It’s the sort of friendship that is so confusing you never quite know what you’re going to get. They’re the ones guaranteed to tell you a new haircut is ‘interesting’ rather than giving a straightforward compliment. They’re the ones who are secretly threatened by your success rather than rejoicing in it. They’re the people who don’t want to integrate you into their lives but will enjoy striking up intimate acquaintances with your other friends that exclude you. They’ll probably find a way of ensuring they’re the centre of attention at any party you have. They will trigger feelings of uncertainty, because they’re capable of both love and withdrawal, sometimes within a single hour. They offer backhanded compliments and passive-aggressive jabs and never seem entirely capable of being happy for you.

In academic circles, these are called ‘ambivalent’ friendships, after the work of the Swiss psychologist Paul Eugen Bleuler, who coined the term ambivalence in the early 1900s to describe the state of having simultaneous conflicting reactions, beliefs or feelings. An ambivalent friendship can therefore be one that contains multiple different impulses – they can be affectionate one day and combative the next. In popular culture, however, the verbiage is more straightforward. We call these people frenemies.

I previously thought that Gwyneth Paltrow had invented the concept after writing a 2009 piece for her Goop newsletter in which she revealed that she had once “had a ‘frenemy’ … This person really did what they could to hurt me.” There was much speculation that she was referring to fellow actress Winona Ryder, who had been reading the script for Shakespeare In Love when Paltrow stumbled across the screenplay in Ryder’s home and asked for an audition. Paltrow got the part and later rubbed salt in the wound by winning an Oscar.

But it turns out that ‘frenemy’ actually came into common usage as early as 1953, when the anti-communist journalist Walter Winchell used it in the Nevada State Journal to describe the tense relationship between America and Russia. In the 1970s, the author Jessica Mitford, sibling of the more famous Nancy, claimed that the portmanteau was in fact invented several decades earlier by one of her six sisters “when she was a small child to describe a rather dull little girl who lived near us. My sister and the Frenemy played together constantly, invited each other to tea at least once a week, were inseparable companions, all the time disliking each other heartily.”

I have, in my time, attracted a fair few frenemies, both men and women. They have varied widely in terms of background and character, but the one common feature is that they made me feel being their friend was a precious and unparalleled gift – the equivalent of the toxic partner who tells you that you’ll “never find anyone who loves you like I do”. It takes years of recovery to realise you won’t, and that this is a good thing because the way they loved you was unhealthy and controlling and not a little narcissistic.

With frenemies, I found it was often very difficult to become their friend because they had absurdly high standards for anyone who came into their orbit. They would criticise a stranger’s clothing, or a colleague’s halitosis, or the fact that the person they’d just gone on a date with had terrible taste in music. All of those should have been red flags, but then I discovered that to be allowed into their hallowed circle after passing these invisible tests – to be deemed ‘worthy’ of their attention – felt as though I’d won a competition I’d never even realised I was entering.

So why did I continue with them, even after realising they were bad for me? In 2003, two psychologists from Utah called Julianne Holt-Lunstad and Bert Uchino conducted an intriguing experiment. They asked 102 participants to wear blood-pressure monitors for three days and to record every social interaction lasting more than five minutes, noting their feelings and responses. Holt-Lunstad and Uchino found that blood pressure was raised when the participants interacted with people they disliked but could not avoid. But the really interesting discovery was that when these people met up with ambivalent friends, their blood pressure was even more elevated than when they spoke to the straightforwardly awful ones.

The study’s authors concluded that this was because we expect very little from people we loathe. Unpredictable friends, by contrast, can encourage your resilient optimism: you give them the benefit of the doubt because sometimes they are kind and loving and generous and sometimes they’re not. They raise your hopes and dash them with no warning.

Holt-Lunstad and Uchino went on to do several more studies in this area. In 2014, Holt-Lunstad wrote that “concurrent positivity and negativity” in ambivalent friendships was associated with higher rates of depression and greater cardiovascular reactivity to stress. Uchino found in 2012 that the more ambivalent ties a person has in their social network, the more likely they are to have shortened telomere lengths. Telomeres are the caps at the end of each strand of DNA that protect our chromosomes, like the plastic tips at the end of a shoelace. They are an essential component in determining how our cells age (and yes, I did have to look that up because I did Single Science GCSE and essentially only understand that chlorophyll is green and the earth is round). Shorter telomeres are strong predictors of mortality across different diseases including cardiovascular disease, cancer, and infectious diseases.

In brief: frenemies are bad for your health. Not just that, but they are actively worse for you than hanging out repeatedly with someone you hate. At the same time, these relationships are difficult to disentangle yourself from, because the good memories affix themselves to you like burrs on a hemline and the bad ones are frequently too subtle to identify as obviously negative. 

For me, the clearest way of identifying an ambivalent friendship is to take note of how I feel after I’ve seen them. If, on paper, we’ve had a perfectly nice evening but I’m left feeling inexplicably drained and a bit low, the chances are they fall into that category. Holt-Lunstad and Uchino would doubtless advise me to cut these friendships out of my life. But, as previously established, I’m conflict-avoidant and, I don’t know if you’ve heard, but I’m also a friendaholic. When a friend offers me even the slightest glimmer of niceness, it’s hard for me to turn my back on them unless it’s a consistent pattern of repeated bad behaviour over decades of knowing them. It takes a lot to push me to a point of no return. It does occasionally happen but the fact that I can count these break-ups on the fingers of one hand tells you everything you need to know.

Friendaholic: Confessions Of A Friendship Addict by Elizabeth Day (£16.99, Fourth Estate) is out now 

Main image: Jenny Smith; Stylist