Some of you may be wondering, how do psychologists test intelligence; or more particularly, how can we say that one person is more or less intelligent than another. The process is actually quite a bit simpler than you'd think. First, I must point out that psychology has three theories about what intelligence means, and how to show intelligence. I will briefly describe those ideas, and let you choose for yourself which one seems like the best! Second, I'll walk you through the process of intelligence testing. Finally, I'll attempt to integrate this back into the main question.

Theories of intelligence

Largely, there are three theories about what it means to be intelligent--and you'll be able to look around you and see this with your classmates. The first theory major theory suggests that it's the total ability of the person which means they are intelligent. It's called Spearmans g. For example one of your friends may be really good at spelling and writing (verbal ability), but has difficulty when trying to read a map (spatial ability). What this theory tries to do say, "well because they are good at one thing, although poor at another they even out." This general theory is very old, but critical in your understanding of what it means to have intelligence testing.

The second is one which attempts to say that people can differ in three ways: one person may be really good at taking lots of information and putting it together meaningfully, another may be really good at seeing how information can be used in the real world, the last may be really good at seeing a complex thing and breaking it down into simpler understandable parts. This is called the Triarchic theory of intelligence. The special thing about this theory is that require that intelligence stay the same over the life span. In other words, you can learn how to use your knowledge in better ways with experience! Your intelligence won't make substantial changes, like from average to genius, but it can move from average to pretty intelligent.

The third theory is that people have lots of different kinds of intelligence. For example one person may be very good at dancing, gymnastics, or understanding how to move their body in specific ways--and seem to learn those tasks very quickly; others may be very good at learning music; others may be very good at solving problems; and roughly 7 more areas. This is called Garners Multiple Intelligence Theory. So when the redditor pointed out that one person can be really good at writing and another at understanding how people feel, he's generally referring to this theory.

Intelligence Testing

So how do we measure intelligence if there are three different, and competing ways of understanding it? Well, what we have done is develop tests that attempt to measure each of these theories, and tend to have multiple tests. We do not use the same test for children with adults; and if the testing is done properly, you would be tested multiple times. What we are looking for is how you perform on a several tests compared to other kids your age. If most kids are able to figure out 10-12 out of 20 problems, lets say, 70%, we can say that's normal performance. But when kids can't solve any more than 7-9 problems, and there are only about 15% of people who fall into that range, we say they are less intelligent at that kind of problem. However, if a kid solves 13-15 problems we say they are more intelligent at that kind of problem. If you return back to graph I showed you, when I was talking about height, that is what intelligence looks like. Those tests attempt to determine, based on lots and lots of kids being tested; how well do you perform on different kinds of problems. Those problems include how well you remember things, your ability to pay attention when distracting things are around, how you solve problems, and many other ways. What all tests try to do is say your mental age (how you perform on those tasks) and how far away are you from your actual age and we call that IQ or intelligence quotient. The equation to perform this analysis is (Mental Age/Actual Age)*100.

So if at age 10 you perform like a 10 year old, we can say you have an IQ of 100

(10/10)*100=100

If at age 10 you perform like a 12 year old we can say you have an IQ of 120

(12/10)*100=120

Psychology argues about how appropriate this method is, but for now that's just to get you familiar with how we measure intelligence.

Putting it all together

So while your cousin may have a lower general intelligence (the first theory), they may be really good at something else. This is why you have people who may seem to behave very strangely and are unable to do some simple things, but can learn a language in just a few weeks, or read a book once and memorize the entire book! Those people are called savants. Other cases, they are able to understand how other people feel.

TL;DR What you need to take away from all of this is that intelligence is something that falls along a wide range. Psychology has several different ways to understand what intelligence means, and testing attempts to determine how the individual person performs when compared to others.