You understand how much you don't know
The smartest folks are able to admit when they aren't familiar with a particular concept. As Jim Winer writes, intelligent people "are not afraid to say: 'I don't know.' If they don't know it, they can learn it."
Winer's observation is backed up by a classic study by Justin Kruger and David Dunning, published in the Journal of Personality and Social Psychology, which found that the less intelligent you are, the more you overestimate your cognitive abilities.
In one experiment, for example, students who'd scored in the lowest quartile on a test adapted from the LSAT overestimated the number of questions they'd gotten right by nearly 50%. Meanwhile, those who'd scored in the top quartile slightly underestimated how many questions they'd gotten right.
You have insatiable curiosity
Albert Einstein reportedly said, "I have no special talents, I am only passionately curious."
Or, as Keyzurbur Alas puts it, "intelligent people let themselves become fascinated by things others take for granted."
A study published in 2016, in the Journal of Individual Differences, suggests that there's a link between childhood intelligence and openness to experience — which encompasses intellectual curiosity — in adulthood.
Scientists followed thousands of people born in the UK for 50 years and learned that 11-year-olds who'd scored higher on an IQ test turned out to be more open to experience at 50.
Smart people don't close themselves off to new ideas or opportunities. Hammett writes that intelligent people are "willing to accept and consider other views with value and broad-mindedness," and that they are "open to alternative solutions."
Psychologists say that open-minded people — those who seek out alternate viewpoints and weigh the evidence fairly — tend to score higher on the SAT and on intelligence tests.
At the same time, smart people are careful about which ideas and perspectives they adopt.
"An intelligent mind has a strong aversion to accepting things on face value and therefore withholds belief until presented with ample evidence," says Alas.
You like your own company
Dipankar Trehan points out that highly intelligent people tend to be "very individualistic."
Interestingly, recent research from the British Journal of Psychology suggests that smarter people tend to derive less satisfaction than most people do from socializing with friends.