The idea of tradable birthrights has been around for a long while. Of course, it must apply to the women, since one man can inseminate hundreds (thousands?) of women. Such a system would be rather like cap-and-trade and would have the same problems being implemented, especially in a nation such as the U.S., where limits on personal freedoms are strongly opposed...

E. Swanson

Yup, it has been around a while. Kenneth Boulding was an advocate back in the 60s and 70s, and maybe the idea wasn't original even then.

We will need to be convinced that the only alternative is forced rationing of birthrights (1 child/family as with China), forced sterilization, etc. before a system of tradeable rights looks good. That may take a long time in the US, since this land is still relatively 'empty'. If I had to guess, I would say that restrictions on immigration is where it would start (immigrant families get tradeable lots on how many they can bring in as a means to control refugees and the idea gains currency....).