In the Biblical account, Eve is the first to eat the forbidden fruit. Yet still the blame gets put on Adam and later on in the Bible it states that sin entered the world through one man.
Why is Adam considered the original sinner?
Genesis 3:6 (NIV) reveals that Adam was with Eve when she took the fruit.
6 When the woman saw that the fruit of the tree was good for food and pleasing to the eye, and also desirable for gaining wisdom, she took some and ate it. She also gave some to her husband, who was with her, and he ate it.
Adam had the opportunity and authority to stop Eve from eating the fruit. While maybe not the best reason, I believe Adam could be considered the original sinner, because he did not act within his power to stop the sin from occuring.
This is something to remember when you are watching someone you have authority over entering into sin.
I still believe Eve sinned, though. 1 Corinthians 8 indicates that even when another causes you to sin, you still own your own sin.
1 Corinthians 8:9-13 NIV
9 Be careful, however, that the exercise of your rights does not become a stumbling block to the weak. 10 For if someone with a weak conscience sees you, with all your knowledge, eating in an idol’s temple, won’t that person be emboldened to eat what is sacrificed to idols? 11 So this weak brother or sister, for whom Christ died, is destroyed by your knowledge. 12 When you sin against them in this way and wound their weak conscience, you sin against Christ. 13 Therefore, if what I eat causes my brother or sister to fall into sin, I will never eat meat again, so that I will not cause them to fall.
From a technical perspective, Adam did sin first since his sin was allowing Eve's sin.
Also, for more on causing others to sin:
Matthew 18:6 NLT
6 But if you cause one of these little ones who trusts in me to fall into sin, it would be better for you to have a large millstone tied around your neck and be drowned in the depths of the sea.
Adam was charged with a responsibility1 and he had an obligation to fulfill (which included protecting his wife). Yet he did nothing. The account of the fall in the garden includes the detail that he was at Eve's side2 the whole time and failed to fulfill that responsibility. He broke a covenant3.
Did Eve sin? Sure. Was Adam ultimately responsible? Yes.
Genesis 2:15-17 (ESV) The Lord God took the man and put him in the garden of Eden to work it and keep it. And the Lord God commanded the man, saying, “You may surely eat of every tree of the garden, but of the tree of the knowledge of good and evil you shall not eat, for in the day that you eat of it you shall surely die.”
Genesis 3:6 (ESV) So when the woman saw that the tree was good for food, and that it was a delight to the eyes, and that the tree was to be desired to make one wise, she took of its fruit and ate, and she also gave some to her husband who was with her, and he ate.
Hosea 6:7 (ESV) But like Adam they transgressed the covenant; there they dealt faithlessly with me.
I Timothy 2:13-15 clarifies that Eve was deceived (the serpent deceived her into thinking she could draw closer to God by gaining knowledge, etc), but Adam knew full well what he was doing was wrong.
I Timothy 2:13-15 For Adam was first formed, then Eve. And Adam was not deceived, but the woman being deceived was in the transgression.
See also II Corinthians 11 (on the deception of Eve)...
II Corinthians 11:2-3 For I am jealous over you with godly jealousy: for I have espoused you to one husband, that I may present you as a chaste virgin to Christ. But I fear, lest by any means, as the serpent beguiled Eve through his subtilty, so your minds should be corrupted from the simplicity that is in Christ.
The fact that Eve was deceived does not make her guiltless. After all, God had explicitly instructed both Adam and Eve not to partake of the forbidden fruit. However, I think this (that Adam with full knowledge, without being deceived, partook of the fruit) is in harmony with Paul's line of thought in Romans 5:12, as he compared Adam with Christ, the antidote for sin.
Eve was made from Adam and all humanity made from Adam and Eve. This means all came from Adam. This bilogical fact seems to support and illustrate the spiritual counsel of God's will that Adam should be a federal head representing humanity. That Adam could obatin death, or eternal life based on the covenant of works, established by the two trees in the garden made his sin of all importance. In the same way Jesus is the new Adam and a new, second final representative federal head of all the new creation giving the meaning of the term 'the Son of man'. Furthermore, as God already knew about his plan for the second Adam when making the first, Adam properly prefigured Christ's authority over the church by his authority over Eve. Eve prefigured the churches life taken out from his side, loved and dear to him and submissive to his rule.
For the husband is the head of the wife as Christ is the head of the church, his body, of which he is the Savior. (NIV Ephesians 5:23)
As Adam was brought under a covenant of works his actions alone were to achieve life or death for all humanity, as all humanity was subjoined under that covenant in him. As it turned out his actions brought death and Eve's only brought her own being treated as somewhat irrelevant in the New Testament.