[Question:]{.underline} If Eve only, and not Adam, had sinned, would we have been born with original sin?
[Answer:]{.underline} St. Thomas Aquinas himself asks this very question in the Summa Theologica (Ia IIae Q.81, a.5). His answer is very clear. Original sin is transferred through Adam and not through Eve, and so consequently, if Adam had not sinned we would not have inherited original sin, whereas if Adam had sinner, but not Eve, we would still have inherited it.
His answer is based upon Sacred Scripture, that states that “by one man”, that is Adam, “sin entered into this world” (Rm 5:12). Scripture does not say by one woman, nor by two parents, but by one man. Consequently, it is a part of our Faith that original sin is contracted only from Adam. St. Thomas explains this by saying that it is through the process of generation that original sin is transferred. However, the mover or the active principle of generation is the father, and not the mother. Hence it is Adam one who transmits original sin to us, although Eve was the first to commit the sin.
Answered by Father Peter Scott, SSPX.