10 signs you're exceptionally smarter than average, according to science

If you're the eldest child, you have a head start. Here are 9 more.

It always strikes my curiosity what research is saying are the things that smart people have in common. By the way, curiosity? It's number 9 on the list. Just saying...

Here's how to know if the tattooed Millennial you just hired for your design team is a genius in disguise.

1. You're the oldest sibling.

Youngest or middle siblings may beg to differ, but this study of 250,000 Norwegian 18- and 19-year-olds published in Science magazine revealed they had an average IQ 2.3 points higher than their younger siblings.

Studies suggest they're smarter not because of genetics but because of environment and family dynamics. For example, the firstborn gets the benefits of full parental attention until little brother comes along months or years later.

More From Inc.com: 14 Coolest Products Everyone Will Be Talking About This Summer

When number two finally arrives, the eldest child is put into the position of "teaching" or tutoring their younger siblings.

Teaching has high cognitive demands, the theory goes. The eldest children need to recall their own knowledge, structure it and think of a good way to explain it to younger siblings, which boosts intelligence for some firstborns.

2. You're on the thin side.

For the record, obese people aren't dumb. But studies indicate that over time, their cognitive function declines.

A 2006 study conducted by French scientists and published in the Neurology journal found that people with a Body Mass Index - a measure of body fat - of 20 or less could recall 56 percent of words in a vocabulary test, while obese subjects with a BMI of 30 or higher could remember only 44 percent.

When retested five years later, obese subjects cognitive recall dropped to 37.5 percent, whereas their thinner counterparts retained their level of recall.

3. You're left-handed.

As reported in The New Yorker in 2013, psychiatrists from the University of Athens determined that lefties have faster and more accurate spatial skills, mental flexibility, and enhanced working memory.

They're also "divergent thinkers" -- a specific kind of creativity that gives lefties the ability to generate novel ideas on a whim.

4. You're tall.

A study by Princeton University says that taller people earn more because they are smarter.

This is backed by another study that says a 6 foot tall person earns, on average, nearly $166,000 more during a 30-year career span than someone who is 5 feet 5 inches, regardless of gender, age and weight.

The height bias may be due to tall people having greater self-esteem and social confidence than shorter people. In turn, tall people are perceived as more leader-like and authoritative.

RELATED: Weird jobs that pay surprisingly well

5. You're a night owl.

A study at the University of Madrid says night owls have higher IQ than the early risers springing out of bed. They also earn more and lead more comfortable lives.

More From Inc.com: The Best Music for Staying Productive at Work, Backed by Science

6. You're an introvert.

Chances are, if you grew up as that awkward, quiet, geeky kid with a social anxiety, you were probably special and didn't know it. The Gifted Development Center says that 60 percent of gifted children are introverted.

It gets better. As an adult, you now leverage your strength for processing, contemplating and thinking things over, which is a trait of highly intelligence people. In fact, more than 75 percent of people with an IQ above 160 are introverted.

7. You were breast-fed.

I don't make this stuff up. A rather unorthodox study in Brazil followed 6,000 people from birth to the age of 30. Breast-fed babies did better than babies who were nursed for a month or less, scoring better on intelligence tests as adults, having more success and earning more.

8. You're a liberal or atheist.

Both types have been shown to be more intelligent than conservatives or religious types, says evolutionary psychologist Satoshi Kanazawa.

The reasoning goes that liberals and atheists tend to show superiority or elitism, which is liked with higher IQ. Aligning your values and beliefs to progressive and nonconformist ideals may be ways to express to others that you're smarter than the average bear.

9. You're curious.

Albert Einstein famously said, "I have no special talents, I am only passionately curious."

Harvard Business Reviewreports that people with higher "curiosity quotient" (CQ) are more inquisitive, generate more original ideas, and this "thinking style" leads to higher levels of knowledge acquisition over time.

CQ, the author states, "is the ultimate tool to produce simple solutions for complex problems."

More From Inc.com: 10 Tips for Working With Social Media Influencers

10. You're funny.

So many studies have been conducted to link intelligence to humor, it deserves its own article. But this 1970s research involving 55 male and 14 female comedians speaks for itself.

Comedians consistently scored much higher on IQ tests than the average population. Male comedians scored on average 138, and female comedians 126. The average IQ score of the generation population? Between 90 and 110. Big difference.

Funny people have also been linked to generating greater creativity, having superior verbal skills, and being more sexually attractive. That's my cue to crack a good joke for my wife.

More from Inc.com:
7 Things Really Successful People Never Do
Why Constant Learners All Embrace the 5-Hour Rule
The 21 Toughest Tech Companies for Getting a Job

RELATED: The best jobs for you according to your zodiac sign