Throughout time, there are few topics that have been examined and expanded upon as much as the biblical account of the fall of mankind from the graces of God. This continual dredging up of a topic that might have, under other circumstances, been spiritual water under the bridge can be attributed to several characteristics of the story. Both the sin and the punishment for the sin, by their very nature, changed all mankind irrevocably. Perhaps even more important for all mankind is the question of blame. Who should we, the descendants of Adam and Eve, blame for the fall, and how should we penalize the guilty party's descendents? Critical and creative ink seas have been spilled for this question, yet we come no further toward understanding the sin, the punishment for the sin, or even the story itself. All we really gain are the works of the thousands upon thousands of authors who have had some agenda to advance, and felt the need to unburden themselves onto the relatively innocent reader. This is one of those works.
Sure, I could try to mislead you, and say that I'm going to be able to accomplish something that countless authors before me couldn't; it just wouldn't work. Instead, I'll try to make my bias plain by declaring what my conclusion is before I go on to support it. Man's fall from grace and his resulting expulsion from paradise was God's fault. In order to support this conclusion, I'm going to review the cases for fault for Eve, Adam, and finally God himself. I will be using three different sources: Genesis 2:4 through 3:24 from The Bible, "Eve's Apology in Defense of Women" from Aemilla Lanyer's Salve Deus Rex Judaeorum, and finally Book 9 from John Milton's Paradise Lost.
Eve is the person who is historically viewed as the instigator of the fall, and countless works have vilified her. Unfortunately, it doesn't stop there; Eve's perceived responsibility for the fall of man has been used as a reason to subjugate women for literally thousands of years. Because this end has been the sole point of many of the works, Eve very rarely gets a fair evaluation of her fault.
In the Bible's account God makes the world, God makes Adam, and then God warns Adam about the tree of knowledge: "You are free to eat from any tree in the garden; but you must not eat from the tree of knowledge of good and evil, for when you eat of it you will surely die." (Genesis 2:16-17). Then God then makes Eve.
You'll notice that Eve is not present to receive God's warning personally. In fact, during Eve's exchange with the serpent, the warning seems to have changed somewhat; it has become "You must not eat fruit from the tree that is in the middle of the garden, and you must not touch it, or you will die." (Genesis 3:3) Apparently Adam has conveyed God's warning, but he has changed it somewhat.
When Eve encounters the serpent, it convinces her that Adam's warning was wrong. The serpent says "You will not surely die, for God knows that when you eat of it your eyes will be opened, and you will be like God, knowing good and evil." Eve isn't completely convinced, but she follows the snake to the Tree of Knowledge of Good and Evil. Eve sees "that the fruit of the tree was good for food and pleasing to the eye, and also desirable for gaining wisdom" (Genesis 3:6) so she eats it. In doing so, Eve ignores Adam's warning. After eating the fruit, Eve "gave some to her husband, who was with her, and he ate it." (Genesis 3:6)
Generally it is thought that Eve disobeys God by eating the fruit. Why? God never told Eve not to eat the fruit from the tree of knowledge of good and evil. God's warning was specifically given to Adam, about Adam. "You must not eat from the tree of knowledge", not "You, and anyone you happen to see must not eat from the tree of knowledge." At worst, Eve is disobeying Adam, who felt the need to change God's warning to make it more restrictive.
Looking toward Milton's account of what happens, it becomes clear what sort of corruption of the story is required to make Eve even somewhat guilty. First, Milton leaves out the beginning portions of creation from his work, so we never really know anything specific about God's command about the Tree of Knowledge. Then he really gets going, as he relates a story that only slightly resembles the account in the Bible.
First, Eve lobbies Adam to allow her to wonder freely without Adam. Adam is reluctant, and he cautions Eve "The wife, where danger or dishonor lurks, / Safest and seemliest by her husband stays, / Who guards her, or with her the worst endures." Eve points out that because no physical harm can come to her, Adam must fear that she is weak: "His fraud is, then, thy fear; which plain infers / Thy equal fear that my firm faith and love / Can by his fraud be shaken or seduced;" Adam is concerned about Satan, because "Subtle he needs must be who could seduce / Angles". Eve says that Satan has underestimated their integrity "But harm precedes not sin: only our foe / Tempting affronts us with his foul esteem / Of our integrity".
As I mentioned previously, this bears little resemblance to the story in the Bible. Milton must do this so that he can separate Adam and Eve, so that the serpent can tempt only Eve. The insinuation is that if Adam were there, then the fall could not have happened, and it was through the weakness of Eve (and her Pride, which was demonstrated in her own estimation of her integrity) that the sin was made. The Bible contradicts this account in Genesis 3:6, where it says "She [Eve] also gave some [fruit] to her husband, who was with her, and he ate it." The Bible's account has Adam with Eve during the entire affair.
