Bible Verses About Polygamy: Mosaic law respecting the firstborn in

If a man have two wives, one beloved, and the other hated, and they have had children by him, and the son of the hated be the firstborn,

And he meaneth to divide his substance among his sons: he may not make the son of the beloved the firstborn, and prefer him before the son of the hated.

But he shall acknowledge the son of the hated for the firstborn, and shall give him a double portion of all he hath: for this is the first of his children, and to him are due the first birthrights.