In certain segments of geeky culture, it’s considered appropriate to generally disbelieve uncited claims. This view has some merit, but it shouldn’t be applied too blindly.
For example, I remember claiming that Bush II was easily more intelligent than the average person. One person refused to believe me unless I could offer some source for the claim.
“He was the president,” I said, “honestly it should be the default assumption.”
“I’m sorry, I can’t believe you without some proof,” he replied.
The United States has had 43 presidents. Of those, 32 had college degrees. Of the 11 who didn’t, 8 served before 1900, 2 attended some college, and the last one (Cleveland) practiced law even though he lacked a degree. For comparison, in 2012 31% of the US population had a bachelor’s degree.
This list (not the hoax) estimates that the lowest presidential IQ was Grant’s at around 120, which is more than one standard deviation above average, and that all but 2 presidents had IQ above 130, or two standard deviations above average.
Based on these data alone, without knowing any specific facts about Bush, how likely is it that he would be more intelligent than the average person? And sure I didn’t have those facts to hand at the time, but is anyone really surprised by them?
My suspicion is that in response, the other arguer would have pointed out contrary evidence: for instance, Bush’s tendency to malapropisms. But Bush was president for 8 years, or almost 3,000 days. Is it really surprising that in that time, he would make several (or even many) verbal missteps?
Here’s my point: different claims require different degrees of evidence before they can be firmly believed. If I make a surprising claim (there’s a dragon in my basement), you might intuitively understand that more evidence than my word is necessary – Carl Sagan popularized this heuristic by saying “Extraordinary claims require extraordinary evidence.” And the corollary is that ordinary claims, like a president having a good intellect, don’t require much evidence at all.