If you search for "thunk" on LEO, you'll find a ton of stuff on this.

It's a humorous, nonstandard past tense of 'think.'

The forum archive (Suche in allen Foren / Search all) is a good place for this kind of question; there are several past threads on 'thunk,' e.g.,

related discussion: Who would have thunk?

Also look for "woulda" while you're at, Petrulik :-)

I usually write it: Who'd a thunk it?

But if The Economist is writing it that way, I may alter my version. ^^
(Was it a quote?)

AE

To me the 'it' sounds a little off; I usually say 'Who'd a thunk' or 'Who woulda thunk?'

#5: Does that mean you'd also say e.g. "Who'd have thought?" and not "Who'd have thought it?"? I think the version with "it" is (more) normal in BE.

Yes, I was thinking it sounded BE -- or rather, 'thunk' doesn't sound BE to me, it sounds (to use a term that came up today) redneck, so actually it sounds like an odd mix.

I say 'Who would've thought?' or 'Who would have thought?'. I think.

This is overthunk on my part, but there's a difference:

A) Who'd a thunk it?
B) Who'd a thunk?

The "it" at the end of A) makes it more provocative (?). The emphasis is on "it", which is the topic. Without the "it" in B), it's a more bland statement ... no big deal. IMHO!

Talking of "it"... I've just realised I left "it" out in my #3 :-)

#3: Also look for "woulda" while you're at it, Petrulik :-)

I use the phrase the way opine does, with "it," but I really have no idea how to write that. "Who'd a" looks completely made up, but "Who'd 'a'" is too fussy. I guess if I had to write it I'd go with "Who woulda."