Okay, I realize I'm pretty late for a reply to this post, but anyway: Yes, I did try to make a rational decision about my diet. In my case that was a step to being a vegan, but I don't think that's very relevant in this context.

I'm surprised no one else commented on this, but maybe that's due to other reasons than lack of people who decide on their diet using rational thinking.

About 2 years ago, I heard of a book called "Eating animals" by Jonathan S. Foer and felt that this might be an okay source to get some insight in a subject I didn't know much about; the live of farm animals. The book provided a lot of insights and sources and the author seemed to try being as unbiased as possible, while giving a very close insight in how animals live under different farming conditions. I reacted very emotionally to the book. So I guess my decision wasn't completely rational, but I do think the arguments are watertight if you look at factory farming and your premises are the same as mine (as in, definition of suffering, whether suffering (of animals) should be diminished, ...). I did make a conscious effort to decide rationally.

Actually the author was in favor of becoming a vegetarian (which I was already, for more vague reasons, when I read the book) but from the arguments alone I immediately thought going vegan was the best option. It was a significant hurdle to go from that thought to being a vegan in practice, with thoughts like "But I like [unvegan food option]!" "Me a vegan? I never associated with that label" "Won't I be a pain for other people to live around?".

Anyway: Feeling the appropriate emotion about the facts doesn't make a decision less rational. I am a very empathic person and that made it easier for me to stay with the choice. Two other people I know switched to a vegan diet after I spoke to them, both involved with rational thinking. So that would make three.