I was under the impression that race is self-declarative in the US. What would prevent me, a white guy, to declare being a black woman if that gives some advan

I was under the impression that race is self-declarative in the US. What would prevent me, a white guy, to declare being a black woman if that gives some advantages? This would not be the first time people fraud, and here it is even hardly fraud if all that is needed is a declaration.