This statement is a very controversial one. It originates with the US Army Historian SLA Marshall, who purported to base his argument on extensive post-combat interviews he did with American servicemen during the war, and which he published in the book "Men Against Fire". It was quite popular for a time, but recieved extensive pushback in later years, only to be resurrected as one of the centerpieces in Dave Grossman's book "On Killing", for which he has been quite criticized. You can fine a more in-depth look at this with this old post that I wrote a year back. The sum of it is that maybe Marshall had some valid points, but his statisics are incredibly suspect and should not be used uncritically.