Marshall's work, wherein he makes the claim that 75% of soldiers do not fire on the enemy, was based on post-combat interviews with soldiers, but no record of any questions about the ratio of fire exists.
In fact, the only record of his interviews at all (besides his books), makes mention of soldiers firing weapons, but nothing whatsoever that could support a hard number of how many men fired or did not fire.
There is no evidence of statistical analysis based on his interviews, no records of questions about whether soldiers fired or not, no questions about ammunition consumption. There is no evidence from quartermasters about ammunition consumption, barrel wear, or any other secondary evidence.
So this number is one that Marshall may have arrived at honestly, but there is simply no evidence to support it.
If you're interested, Robert Engen wrote a very incisive article on the subject in the Canadian Military Journal, and wrote his Masters thesis on the subject.
Engen found (and has the evidence to prove) that - for Canadians, at the very least - did not have this problem. Based on primary sources (written post-combat interviews with Canadian officers), he found exactly the opposite of what Marshall and Grossman claim.
Canadian officers found that their forces fire was very effective, and, if anything, their men fired too much!
So - there's no evidence to support this claim, and there is primary source evidence that it is BS.
If you'd like to read Engen's article in the CMJ or his thesis, here they are:
http://www.journal.forces.gc.ca/vo9/no2/16-engen-eng.asp
http://qspace.library.queensu.ca/bitstream/1974/1081/1/Engen_Robert_C_200803_MA.pdf
In the interest of (a little) balance, Grossman makes a (in my opinion very feeble) defense of his work and Marshall's in the CMJ as well:
This claim comes from 'Man Against Fire' by S.L.A. Marshall, who was a US Army historian for WW2. He's fairly controversial, as is the book, and the 15%-20% figure comes from his book, although it's misquoted.
For one, he's basing it off interviews he did, which isn't exactly a good piece of evidence. Also, it's more specific then is stated. Only 15-20% of American riflemen fired their personal weapons at an exposed enemy soldier. This makes the statistic a lot more understandable, because it excludes crew-served weapons (machine guns), and key weapons (flamethrowers).
While his numbers were initially accepted and frequently quoted, they've been called into question several times. One of the larger things I've seen pointed out in rebuttals of his statistics is that it fails to distinguish between soldiers who can fire, and shoulders who should fire. A medic has a sidearm. Should he be firing at an exposed soldier, or should he be doing his normal duties? What about squad leaders, more focused on directing the battle then taking shots themselves?
This is an excellent breakdown of why his methods were called into question, and includes an interview by the man who accompanied him through his interviews in the Korean war (which came up with a 50% fire rate). Some sample issues with his work include that he didn't interview casualties, only unharmed men who were still ready for action, that he didn't take into account things like weapons jamming, and that his numbers were based more on guesses then anything else. It wasn't a proper survey even, but instead a group discussion he'd pull information out of.
Simply put... it's just not very good history. It's hard to get a proper number for how many soldiers fired their guns, but the 15-20% is more or less impossible to back up, and completely ignores several significant factors.
Probably referring to S. L. A. Marshall's findings, reported in Men Against Fire and other places:
The thing is simply this, that out of an average 100 men along the line of fire during the period of an encounter, only 15 men on average would take any part with the weapons. This was true whether the action was spread over a day, or two days or three...In the most aggressive infantry companies, under the most intense local pressure, the figure rarely rose above 25% of total strength from the opening to the close of an action.
And
It is therefore reasonable to believe that the average and healthy individual--the man who can endure the mental and physical stresses of combat--still has such an inner and usually unrealized resistance towards killing a fellow man that he will not of his own volition take life if it is possible to turn away from that responsibility...At the vital point he becomes a conscientious objector...
Marshall's conclusions have not gone unchallenged, and are still discussed today. Apparently his methods were not very rigorous or scientific, and there's lots of reasons for a soldier to not fire his weapon, even in a close engagement with the enemy. (Do keep in mind that the weapons of the time were powerful enough to make even a fleeting and momentary glimpse of the enemy a good opportunity for a kill. These weren't firing lines 100 yards apart. Even an earnest killer might not have abundant opportunities of actually shooting someone.)
Still, after WWII they led to innovations in training to increase rate of fire.
Ah yes, Marshall's study on combat fire ratios. The answer is: no one knows. There's a lot of explosions and confusions during war, keep in mind 20% of all casualties in WWII was from friendly fire ^ 0 (over the years this decreased as our forces became more organized or alternatively the enemies after WWII we fought were less and less organized then the Axis). Many people have cried bullshit at his studies, his studies were poorly documented, were based on after action interviews, and said things like "Brennan’s account also reinforces the contention of critics of Marshall’s use of statistics, who conclude that Marshall was unscientific in his methodology and that his figures about the percentage of troops firing their weapons were either sloppy, fabricated, or simply guesswork. ... Nevertheless, unlike the recollection of the Army captain who accompanied Marshall in Europe in World War II that he could not recall Marshall ever asking who had fired his weapon, Brennan does recall the journalist occasionally asking that question directly in his interviews in Korea." ^1
I would like to mention a scene in Maus, the comic book / biography in which the author's father is ordered to fire his weapon, so he blindly fires it, partly because he's unwilling to take another life (although later he does kill a German soldier). ^3 In a google search I just made, somewhere between 25,000 to 100,000 rounds were expended per kill in Vietnam. That is because of suppressive fire. But I must say that it's hard to figure out the motivations for suppressive fire, it could be caused by combat stress or opposing enemy suppressive fire causing soldiers to , or it could be a result of a psychological factor that results in people to avoid aiming directly at an enemy (few people want to kill another person).
I also would like to say it's pretty unlikely for soldiers to admit in front of their comrades that they didn't even attempt to kill an enemy. Unfortunately nothing is certain in war, maybe when we attach cameras to the guns of soldiers we could get a better glimpse, but even then... nothing is certain.
I apologize that this post falls a little short of the standards of this subreddit, but this question is probably better asked in r/military/ or r/asksocialscience/ .
On a final note, Marshall's statistics was part of the reason for the adoption of the M16, it was felt that automatic weapons would allow soldiers who typically don't fire their weapons to blind fire their weapons through suppressive fire.
References: 0. [Added this one in at last minute] (https://web.archive.org/web/20070329001132/http://members.aol.com/amerwar/ff/ff.htm)