I have always been prone to homesickness. As a child, I didn’t really enjoy holidays, I dreaded going away on school trips and I hated sleepovers. At the beginning of 2021, when I first started thinking about the history of nostalgia, and in the midst of the pandemic, I moved across the Atlantic from London to Montreal, Canada, for work. Far from home and away from my family and friends, I felt a kind of grief whenever I thought about the life I’d left behind. There was so much to love about my new life but I felt anxious, worrying constantly about the safety and wellbeing of my parents, siblings and friends. What if, due to the time difference, I missed an urgent call or woke up to terrible news? These fears were, of course, unfounded, and they were also ridiculous, childish even. Grownups – married 30-year-olds with mortgages and full-time jobs – shouldn’t miss their mums.

I also tend to be homesick in a weirder, more abstract way – homesick for somewhere I’ve never been. It’s a feeling otherwise known as nostalgia. Melding fairytales with Horrible Histories, as a child I spent hours imagining myself transported back in time to invented and romanticised versions of the past. I was an avid reader of Enid Blyton’s novels and, despite my homesick inclinations, begged my parents to divert me from my 1990s London primary school to a boarding school in 1950s Cornwall. My pleas went unanswered, so I went to my uniform-free state school every day in pleated skirts and white blouses, desperate to return to a world I’d never inhabited.

Growing up, I cut these emotional ties to the past, and history and I developed a new, much more cynical relationship. I did a few history degrees and became hardened to the past – a steely, objective academic who avoided sentimentality. Professional historians tend to have a low opinion of nostalgia and, at first, I absorbed this view. Nostalgia is, for many academics, a hallmark of history amateurs – more the preserve of re-enactors, hobbyists and popularisers. By contrast, we’re supposed to be able to focus a critical lens on the past, see it for what it is, warts and all.

In my personal life, I also became less nostalgic. I like to think of myself as politically progressive and I’m certainly an optimist. But despite holding these lofty ideas about myself, I still sometimes found myself languishing in the romanticism of the past, allowing myself a bit of nostalgia now and again, as a treat.

I’m slightly embarrassed of this because, even outside academia, nostalgia has a poor reputation. For many, it is a fundamentally (small-c) conservative emotion, one held by people unwilling to engage with modern life – the proverbial ostriches with their heads in the sand. It is, according to sociologist Yiannis Gabriel, “The latest opiate of the people.” At best, a mostly harmless condition experienced by antiquarians and sentimentalists. At worst, a kind of reactionary delusion, one blamed for a range of perceived social and political sins. But nostalgia used to be even worse. And you don’t need to travel that far back in time to find it listed as a cause of protracted illness, or even death. In the premodern world, it had the capacity to kill.

Nostalgia was first coined as a term and used as a diagnosis in 1688, by Swiss physician Johannes Hofer. Derived from the Greek nostos (homecoming) and algos (pain), this mysterious disease was a kind of pathological homesickness. It caused lethargy, depression and disturbed sleep. Sufferers also experienced physical symptoms – heart palpitations, open sores, and confusion. For some, the illness proved fatal – its victims refused food and slowly starved to death. In the 1830s, a Parisian man was threatened with eviction from his cherished home. He took to his bed, turned his face to the wall and refused to eat, drink or see his friends. Eventually he died, succumbing to a “profound sadness” and a “raging fever” just hours before his house was due to be demolished. His diagnosis? Nostalgia.

As the 20th century dawned, nostalgia loosened its grip on the medical mind, parted ways with homesickness and morphed into, first, a psychological disorder and, then, into the relatively benign emotion we know today. While they no longer considered nostalgia a physical sickness, early psychoanalysts still had little patience for the nostalgics they encountered on their couches. They accused people with nostalgic tendencies of being neurotic and unwilling or unable to face reality. Much like many political commentators today, they were snobbish, arguing that the middle classes were less likely to be nostalgic than “lower-class” or “tradition-bound” people.

It wasn’t until the 1970s that these views softened. Today, psychologists believe nostalgia is a near-universal, fundamentally positive emotion – a powerful psychological resource that provides people with a variety of benefits. It can boost self-esteem, increase meaning in life, foster a sense of social connectedness, encourage people to seek help and support for their problems, improve mental health and attenuate loneliness, boredom, stress or anxiety. Nostalgia is even now used as an intervention to maintain and improve memory among older adults, enrich psychological health and ameliorate depression.

Nostalgia is now supposed to be pleasurable for the individual experiencing it, but its reputation as an influence on politics and society is not so honeyed. Populist movements worldwide are repeatedly criticised for their use and abuse of nostalgia. The images these movements paint of the past are condemned for being overly white and overly male. It’s also seen as the preserve of those who are retrograde, conservative and sentimental. Writers lambast those who voted for Trump and Brexit for their nostalgic tendencies and it remains, strangely, a kind of diagnosis – an explanation for what the critic sees as wayward or irrational acts. As the historian Robert Saunders put it, in reference to Brexit, the prevailing rhetoric marked out the Leave vote as, “a psychological disorder: a pathology to be diagnosed, rather than argument with which to engage”.

This tendency is as widespread as it is strange. Not least because nostalgia is a feature of leftwing political life, just as it is of conservatism and populism – think of the NHS, for example. It is also strange because, if you take present-day psychology seriously, everyone is nostalgic, pretty much all the time.

Most experts agree nostalgia is a predominantly positive emotion that arises from personally salient, tender and wistful memories. And nostalgia is more than just benign; it can be actively therapeutic. As one psychologist put it, during moments of nostalgic reflection the mind is “peopled”. The emotion affirms symbolic ties with friends, lovers and families; the closest others come to being “momentarily part of one’s present”. People with nostalgic tendencies feel more loved and protected, have reduced anxiety, are more likely to have secure attachments and are even supposed to have better social skills.

Maybe I’d have felt less unhappy if I’d spent more of my time abroad indulging in nostalgia. Rather than wallowing in sadness and thinking of all the people I wasn’t with, I could have used those memories to remind me that I have friends and family to miss.At the very least, knowing more about the emotion and its history might have enabled me to disentangle my feelings from the assumptions I’ve held about what normal, appropriate emotional responses to change are supposed to look like.

The process of researching nostalgia shifted my intellectual relationship to emotions. Society as a whole, and especially academia, tends to see emotions as irritants. There is now a certain degree of cultural pressure to talk about feelings and to acknowledge trauma and distress publicly (a bit like I’m doing here) and seek help and support when unhappy, anxious, or depressed. But at the same time, some emotional responses are still seen as more appropriate or adult than others; and political and professional decisions seen to be driven by feelings are still taken less seriously than those deemed motivated by reason, rationality or research. As a historian, I’m keen on research. But as a historian of emotions, I’m also keen on feelings. I’m interested in their variety, curious about their range and I take their power seriously. Nostalgia could do with a makeover – it needs rescuing from its associations with the sick, the stupid and the sentimental.

Because the emotion is everywhere, a source of both pain and pleasure, and it explains so much about modern life. Expressions of nostalgia are one way we communicate a desire for the past, dissatisfaction about the present, and, perhaps paradoxically, our visions for the future. Progressive, as well as conservative; not just stultifying, it’s creative, too. Homesickness also needs to be treated with more respect. In its harmful, pathological forms, it must be taken more seriously. And even in its more benign manifestations, like mine, we should see it for what it is. Not as a contaminant, nor a thing that gets in the way of us living our lives, but as evidence for deep feeling – for connection and commitment. Proof that we love and are loved in return.

Nostalgia: A History of a Dangerous Emotion by Agnes Arnold-Forster is published by Picador at £22. Buy a copy for £18.70 at guardianbookshop.com