The truth about the cuts from the mouth of Frank Capra:

I was a Theatre and Dance major in the early 70's and we were delighted to have Frank Capra as a guest speaker for our Film class one night Lost Horizon and It's a Wonderful Life were the two films targeted for the evening. Mr Capra said that the pre-views for LH were puzzling at best. Audiences began to laugh and wouldn't stop. This was not a comedy! Capra was more than worried when the problem solved itself. Somehow, in the transport of the reels from one location to the next, they lost the first two reels. The next preview was seen without them and the film was received with great acclaim (and no laughter.) I believe if you sear his autobiographies you'll find a chapter titled "Lose the first 2 reels."
Unless he was lying to 500 college students and the head of the Department Of the University of Wisconsin, I have corroboration.