" 'Actually that should be credited as a Lennon-Ono song because a lot of it — the lyric and the concept — came from Yoko. But those days I was a bit more selfish, a bit more macho, and I sort of omitted to mention her contribution. But it was right out of Grapefruit, her book. " 'There's a whole pile of pieces about "Imagine this" and "Imagine that." ... But if it had been Bowie, I would have put "Lennon-Bowie," you see. If it had been a male, you know. ... Harry Nilsson — "Old Dirt Road," it's "Lennon-Nilsson." But when we did ["Imagine"] I just put "Lennon" because, you know, she's just the wife and you don't put her name on, right?' "

http://www.npr.org/sections/thetwo-way/2017/06/15/533049527/yoko-ono-to-receive-songwriting-credit-on-imagine-48-years-later