The status of a firstborn son traditionally carries immense privilege, encompassing material wealth, leadership, and genealogical honor. As the firstborn of Jacob's family, Reuben naturally held this position. By all standard conventions, the historical records and genealogies should have begun with him, and he was the rightful heir to both the family's primary inheritance and its royal leadership [רד״ק, מצודת דוד, מלבי״ם].
However, the complex history of Jacob's children fractured these traditional rights. Reuben's privileges were stripped from him following a grave act of disrespect that defiled his father's bed [מצודת ציון]. Commentators differ on the exact nature of this offense. Some interpret the event literally, explaining that Reuben slept with Bilhah, his father's concubine [רלב״ג, ביאור שטיינזלץ]. Others suggest a less literal but still severe disruption, explaining that Reuben intentionally overturned and rearranged his father's bedding to prevent Jacob from being with Bilhah [מצודת דוד].
As a direct consequence of this disrespect, the birthright was taken from Reuben and given to Joseph, Jacob's most beloved son [רד״ק]. The primary approach among commentators is that this transfer was fundamentally practical and economic. Joseph received the double portion of the land inheritance normally reserved for the firstborn, which resulted in his descendants multiplying into two distinct tribes, Ephraim and Manasseh [מצודת דוד, רלב״ג, מלבי״ם, ביאור שטיינזלץ].
Yet, this transfer of the birthright was not absolute. While Joseph gained the material wealth, he was not granted the genealogical title of the firstborn. By birth order, he remained one of the youngest sons, and the official family lineage continued to recognize Reuben as the physical firstborn [רלב״ג, מלבי״ם, ביאור שטיינזלץ]. Although a midrashic tradition suggests Reuben lost even his genealogical title to Joseph, the widely accepted interpretation maintains that Reuben retained his historical place in the birth order [מנחת שי, מלבי״ם]. Furthermore, Joseph was not given the right to rule [רש״י, רד״ק, מצודת דוד].
The loss of Reuben's birthright did not mean that all his potential roles simply shifted to Joseph. Instead, the right to kingship was separated entirely and given to Judah. This was not merely a default result of Reuben's failure, but rather because Judah proved himself to be the most worthy and capable of leadership on his own merit [רש״י].