Why did Jacob adopt Ephraim and Manasseh as his own sons?

To give Joseph a double portion — the legal right of the firstborn — because Reuben, the natural firstborn, had forfeited it. By counting both of Joseph's sons as his own, Jacob gave Joseph two tribes where every other son of Israel received one.

Before Jacob blesses his grandsons, he does something legal. He adopts them.

"Your two sons born to you in Egypt before I came to you are mine; Ephraim and Manasseh shall be mine, as Reuben and Simeon are" (Genesis 48:5). The Hebrew clause is blunt: לִי הֵם (li hem) — "they are mine." The two grandsons born in Egypt, boys who grew up speaking Egyptian, the sons of an Egyptian mother (Genesis 41:45), are now reckoned as Jacob's own sons — leveled with his two eldest.

The legal effect is precise. In Israelite inheritance law, the firstborn received פִּי שְׁנַיִם (pi shenayim) — "a double share" — of the father's estate (Deuteronomy 21:17). Every other son receives one portion. The firstborn receives two. By adopting Ephraim and Manasseh and numbering them alongside Reuben and Simeon, Jacob is giving Joseph two portions instead of one. Joseph will not inherit as "Joseph" — one tribe among twelve. His line will produce two tribes: Ephraim and Manasseh, each counted separately, each receiving a territorial allotment in Canaan. The double portion that belongs to the firstborn goes to Joseph's sons.

But Joseph was not the firstborn. Reuben was. So where does the double portion come from?

The Chronicler explains it in the plainest terms the canon offers: "the sons of Reuben the firstborn of Israel — for he was the firstborn, but because he defiled his father's bed his birthright was given to the sons of Joseph the son of Israel" (1 Chronicles 5:1). Reuben's act with Bilhah (Genesis 35:22) cost him the firstborn's legal standing, and Jacob's own later verdict confirms the transfer: "you went up to your father's bed; then you defiled it — he went up to my couch!" (Genesis 49:4). The birthright vacancy was real, and Jacob filled it with Joseph's line at the deathbed.

"Ephraim and Manasseh shall be mine, as Reuben and Simeon are."

— Genesis 48:5

There is a personal note buried in the middle of the adoption declaration. Jacob pauses to recall Rachel: "Rachel died, to my sorrow, in the land of Canaan on the way, when there was still some distance to go to Ephrath, that is, Bethlehem" (Genesis 48:7). The mention is not sentimental drift. These are Rachel's grandsons. Their mothers' identity matters in a patriarchal lineage — by adopting them into the house of Israel, Jacob is asserting their place in the covenant family despite their birth in Egypt to an Egyptian mother. They are Rachel's grandsons, and they belong to the promise.

The adoption has consequences that outlast Genesis. The twelve tribes of Israel are normally counted by Jacob's twelve sons. But Ephraim and Manasseh appear in the land allotments as separate tribal territories, and "Joseph" appears without a tribal allotment of his own — his line is distributed between his two adopted-sons-who-became-grandsons. The deathbed legal act of a dying old man shaped the map of the promised land.

The full study traces the adoption, the double portion, and the transfer from Reuben to Joseph's line in The Angel Who Redeemed Me.