Turns out that sitting up straight is actually bad for your back.
This article confirms what I have been doing empirically for years. I pinched a nerve in my back when I was 19, and ever since then I’ve had recurring lower back pain. As I’m a programmer, this could be a problem. I found long ago that sitting in a “slouch” — with lower back support — is the best position for me to work in pain-free, and have continued to sit that way in defiance of therapists and busybodies ever since. Now I can have the last laugh!