Nobody wants to be a number. But there is one number that probably says a lot about you, whether you know it or not: your IQ, or intelligence quotient. (President Donald Trump recently bragged to Forbes that he would "win" if his IQ test score was compared to that of his Secretary of State, Rex Tillerson.)

IQ is often dismissed as antiquated, misguided, or less important than personality traits. But according to Stuart Ritchie, an intelligence researcher at the University of Edinburgh, there's a massive amount of data showing that it's one of the best predictors of someone's longevity, health, and prosperity. And psychologists have been able to replicate these findings over and over.

(Richie and I go further into why IQ research makes some people uncomfortable in a Q&A here.)

In a new book, Intelligence: All that Matters, Ritchie persuasively argues that IQ doesn't necessarily set the limit for what we can do, but it does give us a starting point. And the truth is some people start ahead.

Here are nine facts that help explain IQ and why it matters.

(Most of these charts have been pulled from Intelligence: All that Matters. It's a spry, uncluttered read if you're interested in learning more.)

1) Most people have average intelligence

The first thing to know about IQ is that it is a composite score made up of the results of many different tests of reasoning, memory, acquired knowledge, and mental processing speed. These sub-scores are totaled, and are then compared with those of the rest of the population. A perfectly average score is set at 100.

Note: A full IQ test is an hour-plus, intense process. It needs to be administered by a trained tester, and certain portions are timed. Those free quizzes you see online are not legitimate IQ tests.

Like other variable human traits (height, for example), the range of IQ is on a standard curve bell curve.

Most people you meet are probably average, and a few are extraordinarily smart. Just 2.2 percent have an IQ of 130 or greater.

What's fascinating is that people who score well on one of the tests tend to score well on them all. So your score on a task on how fast you can turn off a blinking light (one component of some intelligence tests) is correlated with your scores on verbal and spatial reasoning.

Psychologists call this overlap of scores the "G," or general intelligence, factor.

"The classic finding — I would say it is the most replicated finding in psychology — is that people who are good at one type of mental task tend to be good at them all," Ritchie says.

Where or how "G" exists in the brain isn't well-understood. But no matter how it arises, the G-factor is real in the sense it can predict outcomes in our lives — how much money you'll make, how productive of a worker you might be, and, most chillingly, how likely you are to die an earlier death.

2) Having a higher IQ protects you from death

This is an uncomfortable one: According to the research, people with high IQs tend to be healthier and live longer than the rest of us. This graph represents a study of 1 million Swedish men. The researchers found a threefold difference in the risk of death between the those with the highest IQs and those with the lowest.

There are a few interrelated reasons why this may be. One is the fact that people with higher IQs tend to make more money than people with lower scores. Money is helpful in maintaining weight, nutrition, and accessing good health care.

It could also be that people with higher IQs are smart enough to avoid accidents and mishaps. There's actually some evidence to support this: Higher-IQ people are less likely to die in traffic accidents.

3) IQ is correlated with career success and wealth, but not necessarily happiness

Like mortality, the association between IQ and career success is positive. People with higher IQs generally make better workers, and they make more money.

But these correlations aren't perfect.

Correlations are measured from -1 to 1. A correlation of 1 would mean that for every incremental increase in IQ, a fixed increase in another variable (like mortality or wealth) would be guaranteed.

Life isn't that pretty. Many of these correlations are less than .5, which means there's plenty of room for individual differences. So, yes, very smart people who are awful at their jobs exist. You're just less likely to come across them.

With all the perks of a high IQ — wealth, health, longevity — you'd think the severely smart would be happier for it. But that's not necessarily the case.

"The correlation between IQ and happiness is usually positive, but also usually smaller than one might expect (and sometimes not statistically significant)," Ritchie says.

Also know that IQ, generally, is not related to the personality factors that can also get us ahead in life. Of the "Big Five" personality traits, the only linked to IQ is openness to experience. "To some degree smarter people will seek out more experiences and think about things more and enjoy considering ideas," Ritchie says.

(IQ often beats personality when it comes to predicting life outcomes: Personality traits, a recent study found, can explain about 4 percent of the variance in test scores for students under age 16. IQ can explain 25 percent, or an even higher proportion, depending on the study.)

4) You're probably stuck with what you got

Studies have found if you're a smart kid, you'll be a smart old person.

This chart shows a Scottish study where a group of 90-year-olds were given an IQ test they previously took at age 11.

Even though intelligence generally declines with age, those who had high IQs as children were most likely to retain their smarts as very old people.

5) Intelligence peaks in your mid- to late 20s, and then slowly declines

I'm 26 years old. And I'm probably as smart as I'll ever be in life.

After your mid-20s, your "crystallized intelligence" — i.e., accumulated knowledge — plateaus, while your "fluid intelligence" — the ability to solve new problems — starts to drop. Your mental quickness takes an even steeper dive.

Ritchie says understanding these age-related intelligence declines is one of the most important reasons for studying the biology of IQ.

"If we know the genes related to intelligence — and we know these genes are related to cognitive decline as well — then we can start to a predict who is going to have the worst cognitive decline, and devote health care medical resources to them," he says.

6) Around half the variance in IQ can be explained by genetics

Studies comparing identical and fraternal twins find about half of IQ can be explained by genetics.

But what's odd is that genetics seems to become more predictive of IQ with age.

That is, twins' genes seem to be less important for IQ when they're children compared with when they're adults. The reason why isn't entirely understood.

Intelligence researchers Robert Plomin and Ian Deary suggest it may be due to what's known as "genetic amplification," a process in which "small genetic differences are magnified as children select, modify and create environments correlated with their genetic propensities," they write in a 2015 literature review.

Consider this: A child with a genetic propensity to be smart may choose to spend more time in a library. A tiny 6-year-old may not be allowed to go to the library by herself. But a 16-year-old can.

The idea is as we age, we grow more in control of our environments. Those environments we create can then "amplify" the potential of our genes.

7) Genes are not the only thing that matters in intelligence

Genetics doesn't seal your destiny when it comes to IQ. About half the variability in IQ is attributed to the environment. Access to nutrition, education, and health care appear to play a big role.

But overall, the environmental determinants of IQ aren't as well understood as the biology.

"In terms of environment, it’s much harder to pin things down," Ritchie says. "People’s lives are really messy, and the environments they are in are messy. There’s a possibility that a lot of the environmental effect on a person’s intelligence is random."

8) Humans are getting smarter

Hurray! Mean IQ scores appear to be increasing between 2 and 3 points per decade.

This phenomenon is know as the Flynn effect, and it is likely the result of increasing quality of childhood nutrition, health care, and education. (In his book, Ritchie explains, it might also be the result of an increased emphasis on knowledge as the engine of our economy, which has encouraged the type of abstract thinking IQ exams test for.)

9) IQ is increasing faster in developing countries

Some of the greatest gains in IQ are happening in the developing world, where increases in childhood nutrition (namely via iodine supplements) and access to health care have made the greatest difference in IQ.

There's actually some evidence that the Flynn effect may be waning in the developed world. "It might well be we are running out of these low-hanging fruits [like standardized education and nutrition] that we know improve IQ," Ritchie says.

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One final note: Know that people with higher IQs aren't better at everything. In fact, they're more likely to need glasses to correct for nearsightedness. Nerds!

Proof of evolution you can find on your own body