The moment the other side knows the number you would actually accept, the negotiation is no longer a negotiation. It is a formality.
What the research found
Negotiation theorist Howard Raiffa's foundational work described the opening moves of any negotiation as each side trying to discover the other's reservation price, the point below which they would walk away, while protecting their own. He called this early maneuvering the negotiation dance. The entire value of that dance depends on neither side knowing the other's actual floor. Once one side's reservation price is known, the other side no longer needs to negotiate toward it, they can simply aim just above it.
Why this leaks more often than it should
A reservation price rarely gets handed over deliberately. It leaks through an offhand comment in an earlier, informal conversation, an over-eager tone, or a deadline mentioned in passing that reveals how much pressure someone is actually under. None of these feel like disclosures at the time. All of them function as one.
Where to start
Before an important negotiation, identify the specific number or condition you would actually accept, and treat it as the one piece of information in the entire conversation that does not get said out loud, including in the informal conversations that happen before the formal one starts.
Everything else can be discussed. That number is the one thing worth protecting.
Reading which informal conversations around a negotiation are actually safe, versus where your floor might slip out without you noticing, is a judgment worth being deliberate about rather than assuming. What this gives you is the one number worth guarding on purpose.
There is a specific way to answer a direct question about your bottom line without lying and without giving it away, and it is worth having ready before you are asked.