Whether you're rich or poor, young or old, there's one thing one-third of all couples have in common: They fight about money.

Sniping about spending, saving, bills, budgets, and secret purchases causes more relationship stress than kids, in-laws, and -- you guessed it -- intimacy, according to the latest American Express Spending & Saving Tracker survey.

Nearly half of overall survey respondents (45%) said that their money talks devolved into money arguments. Tiffs were most common among young professionals (under 30, college-educated, with household income of $50,000 or more), with 72% saying they squabbled about money. And, yes, even affluent couples (defined by American Express as those with more than $100,000 in household income) don't always see eye-to-eye on the family finances: 44% fessed-up to feuding about finances with their betrothed.

It's no wonder we're all fighting about finances and hiding our spending flaws: Fewer than half of Americans even remember having a conversation about money before marriage. And 12% say that they've never had a money talk. Ever.

Then again, even if you and your partner are on financial speaking terms, are you sure they're telling the truth?

How to commit financial infidelity
One of most common lies among couples is not that those jeans actually are unflattering, but what those jeans really cost.

It's not uncommon for twosomes to disagree over purchases (46% of respondents say they have). Plenty admit to fibbing about the amount they spend (more than a quarter of couples do). Others go so far as to deny any existence of the purchase in the first place: 30% say they've bought stuff completely on the sly.

Stealthy shoppers are creative about concealing the evidence. Here are some of the ways they said they kept their partner from discovering their financial cheats:

  • Ensuring the item arrived when their partner was out of town.
  • Concealing purchases in grocery bags to bring the camouflage contraband into the house.
  • Sneaking out at night to make the acquisition (and then hiding it under the bed).
  • Burying purchases in the backyard.

How to handle money with your honey
Want to live financially happily ever after? Fudging the numbers or trying to bury your money problems in the backyard ain't gonna cut it.

Managing your money relationship is a two-person job. You've got to take equal responsibility for keeping your joint finances on track, even if one of you does most of the heavy lifting. (If that's the case, I suggest the other person volunteers to be the permanent dishwasher.)

Here are six ways to keep things happy on the home front.

1. Set a formal date to sit down and discuss finances
If this is a prickly topic in your household -- and it is for 91% of the Americans Amex surveyed, who said they avoid money talks -- you really don't want to try to have this conversation on the fly. Or in public. Here's an agenda that'll help keep the conversation flowing (and even a little fun!). After your first talk, set up quarterly meetings to track your progress.

2. Don't dwell solely on the downer stuff
... like credit card debt, spending, or your ravaged investments. Instead, pick money goals that get you both excited, such as buying a home, a car, planning a big vacation, buying a new couch. Set short-, mid-, and long-term priorities and set achievable milestones (and a reward for achieving them) for each goal.

3. Establish financial ground rules
Set money rules so that not every little thing is an issue. For example, give each other spending space by setting a weekly amount each of you can spend on whatever you want without consulting your partner. (In other words, institute a spouse allowance.) Also, set limits on what you can spend on household purchases without consulting your partner. That dollar threshold for the average American, according to the survey, is $275. That said, with around one-third saying they circumvent that rule by lying, you'll want to...

4. Be careful about mingling your credit reputations
While there's no such thing as a "couples" or "joint" credit report, if you both are listed as joint account holders, any mismanagement will mar both of your credit files. Even if you're each pristine borrowers, maintain some credit autonomy (having a card in your name alone; keeping your oldest account from your single days active).

5. Hire a neutral third party
A trusted financial advisor (make sure they're fee-only, not paid by commission) can be a relationship saver: They can help identify issues, map out a plan of attack, and, best of all, take the role of "bad cop" so you can keep the peace at home! (To find such an advisor near you, use this map to search the Garrett Planning Network's group of fee-only advisors. Those identified with the Motley Fool logo are offering a limited-time 10% discount to our readers.)

6. Deal with financial infidelity
And finally, if you're not going to fess up to past money blunders, at least vow to put all your cards on the table from this day forward until death do you part. Here's advice on how to recover from financial infidelity and open your hearts and checkbooks to each other once again.

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