St. Augustine's view on transmission of original sin?

The answer can be simply stated: original sin is propagated from parent to child through sexual intercourse.

But to fully appreciate it, I recommend reading a good theological treatise on St. Augustine's doctrine of the Original Sin to understand how the transmission relates to other aspects of St. Augustine's doctrine of the original sin, and what motivated him to construe it that way.

An example of such a treatise from the Catholic perspective can be found in Villanova University Professor Jesse Couenhoven's 2010 journal article published in Augustinian Studies: St. Augustine's Doctrine of Original Sin (pdf here) in which he discusses the following 5 tenets of the doctrine:

In brief, the five elements of the doctrine of original sin are as follows: (1) the source of original sin is a primal sin in the garden of Eden. (2) All human beings share in this sin because of our solidarity with Adam, the progenitor of the race. The results of the primal sin are twofold. (3) From birth, all human beings have an inherited sin (original sin itself), which comes in two forms: common guilt, and a constitutional fault of disordered desire and ignorance. (4) In addition, Augustine holds that the human race suffers a penalty because of sin—human powers are weakened, and we will experience death. (5) Finally, Augustine speculates about how both sin and penalty are transmitted from generation to generation. Each of these points are explored below, though I focus on the third, the conceptual heart of the doctrine.

Below are some topic headings as I skimmed through the article quickly.

1. Primal sin: source of original sin (peccatum originis originans), to be distinguished from condition of original sin (peccatum originatum)

   • original innocence (cf Ecc 7:29)
   • how Augustine struggle with the inexplicability of primal sin, which he later attempted to explain with his doctrine of Original Sin

2. Solidarity with Adam: how Augustine explains the connection with our sin with Adam's sin

   • inherited sin as participation in the common guilt and as constitutional fault (i.e. an inherited state of disordered desire and ignorance)

3. Sin as concupiscence (i.e. perversity and lack of order, a turning away from the Creator who is more excellent toward creatures which are inferior to Him), often misunderstood as blaming the body for sin because of Augustine's use of the descriptor "carnal". Under this heading, the role of disordered sexual desire.

   • connection between carnal concupiscence and original sin, and connection to the term "law of sin"
   • how according to Augustine, post baptism carnal concupiscence remains in the baptized

4. Penalty of primal sin arising from "inherited sin" vs. penalty for our own sins

   • how the penalty displays itself as wounds to human nature manifested in ignorance and disagreement between flesh and soul
   • penalty as punishment for sin and penalty as pedagogical for sinners

5. Transmission of original sin

   • Augustine takes the cue from Ps 51 that original sin is propagated by sexual intercourse, although he wasn't completely satisfied since it doesn't address all issues outlined above
   • Augustine doesn't want to depend on a particular theory of ensoulment, such as Tertullian's "traducianist" theory vs. "creationist" theory (see my answer to another question)
   • A consequence of not relying on an imputation theory is Augustine's emphasis on sexuality being vital to our ontological solidarity with Adam:

     we were in Adam because we were in his seed. Sex matters because that is how human nature is propagated.

   • How Augustine attributes even more to sexuality by giving sexual desire a significant role in original sin's transmission, claiming that

     carnal concupiscence, though originally the daughter of sin, is also the mother of sin ... lust becomes causally involved in the transmission of original sin ... Sexual lust thereby becomes not merely a symbol of carnal concupiscence, but its cause.

   • In addition, Augustine endorses a theory of the social transmission of sin:

     Augustine further claims that parental sins can increase the original sin of their children, and parental righteousness can lessen it. ... Adam's sin had singular power, guaranteeing the presence of original sin in all infants. But parents, too ... ensnare children in their own guilt.

   • Another aspect of holding that original sin is transmitted by lust suggests, however, that the rest of the race suffers not Adam's sin, but the consequences of Adam's sin.

Further resources

• For the primary source, read St. Augustine's book On Marriage and Concupiscence
• For a plausible but non-traditional Biblical and pre-Augustinian early church view of the meaning of the "Fall of Adam", read a 2017 blog article series on Original sin by Joel Edmund Anderson, an OT scholar and OT instructor. The series includes a very short summary of St. Augustine's doctrine (Part 2) but then offered an alternative based on St. Irenaeus's view of our current humanity as "first step" of God's over-arching plan (Part 3) and how Adam is simply a representative of this current creation as "man of dust" while Christ is the "next step" / "fulfillment" of that very plan (in Part 4):
  • Part 1 : Adam, Augustine, and the Doctrine of Original Sin
  • Part 2: Augustine and the Doctrine of Original Sin: Background About the Ultimate STD
  • Part 3: The Problems with the Doctrine of Original Sin
  • Part 4: Romans 5, I Corinthians 15, Ancestral Sin, and the “Natural” Order of Things