Thirty seconds can wipe out completely your ability to correctly sum up a number. The numerosity adaptation effect is so strong that it overshadows your ability to eyeball numbers — and it works whether you know about it or not.
Take a look at the graphic below, or check it out here. Look at the cross between the circles at the top. Stare at it for thirty seconds. Then stare at the cross between the circles at the bottom. You should get the feeling that the circle on the lower left has a lot more dots than the circle on the lower right. Even knowing that the number of dots is identical shouldn't stop the malfunction of your mathematical intuition.
No one entirely knows why this is, although they've managed to discount things like the form or color of the circles and bars. It's possible that your brain adapts its expectations of what should be on the left and right hand sides of your field of vision. Although you intend to compare left and right, which have the same number up dots, you really compare each side to what you saw before. Your experience gets you thinking that the left hand side has "more," because the bottom circle has more dots than the top one, while the right hand circle has "less," because its number of dots was reduced in the lower bar.
What I want to know is how anyone realized this in the first place. Does anyone remember seeing anything like this in their everyday life?
Top Image: Angela Monika Arnold
Via Annals of the New York Academy of Sciences and Acta Psychologica Sinica