The trivial answer to the subject question is 'Doctor of Philosophy', which doesn't help us much. I was prompted to write about it by responses to Chris's comments on the pathetic petition over on Chris Colose's blog, wherein a reader seemed to think that once one had a PhD, one had received a grant of omniscience.
Ok, not quite. Rather, to quote Stephen (11 June 2008) directly: "...a person with a PhD is more apt to think critically before making a decision. Granted, it’s not true in every case, obviously, but someone with a PhD in any scientific field is statistically more likely to look at all of the information available to them." Unfortunately he never gave us a pointer to where those statistics were gathered -- the ones that supported his claim that PhDs in a scientific field were 'statistically more likely' .... I'm minded of the observation that 84.73% of all statistics on the net are made up.
The details of what a PhD means vary by advisor, school, and era. But for what's at hand, the finer details don't matter. One description of doctoral requirements is "an original contribution to human knowledge". This much being true whether we're talking about science or literature. The resulting contribution should (more so these days than a century ago) be publishable, and published, in the professional literature. Notions vary, but there's also a principle that someone who earns (or is a candidate to receive) a PhD should conduct the work with significantly less guidance than an MS candidate. And far less than an undergraduate. Again, true whether science or literature.
One thing you don't see there is 'more apt to think critically' about everything they comment on. You also won't find 'look at all information' about everything. The about everything is my addition, not the exact quote. But for the comments to be meaningful, they have to apply to the specific thing at hand, whatever it is that's at hand, whether it's the pathetic petition or other things allegedly about science.
The posession of a doctorate says, instead, that the owner is likely to be capable of making an original contribution to knowledge, without too much guidance from someone else. This is a pretty good sign. But it hardly means that the owner has been turned in to Mr. Spock. PhD holders are human still. We all have capabilities, PhD or no. And we humans don't always exercise the highest of our abilities. The area where you can bet (if not guarantee) that a PhD holder is more apt to think critically and consider all information is the area of their professional work. Outside that ... you're much better off to either ask if they brought full ability to bear, or to assume that they didn't.
In saying that, remember, I do have a PhD myself. It is possible that a PhD-holder is bringing full abilities to bear. If so, then they can fare better than most non-PhDs in evaluating things which claim to be science. I did, for example, take such a look at a couple of different scientific papers regarding left-handedness (I'm left-handed and interested in the topic). I thought they were very bad, for a number of reasons of 'how you do science'. The serious work, however, was done by the people who were in the field (PhD or no) and wrote the rebuttal papers for the peer-reviewed literature. They named many things that I got, and many more besides. Being a scientist got me about 1/3rd of the way through the list of errors that the original authors had committed.
On the other hand, many of the things I looked at and for in evaluating those papers were things I'm discussing here and seriously believe a jr. high student can learn to apply regularly.
So where are we? As Chris suggested in his response to Stephen: "From experience with my professors though, I wouldn’t ask many of them a question outside of their field, at least beyond 101 level stuff, so maybe not. But at the same time, none of them would go off signing petitions about things they know little about." Those are both good rules of thumb. Outside the professional field, a PhD-holder can't be presumed to know more than (or, depending on field, even) 101 level stuff. But most of us know this about ourselves, so toss the junk mail petitions where they belong when they arrive.