Financial infidelity
By Cheryl Madliger • Bankrate.com

You buy yourself a new outfit or treat your colleagues to a round of drinks at happy hour and "forget" to mention it to your spouse or partner. While it might seem innocent enough, this kind of deception is a subtle form of financial infidelity -- an issue that's growing more and more common.

While the nature of the deception varies, hiding purchases, keeping secret bank accounts or stores of cash, lying about earnings, debts or bills, or not telling your partner about recreational spending (from gambling to dining to entertainment) can all cause a stir.

However you define it, financial infidelity and issues around spending, earning disparities and savings can be devastating to a relationship, says Tina Tessina, a psychotherapist in Long Beach, Calif., and author of "Money, Sex and Kids: Stop Fighting about the Three Things That Can Ruin Your Marriage."

"Money is one of the biggest generators of problems, arguments and resentment in long-term relationships," she says.

And while stereotypes about women as the main perpetrators exist, Mimi Lee, a financial adviser in Toronto says both men and women are guilty.

"I have conversations with clients, both male and female, regarding [financial infidelity]," she says. "This is very common."

Lee, who cites a survey by the National Endowment for Financial Education that found that nearly a third of all couples deal with the issue, says there are lots of reasons partners might hide things from one another -- many of which involve good intentions.

For example, a partner might withhold financial information as a way to "protect" their other half, says Lee.

"They take up all the financial obligations so their partners don't have to worry about money," she says, though in many cases people lie about money for self-serving reasons. In other instances, people hide debts to avoid scaring a partner away, save backup money in case of a breakup, or withhold information about earnings for fear of a partner spending more money if they knew how much was available, she adds.

Top warning signs
Regardless of motivation, there are certain behaviours that you can look for if you suspect your partner might be trying to hide something from you. Besides secretive behaviour in general, here are the top warning signs of financial infidelity:

1. Reluctance to talk about money: While finances might not make for the most exciting conversations, noticeably avoiding the topic can signal that your partner has something they'd rather not share or that they don't feel comfortable discussing finances -- a problem in itself.