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 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 a 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.
1. You took music lessons
Research suggests that music helps kids' minds develop in a few ways:
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.
2. 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 reports: "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."
3. You're thin
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 say 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 say environmental factors are at play, since the relationship between BMI and smarts was mediated by socioeconomic status.
4. 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.
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.