This is a nice little piece from Tim Harford about causation and correlation. I particularly liked this:
"It’s not clear why Huff and Fisher were so fixated on the idea that the growing evidence on smoking was a mere correlation. Both of them were paid as consultants by the tobacco industry and some will believe that the consulting fees caused their scepticism. It seems just as likely that their scepticism caused the consulting fees. We may never know."
I hope that Harford is being gently wry here rather than downright sarcastic: it is well worth remembering that there are plenty of people who will say things whether or not they were being paid to say them and regard the money as a pleasant bonus.
In my own line of work I find that there's a pretty good correlation between the expert witnesses who support claimants and the expert witnesses who are paid by claimants (and a pretty good correlation on defendants' side too), but that's not to say that expert witnesses simply say what they are paid to say - indeed, we make them swear that they won't do so. Now, who paid me to say that?