How do you know if someone's smart?
Without administering an impromptu IQ test, there are certain clues you can use to gauge a person's relative intelligence.
Business Insider combed through decades of scientific research and highlighted 13 surprising signs of braininess.
Keep in mind, however, that "intelligence" is often measured through tests that have been widely criticized for putting certain social groups at a disadvantage and for minimizing the importance of creativity. Psychologists are constantly finding newer, more effective ways to measure cognitive ability - meaning the signs are ever evolving.
1. 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.
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."
For this and other reasons, firstborns tend to be more successful (but not that much more successful) than their siblings.
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.
5. 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.
6. 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."
7. You're left-handed
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.
8. You're tall
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 write: "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."
9. 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.
10. You're politically liberal
Kanazawa led another study that found the farther left you fall on the political spectrum, the smarter you are.
Among thousands of American adults, those who scored higher on IQ tests as kids and adolescents were more likely to report being politically liberal.
11. 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 suggest that reading from an early age increases both verbal and nonverbal (e.g. reasoning) ability, as opposed to the other way around.
12. You worry a lot
A growing body of research suggests that anxious individuals may be smarter than others in certain ways.
In one recent 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.
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.
13. You're funny
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.