Couples argue about this without ever separating the two, and they are not the same thing. Privacy is an area of your life your partner does not have automatic access to — a journal, a therapy session, a conversation with your sister. Secrecy is actively keeping something from a specific person because of what they would do if they knew. One is a room with a door. The other is a thing hidden behind it.
Research on secret-keeping has landed somewhere counterintuitive: the damage is mostly not in the moment of concealment. Michael Slepian's work found that people are rarely actively hiding their secrets; what wears them down is how often the secret comes to mind when nobody is asking. The cost is carried alone, between conversations, which is also why 'they'll never find out' is such a poor defence. The finding out is not the expensive part.
The practical test is not how big the thing is, it is who it is being kept from and why. A password you have simply never shared is different from a password you changed last month. A dinner you did not mention is different from a dinner you would have mentioned if it had been anyone else. If the reason you are not saying it is a prediction about your partner's reaction, it has stopped being private, whatever the size of it.
This cuts both ways. A relationship in which nothing may be unshared is not more intimate, it is more supervised, and people in that arrangement get quieter over time, not closer. Two adults are allowed interior lives. What they owe each other is not total access but the absence of the second category — no thing being held back specifically because of what the other person would feel about it.
Each name one thing you keep to yourself that is genuinely just yours, and say plainly that it is not a secret. It is a strangely relieving conversation, and it makes the real thing much easier to spot later.
