Histories

therefore it was neither reasonable nor just that anyone should have the royal privilege before him. At Sparta [22.4417,37.0667] (Perseus) Sparta too (advised Demaratus) it was customary that if sons were born before their father became king, and another son born later when the father was king, the succession to the kingship belongs to the later-born.

Xerxes followed Demaratus advice, and Darius judged his plea to be just and declared him king. But to my thinking Xerxes would have been made king even without this advice, for Atossa held complete sway.