Only a third of couples would feel happy sharing their PINs with their other halves, according to new research from anti-fraud company Defender Note.
In fact, more than half of us keep our PINs completely to ourselves.
Morgan Rothwell at Defender Note said: "It's a little surprising that so many couples don't trust each other with their PIN, but people are right to be careful when it comes to sharing their personal information."
There have been a number of cases of people scammed out of money by those close to them after sharing their PINs.
Last November, for example, we reported on the case of Beryl and Melvyn Bevan, an elderly couple whose carer stole their money after persuading them to hand over their bank card and PIN.
And earlier in the year, we revealed how a Lancashire carer paid to help people with learning difficulties manage their finances had stolen more than £80,000 from her vulnerable clients.
Women are more likely to give their PINs to loved ones than men, Defender Note's researchers found.
Some 35% of the female respondents to the study said they would share their PIN with their partner, compared to just 31% of male respondents.
Unfortunately, all this privacy could well prove pointless should their bags or wallets fall into the wrong hands.
A worrying 5% of people store their PIN on their phone to help them remember it, while 2% keep a note in their wallet or purse.
Defender Note is therefore urging consumers to choose PINs that are hard to guess and avoid writing them down where they could be found.
"Criminals continue to come up with new and covert ways of defrauding consumers, Rothwell said.
"In the last year alone, there's been 26 per cent rise in card fraud in the UK, with last year's total bill reaching £755million."