Stupid people tend to overestimate their competence, while smart people tend to sell themselves short. As Shakespeare put it in As You Like It: "The fool doth think he is wise, but the wise man knows himself to be a fool."
That conventional wisdom is backed up by a Cornell University study conducted by social psychologists David Dunning and Justin Kruger. The phenomenon is now known as the Dunning-Kruger effect.
So, if you're not too sure about your own intellect, it actually might be an indication that you're pretty intelligent — thoughtful enough to realize your limitations, at least.
Here are some subtle signs that you are considerably smarter than you think.
You took music lessons
Research suggests that music helps kids' minds develop in a few ways. A 2011 study found that scores on a test of verbal intelligence among 4- to 6-year-olds rose after only a month of music lessons.
A 2004 study led by Glenn Schellenberg found that 6-year-olds who took nine months of keyboard or voice lessons had an IQ boost compared with kids who took drama lessons or no classes at all.
Meanwhile, a 2013 study, also led by Schellenberg, suggested that high-achieving kids were the ones most likely to take music lessons. In other words, in the real world, musical training may only enhance cognitive differences that already exist.
You're the oldest
Oldest siblings are usually smarter, but it's not because of genetics, one study found.
Norwegian epidemiologists used military records to examine the birth order, health status, and IQ scores of nearly 250,000 18- and 19-year-old men born between 1967 and 1976. Results showed that the average firstborn had an IQ of 103, compared to 100 for second children and 99 for third children.
The New York Times reported: "The new findings, from a landmark study published [in June 2007], showed that eldest children had a slight but significant edge in IQ — an average of three points over the closest sibling. And it found that the difference was not because of biological factors but the psychological interplay of parents and children."
For this and other reasons, firstborns tend to be more successful (but not that much more successful) than their siblings.
For a 2006 study, scientists gave roughly 2,200 adults intelligence tests over a five-year period and results suggested that the bigger the waistline, the lower the cognitive ability.
Another study published that same year found that 11-year-olds who scored lower on verbal and nonverbal tests were more likely to be obese in their 40s.
The study authors said that smarter kids might have pursued better educational opportunities, landed higher-status and higher-paying jobs, and therefore ended up in a better position to take care of their health than their less intelligent peers.
Meanwhile, a more recent study found that, among preschoolers, a lower IQ was linked to a higher BMI. Those researchers also said environmental factors were at play, since the relationship between BMI and smarts was mediated by socioeconomic status.
You have a cat
A 2014 study of 600 college students found that individuals who identified as "dog people" were more outgoing than those who identified as "cat people," according to a test that measures personality and intelligence.
But guess what? Those same cat people scored higher on the part of the test that measures cognitive ability.
You were breastfed
2007 research suggests that babies who are breastfed might grow up to be smarter kids.
In two studies, the researchers looked at more than 3,000 children in Britain and New Zealand. Those children who had been breastfed scored nearly seven points higher on an IQ test — but only if they had a particular version of the FADS2 gene. (That version of the gene was present in roughly equal numbers among kids who were and weren't breastfed.)
Figuring out the exact mechanism of this relationship between FADS2, breastfeeding, and IQ will require further study, the scientists noted in their paper on the finding.
You've used recreational drugs
A 2012 study of more than 6,000 Brits born in 1958 found a link between high IQ in childhood and the use of illegal drugs in adulthood.
"In our large population-based cohort study, IQ at 11 years was associated with a greater likelihood of using selected illegal drugs 31 years later," wrote researchers James W. White, Catharine R. Gale, and David Batty.
They conclude that "in contrast to most studies on the association between childhood IQ and later health," their findings suggest "a high childhood IQ may prompt the adoption of behaviors that are potentially harmful to health (i.e., excess alcohol consumption and drug use) in adulthood."
Left-handedness used to be associated with criminality, and researchers are still unclear as to whether and why there are slightly more lefties among criminal populations.
More recent research associates left-handedness with "divergent thinking," a form of creativity that allows you to come up with novel ideas from a prompt — at least among men.
In her review of a 1995 paper, New Yorker reporter Maria Konnikova writes:
"The more marked the left-handed preference in a group of males, the better they were at tests of divergent thought.
Left-handers were more adept, for instance, at combining two common objects in novel ways to form a third — for example, using a pole and a tin can to make a birdhouse. They also excelled at grouping lists of words into as many alternate categories as possible."
A 2008 Princeton study of thousands of people found that taller individuals scored higher on IQ tests as kids and earned more money as adults.
The researchers wrote: "As early as age 3 — before schooling has had a chance to play a role — and throughout childhood, taller children perform significantly better on cognitive tests."
