Watching Netflix’s limited series adaptation of East of Eden, one scene stuck with me long after the credits rolled.
Cathy Ames (Florence Pugh) slips out of Adam’s (Christopher Abbott) bed to sleep with his brother, Charles (Mike Faist). It is a calculated move that happens right before the couple packs up for California, and it casts a massive question mark over the twins, Aron (Joe Anders) and Cal (Joseph Zada).
As we watch the boys grow up, seeing how radically different they turn out, I found myself asking the exact same question Adam eventually wrestles with: who is their real biological father?
(Warning: Spoilers ahead for the plot and ending of Netflix’s East of Eden.)
Did Cathy Sleep With Charles in East of Eden?

To untangle this mystery, we need to look closely at the exact sequence of events before Adam and Cathy left the Trask family farm. Charles spent weeks warning Adam about Cathy, sensing the darkness beneath her innocent act, yet an undeniable, mutual fascination pulled the two of them together anyway.
Right before their departure, Cathy drugs Adam’s tea with a heavy sleeping draught. The moment Adam is out cold, she walks into Charles’s bedroom and seduces him.
Not long after they hit the trail west, Cathy realizes she is pregnant. She tries to abort the pregnancy with a knitting needle once they reach California, but she fails, eventually delivering fraternal twin boys.
Because she was intimately involved with both brothers in the exact same conception window, basic biology tells us either man could easily be the father. There is even the rare biological phenomenon of heteropaternal superfecundation, where fraternal twins have different fathers, which feels almost too fitting for the mythic duality at play here.
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How Cal and Aron Mirror Adam and Charles’s Sibling Rivalry

As we watch Aron and Cal grow into young men, the physical and emotional parallels between the two generations feel impossible to ignore.
Aron clearly inherited Adam’s fragile idealism as he is fair, charming, and desperate for moral purity, living in an untarnished world where he simply cannot survive the devastating reality of his mother’s secret life. Cal, on the other hand, bears every hallmark of Charles’s volatile edge. He is darker, shrewd with money, prone to violent impulses, and tortured by an unrelenting desire to earn love from a distant father.
What really convinced me of Charles’s enduring imprint on the family was Cathy’s final downward spiral in Salinas. Long after Charles’s death, she is literally haunted by visions of his ghost, treating him as the only man who ever saw through her illusions.
When she finally pens her will before her deliberate morphine overdose, she leaves her fortune exclusively to Aron. Cal’s confrontational pity during their brothel meeting unnerved her too deeply, forcing her to look at a mirror of the very darkness she and Charles shared.
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Why East of Eden Leaves Aron and Cal’s Paternity Unsolved

After turning the ending over in my head, I realized neither John Steinbeck nor showrunner Zoe Kazan ever intended to give us a definitive biological answer. And that is entirely the point.
When Adam confronts Cathy years later at her brothel, she taunts him with the possibility that Charles fathered the boys, weaponizing his doubt. But the story refuses to confirm it because handing us a neat genetic explanation would completely destroy the central message of timshel.
If Cal’s brooding nature and rage were purely Charles’s DNA, or if he was simply cursed by Cathy’s blood, he would have no agency over his soul.
Steinbeck leaves the paternity permanently ambiguous because bloodline is not destiny. Whether Adam or Charles provided the seed does not matter; what matters is the choice Cal makes when his father looks him in the eye and whispers, “thou mayest.”
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