Thirteen million Americans have a bank or credit card account that’s hidden from their spouse or partner, according to a new report from CreditCards.com.

Millennial couples were twice as likely as seniors to say they have a secret, separate bank account. Why?

“Sometimes you have to learn things the hard way,” said Matt Schulz, senior industry analyst at CreditCards.com. “It might just be a sign of inexperience in a relationship.”

Or maybe more experience.

MaryLeigh Bliss, chief content officer at Ypulse, a youth market research agency, said she’s not surprised by the finding.

She said millennials are waiting longer to get married, so they have more relationships before marriage. And they grew up in a time when divorce became more common.

“There might be a little part of them — especially if they are a child of divorce — that wants to have that safety net for themselves,” she said.

At 32, Bliss is a millennial. She is married. She said she and her husband have separate, though not secret, accounts. The reason: they don’t want the red tape and hassle of transferring everything into a joint account.