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In research-speak, such friends are called "ambivalent." They can be caring and warm, but maybe they're competitive, critical or frustrating.
And depending on how we interact with them, the relationship can raise our heart rates and blood pressure, which could lead to heart disease, according to Brigham Young University and University of Utah researchers.
They teamed up on a study that will be published today in the journal Annals of Behavioral Medicine.
They found that when people discuss past negative events with an ambivalent friend, their health suffers. In fact, their blood pressure and heart rate rises by merely being in the presence of such friends.
"Is it that these people aren't effective sources of support when we need social support in times of stress, or is it that these people are sources of stress themselves? We found evidence for both," said Julianne Holt-Lunstad, a BYU assistant professor of psychology and lead author of the study.
Even when participants discussed positive events with ambivalent friends, their health may have suffered. She found their blood pressure was lowest talking about good times. That sounds good, but researchers believe the participants distanced themselves from the discussion and thus weren't gaining the positive benefits - like improved self-esteem - that is supposed to come from sharing positive experiences.
"It makes you wonder, if these relationships are potentially detrimental . . . why do we have these friends?" Holt-Lunstad said.
She's researching that now. But she has some theories on why, as she estimates it, roughly half of people's social networks include such friends: They may be relatives, co-workers or neighbors - people who aren't easy to avoid. People who believe in the ideals of forgiveness and acceptance may be more apt to keep such friends.
Plus, these confidants do have their good sides. But those positive feelings for such friends may be what's causing the negative health problems. When people we don't care about let us down, that's easier to deal with than when the ones we care about do it, the researcher noted.
Holt-Lunstad is also studying ways to cope with ambivalent friends, by adding physical and emotional distance, for example.
The BYU-Utah study included 107 college students whose friendships were rated by experimenters as supportive or ambivalent. They were randomly assigned to bring one of those friends in to talk about a past positive or negative experience. The participants weren't told how their friends were categorized and their heart rate and blood pressure were monitored.
John Cacioppo, a psychology professor at the University of Chicago, said the Utah study furthered the research on the health effects of ambivalent friendships by showing in what circumstances they are detrimental.
He was not involved in the study but is interested in the topic.
"The [traditional] argument is that negative relationships can be caustic, but you don't tend to stay in negative relationships. You avoid [them], just like a painful sting," he said.
With ambivalent relationships, "you sit and maintain [them] over time and negative, caustic aspects accrue."