So the short answer to this is that "Nobody Knows".

The long answer is that S.L.A. Marshall, a historian from that period specializing in group interviews of soldiers, had a thesis in the book "Men Against Fire" that less than 25 percent of soldiers would fire their weapons, and that they did not fire because of their inherit humanity as citizens of a democratic society.

This thesis has been increasingly questioned in the last 30 years. Basically the core of this argument is that [there are no statistics documenting Marshall's research.] (http://strategicstudiesinstitute.army.mil/pubs/parameters/articles/03autumn/chambers.pdf) It's a rather dramatic claim, so it should really require good evidence, and the evidence isn't very good.

Here's a previous thread discussing this http://www.reddit.com/r/AskHistorians/comments/1gvpdc/does_sla_marshalls_observation_that_14th_of/