Here's an earlier submission that explains it a bit: Friends don't let friends clap on 1 and 3.
(Come to think of it, this seems very song-dependent. For instance, I definitely find myself clapping on 2&4 to "Thriller" and "I Heard It Through The Grapevine". But I think I'd still clap 1&3 to some of my favorite classic rock and metal songs. Also, I felt 1&4 pretty strongly in that Harry Connick Jr. clip. Maybe they're just clapping too fast: if they were only doing half the claps, I think 2&4 would feel more natural.)
https://www.youtube.com/watch?v=sOnqjkJTMaA&t=5m0s
Curious what you mean by half the claps? Would this involve clapping only on every fourth beat, only on every second beat, or am I completely misunderstanding?
I can feel the backbeat pretty strongly in other music, but this clip sounds very 1&3 to me. It would be musically foolish to automatically prescribe 2&4 to every blues and jazz tune as an axiom. At that point, it just becomes a shibboleth.
Frankly, I think they're just clapping twice as fast in that clip. 2&4 sounds weird and off-rhythm at that speed (to me), but totally normal with half the claps.
(Also: now I'm going to be self-conscious about my clapping until the end of time. Thanks a lot!)