FORTUNE -- Dear Annie: This is kind of a weird question, but how do you tell an office friend that she's damaging her professional image by going on and on about her personal life? I work with someone who is bright, talented, and capable, but other people here -- including our boss -- are starting to roll their eyes (and leave the room, if possible) every time she opens her mouth, because she shares so much about her home life, her kids, what she did over the weekend, etc. Last week she came back from vacation and she hasn't stopped talking about her family's road trip, complete with about 900 pictures.
We work for a manager who says very little about his life outside the office (although he does have the usual framed family photos on his desk and kids' crayon drawings on his walls, but that's about it), so my other colleagues and I follow his lead, the sole exception being this one teammate. I'd like to tell her this oversharing is a habit that could wreck her career here (if it hasn't already), but I don't want to hurt her feelings, since I do have to work beside her every day. What do you suggest? --TMI in Texas
Dear T.M.I.T.: This is a sticky one, because talking a bit about one's personal life now and then "can be really helpful in building solidarity on a team. It helps people discover things they have in common," notes Michael Crom, an executive vice president at Dale Carnegie Training, who adds that his firm's consultants often run across people like your coworker. "But too much talk about extracurriculars is hazardous. It makes you seem unprofessional, or just not focused on the work. There are only so many baby pictures your colleagues want to see."
Crom speculates that a rise in TMI at work can probably be traced back to Facebook (FB) and other social media. "There's a level of openness now that just didn't exist five or 10 years ago," he says. "It's partly generational. Young people coming into the workforce are used to putting things out there in public that used to be considered private, and they may not realize that too much of that just isn't appropriate in most businesses."
At the same time, Crom's company has done extensive research showing that employees are more engaged, and more likely to stick around, if their bosses take some interest in their personal lives and reveal a bit about their own. "People want a closer relationship with coworkers and especially with bosses," he says. "We've found that a warm personal rapport is crucial to retaining top employees."
Barbara Pachter agrees. A communications consultant who has counseled executives at Pfizer (PFE), Merck (MRK), Microsoft (MSFT), and other big companies, she's also the author (with Denise Cowie) of a new book called The Essentials of Business Etiquette: How to Greet, Eat, and Tweet Your Way to Success. "You do have to share a little," Pachter says. "Being too distant can be just as offputting as sharing too much." One manager she coached "came in on a Monday morning with a wedding ring on. He had never mentioned to anyone that he was getting married," Pachter recalls. "His team was furious. They froze him out."
So how do you know how much personal chat is enough? Finding that fine line requires sensitivity to the prevailing culture where you work. It sounds as if you and your colleagues, except for Chatty Cathy, have figured this out. If nobody else is going on at length about their kids or trying to show everyone their vacation snapshots, it's obviously wise to refrain.
Beyond that, Pachter has two rules: First, she says, "If you have strong political beliefs, they're best kept to yourself. Politics can change someone's whole opinion of you, often for the worse -- and, considering it's extraneous to the job you're doing, is it worth it?"
And second, she says, "Never, ever share anything that could be used against you later. Especially, don't talk about any situation where you may have acted less than ethically." In her consulting work, Pachter is frequently amazed at some of the things people brag to coworkers about. "There are people who actually believe it makes them look clever if they reveal that, for instance, a store clerk gave them too much change and they took it without saying anything," she says. "Often, people just don't realize how they're coming across to colleagues -- and some people just talk too much, period."
Which brings us back to your dilemma with your teammate. "You must speak up and let her know" that her behavior is making her persona non grata around the office, Pachter says: "If the situation were reversed and you were doing something that was making people roll their eyes and try to avoid you, wouldn't you want someone to warn you?"
Assuming you would, "start with that. Ask this coworker if she's open to some feedback, and explain that you'd want to hear this if you were in her place. Then describe the effect that her constant personal talk is having on her credibility as a professional, and suggest she put away the vacation photos and talk less about her home life."
It might help to cushion the criticism by stressing that you do, as you note, regard her as bright and capable and you'd hate to see this one quirk hold her back. "Say you're concerned about her reputation," Michael Crom advises. "You could point out that the rest of your colleagues tend to reserve most of their personal talk for lunch hours and other break times" —-- and that your boss seems to prefer that. Good luck.
Talkback: Have you ever worked with someone who talked too much about her life outside work? Do you think the tendency toward TMI is spreading? Leave a comment below.
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