Thanks for the ping /u/the_howling_cow! I've actually gone and done a little expansion on the older answer since it didn't quite touch on the latter half of the question, although of course I don't have Glenn handy this moment, so do check back tonight as I might try to throw some more in beyond!
The most important thing to understand with this is the origins and context of that 25 percent number. During World War II, S.L.A. Marshall studied the combat effectiveness of the American GI. Based on hundreds of after action interviews conducted with rifle companies in Europe, he came to his famous conclusion that:
In an average experienced infantry company in an average stern day's action, the number engaging with any and all weapons was approximately 15 per cent of the total strength. In the most aggressive infantry companies, under the most intense local pressure, the figure rarely rose above 25 per cent of the total strength from the opening to the close of the action.
This was a huge deal, and as you allude to set the US military on a path of overhauling their training in an effort to raise firing rates and the general willingness to shoot the enemy.
Many people are probably best aware of the whole matter through the Black Mirror episode which is a direct reference to these findings, the title borrowing from Marshall's book, published in 1947 as Men Against Fire: The Problem of Battle Command in Future War.
The problem is that his book is based on bullshit. It is generally concluded at this point that he conducted far fewer interviews than he claimed, and that he never actually recorded the information on which he claimed to have based these statistics. No notes, correspondence, or other papers which can be used to reconstruct his data survive, and what information he did offer in the years after is misleading and contradictory. Roger Spiller's 1988 article is one of the most important of several take-downs of Marshall's work, and from which I will quote:
In Men Against Fire Marshall claims to have interviewed "approximately" 400 infantry rifle companies in the Pacific and in Europe, but. that number tended to change over the years. In 1952, the number had somehow grown to 603 companies; five years later, his sample had declined to "something over 500" companies. Those infantry companies-whatever their actual number-were his laboratories, the infantrymen his test subjects, and at the focal point of his research was the ratio of fire. "Why the subject of fire ratios under combat conditions has not been long and searchingly explored, I don't know," Marshall wrote. "I suspect that it is because in earlier wars there had never existed the opportunity for systematic collection of data". (Italics added by Spiller).
Opportunity aplenty existed in Europe: more than 1200 rifle companies did their work between June 1944, and V-E day, 10 months later. But Marshall required by his own standard two and sometimes three days with a company to examine one day's combat. By the most generous calculation, Marshall would have finished "approximately" 400 interviews sometime in October or November 1946, or at about the time he was writing Men Against Fire.
This calculation assumes, however, that of all the questions Marshall might ask the soldiers of a rifle company during his interviews, he would unfailingly want to know who had fired his weapon and who had not. Such a question, posed interview after interview, would have signalled that Marshall was on a particular line of inquiry, and that regardless of the other information Marshall might discover, he was devoted to investigating this facet of combat performance. John Westover, usually in attendance during Marshall's sessions with the troops, does not recall Marshall's ever asking this question. Nor does Westover recall Marshall ever talking about ratios of weapons usage in their many private conversations. Marshall's own personal correspondence leaves no hint that he was ever collecting statistics. His surviving field notebooks show no signs of statistical compilations that would have been necessary to deduce a ratio as precise as Marshall reported later in Men Against Fire. The "systematic collection of data" that made Marshall's ratio of fire so authoritative appears to have been an invention.
Now, the irony is that Marshall probably wasn't wrong. Even Spiller, in his piece, concludes that Marshall likely was trying to create a scientific backing from what he felt more intuitively from "his own experiences and observations of war". He certainly conducted interviews, and certainly talked with many soldiers, and even if he made the numbers up, he did have the sense that many soldiers were refusing to fire, and this needed to change. So he was full of shit and in complete violation of academic ethics... but he was motivated by what he saw as good reasons, namely a boot to the butt of the US Army to improve things. In reality, we really have no idea what the number was, as at absolute best it was a rough guess, and he could have been quite far off both in numbers and reasons. In any case, it is a fairly serious problem with Marshall's work, one which casts a serious shadow on it and one which any subsequent researcher using it must grapple with.
Which brings us to the other issue I want to address... Marshall was in large part discredited after Spiller's article, and at best used with caution. And regardless, since Men Against Fire was published, better, more honest works which explore the same issues of combat motivation have come out such as Glenn's Reading Athena's Dance Card: Men Against Fire in Vietnam, Kellet's Combat Motication: The Behavior of Soldiers in Battle, or Engen's Strangers in Arms: Combat Motivation in the Canadian Army, but these don't pique the conventional psyche. Two factors are of note. One is the appeal it has for the image of the citizen-soldier, a reluctant warrior. In his article on combat motivation, Daddis lays a fair bit of blame at the feet of Stephen Ambrose and his Band of Brothers for its fixing this in modern American minds. I won't disagree.
Bigger though is Lt. Col. Dave Grossman, whose book On Killing was, in comparison to more academic works on the topic, a much more accessible work for the general reader. It also commits the same sins as Marshall, although perhaps more inexcusable in that it is so obvious, in that he has his conclusion and tries to fit evidence to it. He cherry picks information, ignores what he doesn't agree with, possibly makes up citations (I have spent years trying to find evidence of his claim that the British army did laser reenactments of historical battles in the 1980s, without luck. He has never returned my emails) and for our purposes here, uncritically used Marshall's numbers as accurate and uncontroversial. I would tentatively argue that Grossman is single-handily responsible for resurrecting Marshall and giving his work a new, and undeserved life. Although he did eventually publish a response to critics, which you can read here, but is fairly evasive and doesn't really say anything in his defense. Grossman had parlayed his success into more books, and a post-military career as a lecturer to police and military groups which is not really for discussion here due to the 20 year rule, but suffice to say, he has no real motivation to be academically honest at this point, and in my estimation, it shows.
Anyways though, to ham on the initial point, SLA Marshall made things up, but that doesn't mean his impact was any less real. The important of training, of better training, was the underlying purpose of writing his book, and if we want to be the most charitable we can, the reason he likely felt justified in all of that fudging that he did to get the conclusions he wanted, namely, a neon-sign about the need to improve. The heart of his study to him wasn't the "25 percent" he is known for, but probably this:
Since more than a century ago, when the rifle bullet began its reign over the battlefield and soldiers became aware that the day of close-order formations in combat was forever gone, all military thinkers have pondered the need of a new discipline. It has been generally realized that fashioning the machine to man's use in battle was but half of the problem. The other half was conditioning man to the machine. The mechanisms of the new warfare do not set their own efficiency rate in battle. They are ever at the mercy of training methods which will stimulate the soldier to express his intelligence and spirit.
Additional question expanding on this - I have often heard the Prussian style of schooling was directly related to soldiers running from battle. Was the US education system, which is modeled after the Prussian system, changed at all the try to have a long term higher willingness to fire?