Final answer:
All parties involved in the sale of residential properties built before 1978, including the sellers, buyers, and any real estate agents involved, must sign the lead-based paint disclosure, as required by United States federal law.
Step-by-step explanation:
In regards to who must sign the lead-based paint disclosure, the United States federal law requires that all parties involved in the sale of residential properties built before 1978 must sign the disclosure form. This includes the sellers, buyers, and all real estate agents involved in the transaction - this may refer to the listing broker, selling broker, or transaction broker, depending on the specifics of the transaction.
It's critical to note that this requirement is part of the Residential Lead-Based Paint Hazard Reduction Act of 1992, also known as Title X, which aims to protect families from exposure to lead from paint, dust, and soil. Compliance with this regulation is not optional, and failure to follow can lead to significant fines and other penalties.