When I was an undergrad I wrote my term paper for US Military History on S.L.A. Marshall, so I'm displeased to see the news doing it's normal shitty job of covering him. To set the record straight, there are two big things you need to know about S.L.A. Marshall:
1. He was a brigadier general and official army historian who did a lot of post-battle interviews with American soldiers in WWII Europe, concluding that about 80 percent of soldiers weren't firing in combat. Partially thanks his recommendations* for how to fix the problem of low fire ratios, we saw a titanic shift in how infantry platoons fought between WWII and Korea: 1. Training changed to condition people to fire on humans (training marksmanship on pop-up, man shaped targets vs. stationary, bullseye type targets, for example), 2. Platoon organization changed to allow closer management of individual soldiers (the smallest unit in a platoon shifted from a squad of 12 or so people to a fire team of only four people), 3. How platoons were equipped changed (Marshall noticed that soldiers were much more willing to use crew operated weapons than personal weapons on the enemy. Because of this, as well as an increased doctrinal emphasis on suppressing fire, the number of squad automatic weapons per soldier was vastly increased, from one per squad to one per fire team). Marshall recorded much higher fire rates in Korea and Vietnam (well into the 90th percentiles), and by the gulf war fire rates when confronted with the enemy were above 99 percent. So all these reforms worked (at a possible psychological cost for vets, according to the work of retired LTC Dave Grossman, a shrink).
2. Except maybe they didn't, because Marshall's "statistics" are probably bullshit. He never published his data, and as his people began going through his archives after he died most concluded that there was no data. He interviewed lots of soldiers, sure, and his qualitative summaries of their reports have a lot of power, but his quantitative figures about what percent fired in combat might have been pulled directly out of his ass.
So there is sort of a mixed history there. On the one hand, his reforms were adopted so rapidly and so widely by militaries after WWII that I have to think they rang true to the battle hardened veteran officers and non-coms of the day. And the fact that infantry platoons still fight along the lines he outlined proves that his recommendations stood the test of time. So there is probably a lot of truth to his observations.
On the other hand, his stats were possibly full of shit and should only be quoted with heavy caveats.
The media tends to only report one story or the other, so we get this article saying that we know for a fact that only 20 percent of soldiers fired on the enemy in WWII. We don't! Alternately you'll get articles saying that Marshall is a proven fraud and that we should conclude fire rates were as high in WWII as they were in Korea. We shouldn't! It's a more mixed story than that: there is a lot of evidence that S.L.A. Marshall was dishonest, but also a lot of evidence that he was right.
Anyways, his wiki page summarizes some of this if you'd like to know more.
*I want to emphasize that he was only one of many influences that brought about the shift I'm talking about. The marines first tried out the fire team while occupying Nicaragua between WWI and WWII, for example.