Before the fall, before eating of the tree, Adam and Eve were dumb. I should rephrase; not dumb, but they were moralless. They didn't have the knowledge of good and evil. I used to argue that they were not smart, that their dialogue and choices imply a level of intelligence of your average pre-pubescent child, but now I'm not so sure.
Reason's To Belive Ministries has a pdf about choices, and how to make better choices. Of course, in their discussion of choice, they talk a lot about the choice Eve made in the Garden of Eden.
When the serpent came to Eve and told her that, if she ate the fruit, she would not die, and instead become like God, eyes open to the knowledge of good and evil.
Christians often accuse Satan of being a liar. But pay attention here: the serpent did not lie. All he said was correct: God was lying; Adam and Eve didn't die and their eyes were opened.
Eve chose to disobey God. True. But how is she responsible without the knowledge of good and evil? At the time, she has to make a choice: who is likely to be telling the truth? Without good and evil, she has no concept of loyalty to her loving God. And it turns out, she's smarter than I typically give her credit for: she chooses correctly. Unfortunately, she's still not that smart; she forgets that God has the power to make her life miserable as punishment for disobeying him. Oh well, ce la vie.