What percentage of the world population is able to understand abstract concepts? I’ve heard it in the neighborhood of 20%.

I don't know about World populations but this site seems to suggest 24% of adolescents in this country, presumably:

"“Data at ages 12, 13, and 14 suggests that 76 percent of children will sit through three consecutive years of being unable to handle material presented at the level of formal reasoning. It is known that most of the material prepared for junior high schools uses the level of formal reasoning. It is, therefore, possible that such children are “turned-off” by receiving many inputs at that level.”
http://www.mdk12.org/practices/good_instruction/projectbetter/middlely/mly-3-4.html

I very seriously hope those numbers are lying. If only 20% of the human population is capable of abstract thinking, the majority of us must be complete morons. Anyone that has ever taken statistics in college can tell you that you can make number say almost anything.

I find it very difficult to take most of these test very seriously. The mind is such a complex thing. We are not able to determine much no matter how many test have been tried to determine intelligence or lack of it. I have taken the Meyers Briggs personality test several times. I tested out slightly differently each time. I guess that I must have multiple personalities or the test are just not all that conclusive.

Wildflower:
I am not sure about the numbers, but first of all we would have to find out about the percentage of people with age over 12. According to cognitive development theory, abstract thinking is not reached until that age.
Piaget´s theory of cognitive development.
Also, people who don´t receive any education (who can´t reaad or write) do not reach this stage.
Here in Guatemala the illiteracy rate is more than 75% of the population. So from that 25% remaining, only from Guatemala, 42% are below the age 0f 14, which means only 68% would be able to have abstract thought (according to the theory, so, let´s say, in Guatemala, 9% of the population can have abstract thought. CREEPY!!!
This is a third world country and most countries in the world are third world countries.
Tell me what you think...

I guess it would depend on what you call "abstract". I would guess closer to 98% myself, considering that written language is a form of abstracting. The whole may not be literate, but the whole world is capable.

Have to agree with mahal - what was the definition of abstract?
Also need to know if it is a critical absence in people? Do they need to use such abilities?

Sometimes abstract concepts endorsed by groups with a specific point or expectation may not even be valid. Abstract reasoning can be useful, but it can also lead to wrong conclusions.
Just as it is possible to imagine that it is a natural occurance that doesn't challenge or require the ability since, by the argument, it is the abstract individuals who are the abnormality. If the results are correct, it may not be something desirable (though I notice from the responses so far that most people desire to possess the ability.)
In order to answer this one we would need the tools of formal reasoning (thank you, frankvan) to know what methods were used to obtain the results, who was tested, what the actual results showed and how the results were applied - along with subtle things like hidden agendas, expectations, funding to a purpose, etc.

Picture a million dollars.

Understand written text.

See a painting, know what it's of.

Hear a recording, recognize that it's not live.

Read a book, follow the story.

There, you just thought abstractly. I think its 100% of the human population that is not otherwise impared.