I had a boyfriend who worked in a coffee shop, swore constantly, and had a genius level IQ. There was nothing obvious about him that screamed high intelligence. In fact, it's really very difficult to measure intelligence, which might be why there's always some study or test to try to measure it.

Intelligence can mean different things to different people, but generally, it's thought of as having the ability to perceive information, having the capacity for logic, understanding, self-awareness, creativity, and possessing the ability to problem solve.

When you're very intelligent, you're thought of as having a big brain or being very smart, and people assume you must have been highly educated.

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We think people are highly intelligent if they can do difficult math problems in their head or create machines to save the world. But actually, some of the characteristics of people with extremely high intelligence aren't generally associated with having a high intellect.

Here are 8 unusual signs and personality traits of people with extremely high intelligence.

1. Mental illness

In the movie (which is based on a true story) "A Beautiful Mind," the lead character, John Nash, is a brilliant mathematician dealing with schizophrenia. The link between mental illness and intelligence is highly controversial, but some of the most gifted minds in history — like Edgar Allan Poe, Jackson Pollock, and Amy Winehouse — were all bipolar.

The specific reason for this link isn't clear, but a study found that a certain protein associated with memory and curiosity in mice was also associated with bipolar disorder and schizophrenia in humans.

Another study suggested that the ability needed to figure out complex math equations and process information quickly may also put people at risk for mania.

2. Swearing

If someone is cursing you out, it may not feel as if they're super-intelligent, but science says otherwise.In a study published in Language Studies found that people who swear have larger vocabularies (a trait associated with high intelligence) than those who don't.

3. Risk-taking

A recent study completed in Finland found that individuals who are open to new challenges and who aren't afraid to take risks tend to be more intelligent. The study used a driving simulation, and found that participants who made riskier decisions during the test had more white brain matter — an area of the brain associated with cognitive function.

4. Laziness

A study from Florida Gulf Coast University found that smarter people are lazier because they have long attention spans. Big-brained people don't have to constantly be doing something; they can sit and think as their brain gets even larger.

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5. Over-acheiving

Everyone knows that the eldest child in a family tends to be an overachiever because they got the most attention from their parents. One study found that eldest children tend to have more rules to follow and more intense parental supervision, especially where homework and grades are concerned.

6. Anxiety

A study at SUNY Downstate Medical Center in New York found that people with anxiety had higher IQs than those who didn't suffer from anxiety. It makes sense if you suffer from anxiety. You're constantly worrying and thinking up various outcomes for situations, and look at things from the past that could influence various situations.

These people are very good at problem-solving, critical thinking, and abstract reasoning.

7. Being a cat lover

Researchers from Carroll University analyzed 600 student participants and found that cat lovers scored higher on intelligence tests than dog lovers.

Take that, haters.

8. Being an atheist

A review of 35 scientific studies found that people who hold a more naturalistic view of the world generally tend to be more intelligent than people who are religious. As with any study, you can find what you want and discard the rest. Find the evidence that makes you feel good and worthy — that's what someone with high intelligence would do.

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Christine Schoenwald is a writer and performer. She's had articles in The Los Angeles Times, Salon, Bustle, Medium, and Woman's Day.