In 47 years of marriage, there are a few things Sherri Mills hasn't told her husband, Gerald, such as what she really spends on makeup. Or how she indulges the kids.

But these things don't compare with what Ms. Mills calls the 'one real whopper of omission' in her relationship with her husband. It involved her ex-boyfriend, Jim.

In the early years of her marriage, living in the small town of Helper, Utah, Ms. Mills would sometimes run into Jim and his family, with whom she had once been close. Knowing her husband was prone to be jealous and hot-tempered, Ms. Mills would try to avoid her ex-boyfriend or, if that failed, then say as little as possible to him. She knew her husband was suspicious that she might still have feelings for her old flame.

Then Ms. Mills learned that Jim's mother was seriously ill and wanted to see her. Ms. Mills went to the hospital, where the woman made a deathbed request: Would Ms. Mills speak with Jim and encourage him to sell the family-owned business (a bar, then known in Utah as a club) and strive for a more-settled life? The woman believed Jim still loved his ex-girlfriend and would listen.

Ms. Mills decided to try to help, yet she knew she couldn't tell her husband. She made plans see Jim at his sister's house, and brought her toddler son along to make sure the ex wouldn't misunderstand. She stayed 20 minutes, then went home and didn't tell her husband about the meeting for 40 years. 'It kind of ate at me,' says Ms. Mills, now 71 and the owner of a hair salon. 'But I knew I did the right thing.'

If you think this evasion sounds a lot like lying, you are right. But there are other names for the seeminglyharmless lies spouses and romantic partners sometimes tell each other.

Hiding worries or concerns to shield your partner from something unpleasant, or even just to keep the peace? Researchers call this 'protective buffering.' Hiding something to protect yourself, such as how much you spent on your new toy? Researchers call that avoidance. Routine buffering can turn into avoidance or something worse.

Couples often wrongly assume full disclosure is always best, says Sean Horan, a professor of relational communication at DePaul University in Chicago. But even avoidance, when not excessive, can be a 'productive strategy,' he says. Consider how you'd feel if your partner told you any or all of these:

'I left work early for a drink with that friend you hate.'

'I can't stand your brother.'

'Yes, those jeans make you look fat.'

Often the more open partners are with each other, the less happy they are, says Marianne Dainton, a professor at La Salle University, in Philadelphia, whose research focuses on communication in personal relationships. In dozens of studies over the past 20 years, Dr. Dainton has found people often say sharing too much is a source of relationship dissatisfaction.

Does being open lead to dissatisfaction? Or are dissatisfied people more likely to be open? Dr. Dainton says she doesn't know.

Avoiding a topic so as to avoid a fight is often good, Dr. Dainton says. But avoiding topics like financial mismanagement or addiction isn't protective buffering. It's selfish. And as for the biggest lie of all, marital infidelity -- whether to tell or not is an important question, but not the primary one a person will need to address if unfaithful to a spouse, therapists say.

Experts suggest a cost-benefit analysis. Will the information be more harmful to the relationship if disclosed up front, or discovered later? Consider disclosing if withholding gets in the way of intimacy, experts say. But if it will only hurt your partner, then don't tell.

It isn't necessary to mention the crush you have on your co-worker. 'Thought broadcasting is never a good thing,' says Toni Coleman, a McLean, Va., licensed clinical social worker and relationship coach. And beware of 'putative secrets,' the ones you only think you are keeping. Chances are you are already busted.

The buffering method you choose will make a big difference: Active lying is more damaging than avoidance, research indicates.

A study published in the Journal of Social and Personal Relationships in 2009 found 71% of people whose partners actively lied to them said it created distance. Among those whose partner kept a secret but didn't lie, only 43% said it created distance.

'When people overtly lie about something, they can take something innocuous and make it into a bigger problem,' says John Caughlin, the study's lead researcher and professor of communication at the University of Illinois at Urbana-Champaign. Experts say gender isn't a factor -- both men and women withhold information.

Gerald Mills, 67, a retired manufacturing-plant manager, has had his share of secrets over the years. He has hidden new hunting and fishingequipment in his truck and gone out for a drink with friends instead of doing errands. Once when his wife was away, he took their young sons out for a lobster dinner and swore them to secrecy. No need to guess how he got caught.

Ms. Mills, who went on to write a book about her marriage, 'I Almost Divorced My Husband, But I Went on Strike Instead,' says she never doubted her decision to keep the long-ago meeting a secret. (The ex-boyfriend was relieved to hear his mother's wishes; he thought she wanted him to keep the business going.) After the ex died about a year ago, Ms. Mills finally told her husband. 'At the time, I would have been angry and thought she was cheating on me and it would have derailed the marriage,' Mr. Mills says. Instead, 'we went to bed happy.'