Try this simple psychology experiment.
An Interesting Experiment
Imagine four cards on a table. These cards all have letters on one side and numbers on the other. The sides you can see read A, D, four and seven.
That's all you have to picture. A, D, four, seven. Now, someone tells you that if a card has a vowel on one side, then it must have an even number on the other side. Which cards would you turn over to decide whether that is true?
Try it in your mind. You see A, D, four, and seven. Someone says cards with vowels on one side have even numbers on the other. Which cards do you flip...?
Okay, time's up.
What Would You Flip?
Most folks, when presented with this question, choose to flip the A and the four. After all, A is a vowel, so you want to know whether it has an even number on its back. Four is an even number, so you want to know whether it has a vowel on its back. That makes sense. But you had four choices: A, D, four and seven. Do either of the other two choices matter?
Sure, the seven does. Seven is an odd number, so if it has a vowel on its back, you can stop right there--the claim is wrong. But nobody turns over the seven.
Psychologists call this "confirmation bias." Our brains seem to be wired to look for confirming instances--that is, we get an idea of how the world is, and then try to find examples that confirm our idea.
We don't look for examples that disconfirm our idea, even though that's just as useful--sometimes even more so.