Once Eve has "won" her ability to walk freely and unaccompanied, the serpent (Satan) strikes quickly. First the serpent complements Eve, evoking "thy celestial beauty" and calling her "a goddess among gods". These are the prelude to the attacks that use Eve's pride to topple her. The serpent then goes on to tell the most grandiose lies about how it gained speech, and what will happen to Eve if she eats of the fruit. The serpent first makes the sin a "petty trespass". It then goes on to extol the benefits the the fruit, telling Eve that the fruit will make it easier to do good, and that eating the fruit will change them so that "ye [Adam and Eve] shall be gods". The serpent even goes so far as to ask how Eve could hurt god or the tree by eating the fruit, and then finally asks if the admonishment not to eat the fruit is based in God's envy. Finally Eve is convinced that she should eat the fruit, and so Eve falls.
Notice that the serpent in Milton's Paradise Lost is actively lying to Eve. In the bible, the serpent only says "You will not surely die, for God knows that when you eat of it your eyes will be opened, and you will be like God, knowing good and evil." All of these claims are true. They do not die, rather they are cast out of the garden. In Genesis 3:22 God confirms that the latter portion of the serpent's claim is true by saying "The man has now become like one of us, knowing good and evil."
Both Milton and Lanyer improvise situations to explain why Eve then gave Adam the fruit. Lanyer's Eve made a gift of the fruit to Adam out of love: "...Eve, whose fault was only too much love, / Which made her give this present to her dear [Adam]" Milton, consistent with his previous portrayal of Eve, has Eve give Adam the fruit because she can't stand the thought of Adam being happy with another woman (the woman that God would make for Adam after God has killed Eve).
The Bible makes no such pretenses. Where Milton takes four hundred lines to describe Eve giving Adam the fruit, the Bible takes only one. "She also gave some fruit to her husband" replaces four hundred lines of tortured introspection.
So, now we come to Adam. Adam actually "received that strait command / The breach whereof he knew was present death", as Lanyer puts it. The Bible doesn't really cover Adam's actions in any detail, other than the important one. He takes the fruit from Eve, and he eats it. Adam has been specifically warned not to eat the fruit, and he does anyway. In this, he obviously has directly disobeyed God. There isn't much more than that in the Bible.
This lack of information in the Bible hasn't stopped others from speculating wildly, though. Lanyer, for instance, points out that where Eve acted in weakness, Adam could have acted in strength and refused the fruit. Where Eve was convinced to eat the fruit, Adam simply ate it. Further, Lanyer points out that Adam, the lord of all the world could have stopped Eve from eating the fruit, and prevented the whole thing.
Predictably, Milton has a different interpretation of the events. As you have already seen, Adam acted to protect Eve from Satan's influence. If Adam had his way, Eve would not have been left unprotected, and Satan wouldn't have been able to convince both of them to disobey God. Later, when Eve returns with fruit from the tree of knowledge, Adam realizes what has transpired. Adam casts about miserably for several hundred lines about what to do. Finally he decides that if he eats the fruit, he will be allowed to be with Eve, and accompany her in her punishment. Adam eats the fruit (and disobeys God) for the love of Eve.
This casts rather a different light on Adam's sin. I maintain that it doesn't really matter. As I pointed out earlier, only Adam was told not the eat of the fruit of the tree of knowledge, so it was his responsibility to obey God's wishes, what ever they were.
This, of course, assumes that God was completely strait forward in his request. The combination of events that transpired were, by anyone's account, somewhat odd. First, God made the world, and all the creature that populated the world. He then made the Garden of Eden, building in the possibility of sin by including the tree of knowledge in the first place. It would have been trivial in the extreme to simply forgo the creation the tree of knowledge, thus precluding the possibility of sin. To compound the problem, he created the serpent, who possessed the ability to convince Eve to eat of the fruit. And this doesn't even address the primary crux of the problem: Why create Man and Woman to that they have the ability to sin or disobey God at all?
Well, it could have been a mistake, but that is somewhat contrary to the normal conception of God as an omniscient, omnipotent being. So, if one accepts that God is all knowing and all powerful, one must also accept that he designed Man so that he had the ability to sin. Because the fruit of the tree of knowledge of good and evil conveyed... errr... knowledge of good and evil to Adam and Eve, it is fair to postulate that before eating of the fruit, they didn't possess this knowledge. This implies that God created Man without the knowledge of good and evil, but did not construct him in such a way as to preclude the knowledge of good and evil. Without this knowledge, they could not distinguish good from evil, and so they could not have knowledge of sin. As presented, the first sin was a question of obedience, not evil.
So here is the unsupported conclusion: God wanted the fall to occur. Without the fall, man could not know good from evil, and therefor could not know that obeying God was any better than not obeying God. Without the original sin, man would have no idea what sin was at all, and could not avoid it without the direct command of God as his guide. God completed his gift of free will by imbuing man with morals, and then releasing (or banishing, depending on your point of view) him to the world so that Man could live outside the gilded cage of Eden. "If you love something, set it free, and if it comes back to you it's yours."