You drink alcohol regularly
Evolutionary psychologist Satoshi Kanazawa and colleagues found that, among Brits as well as Americans, adults who had scored higher on IQ tests when they were kids or teens drank more alcohol more often in adulthood than those who had scored lower.
You learned to read early
In 2012, researchers looked at nearly 2,000 pairs of identical twins in the UK and found that the sibling who had learned to read earlier tended to score higher on tests of cognitive ability.
The study authors suggested that reading from an early age would increase both verbal and nonverbal (i.e. reasoning) ability, as opposed to the other way around.
You worry a lot
A growing body of research suggests that anxious individuals may be smarter than others in certain ways, according to Slate's coverage of several different studies on anxiety.
In one study, for example, researchers asked 126 undergrads to fill out questionnaires in which they indicated how often they experienced worry. They also indicated how often they engaged in rumination, or thinking continuously about the aspects of situations that upset them, as psychologist Edward Selby reported in Psychology Today.
Results showed that people who tended to worry and ruminate a lot scored higher on measures of verbal intelligence, while people who didn't do much worrying or ruminating scored higher on tests of nonverbal intelligence.
In one study, 400 psychology students took intelligence tests that measured abstract reasoning abilities and verbal intelligence.
Then they were asked to come up with captions for several New Yorker cartoons, and those captions were reviewed by independent raters.
As predicted, smarter students were rated as funnier.
Tomas Chamorro-Premuzi, business psychology professor at University of London, wrote a post for Harvard Business Review in which he discussed how the curiosity quotient and having a hungry mind makes one more inquisitive.
Regarding the importance of CQ, he wrote that, "It has not been as deeply studied as EQ and IQ, but there's some evidence to suggest it is just as important when it comes to managing complexity in two major ways.
First, individuals with higher CQ are generally more tolerant of ambiguity. This nuanced, sophisticated, subtle thinking style defines the very essence of complexity.
Second, CQ leads to higher levels of intellectual investment and knowledge acquisition over time, especially in formal domains of education, such as science and art (note: this is of course different from IQ's measurement of raw intellectual horsepower)."
A Goldsmiths University of London study found that intellectual investment, or "how people invest their time and effort in their intellect," plays a major part in cognitive growth.
A study published in Psychological Science by the University of Minnesota Carlson School of Management's Kathleen Vohs revealed that working in an untidy room actually fuels creativity.
In the study, 48 participants were asked to come up with unusual uses for a pingpong ball. The 24 individuals working in neat rooms came up with substantially less creative responses than the individuals working in cluttered rooms.
So if you are a pack rat, tell everyone you're just fueling your sense of creativity and innovation the next time someone tells you to clean up your act.
You didn't have sex until after high school
High schoolers with higher IQs are more likely to be virgins than those with average or lower IQs, according to a study from the University of North Carolina Chapel Hill. The core sample looked at 12,000 teens from the 7th to the 12th grade.
Not only were the teens with the higher IQs more likely to be virgins, they were also less likely to kiss or hold hands with a romantic partner.
A number of explanations have been put forward by the science blog Gene Expression to explain this gap, including suggestions that smart people possess lower sex drives, are more risk-averse, or simply less able to find sexual partners.
You're a night owl
One study published in the The Official Journal of the International Society for the Study of Individual Differences found that, when all other variables are factored out, night owls tend to beat out early birds in terms of intellect.
It concluded that ethnographic evidence indicates that "nocturnal activities" were rarer in the ancestral environment. That means that more intelligent individuals are more likely to stay up late because smarter people are more likely to "espouse evolutionarily novel values."
You don't always have to try hard
This isn't to say that laziness is a sign of being smart. But it is fair to say that smart people simply don't always have to try as hard as "strivers" who fight to build up their skills — at least in certain fields.
In an opinion piece for The New York Times, psychologists David Hambrick and Elizabeth Meinz cited a Vanderbilt University study of highly intelligent young people.
The study tracked 2,000 people who scored in the top 1 percent of the SAT by the age of 13.
Hambrick and Meinz wrote that, "The remarkable finding of their study is that, compared with the participants who were 'only' in the 99.1 percentile for intellectual ability at age 12, those who were in the 99.9 percentile — the profoundly gifted — were between three and five times more likely to go on to earn a doctorate, secure a patent, publish an article in a scientific journal or publish a literary work. A high level of intellectual ability gives you an enormous real-world advantage."
They concluded that while striving to be smarter is commendable, there are certain innate abilities that can't always be learned.
Drake Baer and Chelsea Harvey contributed to a previous version of this article.
This article was originally published by Business Insider.
More from Business Insider: