In the 1980s, when data from the world’s longest-running study on happiness started to show that good relationships kept us healthier and happier, the researchers didn’t really believe it. “We know there’s a mind-body connection and we all pay lip service to it,” says Dr Robert Waldinger, the director of the Harvard Study of Adult Development, which has been running for 84 years. “But how could warmer relationships make it less likely that you would develop coronary artery disease or arthritis? How could relationships get into the body and affect our physiology?” Then, other studies started to show the same. “We thought: OK, we can begin to have confidence in this finding.”

It was still a surprise, says Waldinger, but so convinced is he of this fundamental truth that the new book he has co-written with Dr Marc Schulz, The Good Life, focuses mainly on relationships and how to improve them. There are other components, of course, and they tend to be similar across countries, cultures and social grades (he points to the UN’s annual World Happiness report). These include good health and a healthy life expectancy, plus the freedom and capacity to make significant life decisions. Trust is important, he says – not just in friends and neighbours, but also in governments. “One interesting thing that people mention around the world is generosity and opportunities to be generous,” says Waldinger.

Money – or, rather, economic security – is important. “We are less happy when we struggle for food security and housing and all that, which is obvious,” he says. What is less obvious is that, above a certain income level, happiness doesn’t go up by much, at least according to a 2010 study that set the threshold for US households at $75,000 (£49,000). The enduring factor is relationships with other people. Waldinger has boiled down his definition of a good life to this: “Being engaged in activities I care about with people I care about.”

Waldinger, a professor of psychiatry at Harvard medical school and a practising psychiatrist, became director of the study in 2005; he is the fourth steward of the research, which began in 1938. Originally, there were two unrelated studies – one group of 268 students at Harvard, another of 456 boys from deprived areas of Boston – but they later merged. Over the years, whole lives have been recorded in real time: health, employment, details about friends and spouses, religious beliefs, how they voted, how they felt about the births of their children, what they worried about in the middle of the night. The list seems endless.

“I’m sort of a voyeur,” says Waldinger, beaming through my screen when we talk on a video call. “I’ve followed all these lives – you can take someone’s folder, thousands of pages, and you can flip through a life. Yes, we do a lot of sophisticated number crunching, but being able to read a life is pretty amazing.”

The study has its limits, he acknowledges. All the original participants were male (Waldinger introduced women by including their partners and children) and white, although this will change gradually as the more diverse third generation is brought in. For the book, he and Schulz include many other, more diverse, studies from around the world, but he stresses that they all show a similar pattern: the more socially connected you are, the more likely you are to live longer and live well.

Loneliness is now considered to be as bad for your health as smoking – and there is a loneliness epidemic. “The best hypothesis for which there’s good data is the idea that relationships help us manage stress,” says Waldinger. “We know that stress is a part of life. What we think happens is that relationships help our bodies manage and recover from stress. We believe that people who are lonely and socially isolated stay in a kind of chronic fight-or-flight mode where, at a low level, they have higher levels of circulating stress hormones like cortisol, higher levels of inflammation, and that those things gradually wear away different body systems.”

Can we really learn about happiness from white men, some incredibly privileged (John F Kennedy was a participant), born in the US in the 1930s? Yes, says Waldinger: “So much of this is about the basic human experience, which does not change.”

Waldinger subscribes to the theory that happiness falls into two categories. Hedonic wellbeing can be summed up as “am I having a good time right now?” he says. Then there is the Aristotelian idea of eudaimonic wellbeing: “That sense of life being meaningful and basically good.”

We don’t necessarily enjoy the things that contribute to eudaimonic wellbeing. The example Waldinger likes to give is having to read the same story to your child at bedtime when you are exhausted after a hard day. “Are you having fun? Is it hedonic wellbeing? No. But is reading that book for the seventh time the most meaningful thing you could do right then? Yes. Often, there’s this difference between what’s fun right now and what we are invested in.” Everyone needs a bit of both, he says. The problems tend to come from chasing only hedonic happiness, rather than the more mundane, but ultimately more meaningful, kind.

We are also not very good at knowing what will make us happy. It is partly cultural – we receive messages constantly that we will be happy if we buy something, or if we have more money, or if we succeed at work. “There was this really interesting survey where they asked millennials what they thought they were going to need to have a happy life, and fame was a really prevalent goal,” says Waldinger.

But it is also due to human nature. When researchers in one study asked people to talk to strangers on a train on their morning commute, those who had predicted it would be a negative experience discovered it was the opposite. “Talking to strangers is a little risky,” says Waldinger. “Even calling a friend is risky, because you don’t know whether your friend is going to want to hear from you. Human relations always have that element of unpredictability.” This is why staying in alone rather than going out can feel preferable. “If I stay home and watch something on Netflix, it’s a predictable evening for me. Part of it is this path of least resistance – away from relationships and towards something more predictable and manageable.”

Waldinger’s parents were from the same generation as the study’s first cohort. He had a happy childhood, although there were times when his mother, Miriam, didn’t seem content – she was a clever woman who was unfulfilled as a housewife. They lived in Des Moines, Iowa – “midwest, small town” – and the family was Jewish. Waldinger’s father, David, went to law school, but couldn’t get a job when he left. “That’s what life was like for Jewish professionals in the United States in the 1930s.” He went into business instead, but he didn’t love it; the lesson his son learned was to pursue work that was enjoyable and meaningful.

How aware was Waldinger of antisemitism as a child? “A bit,” he says. “We were not significantly discriminated against, but it was there.” It was under the surface, but in day-to-day life, he says, people were basically decent to each other. “That’s one of the things that’s so hard now, because the right wing in the US and around the world is taking the lid off some of these prejudices – racism, antisemitism – and that’s what I find so disheartening. It’s there to be tapped, it always has been, but in many times we’re able to keep the lid on it.”

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He didn’t want to be a doctor; he wanted to be an actor and did drama alongside his academic studies. Before going to medical school, he came to the UK, where he had a fellowship at the University of Cambridge, and continued theatre. “I had such a good time, but I knew I wasn’t good enough to be a professional. I was too thin-skinned; I wouldn’t be able to take the rejections.” (Anyone who has watched Waldinger’s 2015 TedX Talk, which has had more than 44m views, will notice how that early theatre experience has translated into stage presence.) Once he became a doctor, though, he found that he loved psychiatry. “I was just fascinated by people’s lives and how their minds worked.”

He looks incredibly happy – and he says he is. “I’m in my early 70s and basically my health is OK. I’ve done my best to take care of myself, but that’s not the whole story. My happiness depends in part on luck, it depends in part on privilege. I have a partner and it’s a good partnership.” He and his wife, Jennifer, a clinical psychologist, have been married for nearly 37 years and have two grownup sons.

Waldinger is also a Zen master, having discovered the Buddhist practice in his 30s. He leads a weekly Zen group and does his own daily 25-minute meditation. “My wife calls it my great big hobby,” he says. How important is religion or spirituality to happiness? The study has found that religious people are not more or less likely to be happy, but that they find faith a solace in times of stress.

He hasn’t always been happy, of course. The times he describes as less happy are characterised by disconnection from other people. As a smalltown boy who got a place at Harvard, he was miserable and lonely for at least his first year, until he made friends. Later, when his children were small, his parents died. “It was a really difficult time for a couple of years,” he says. “That was one of those life crunches. People go through those times and it can be really hard to sustain your happiness.”

It is unrealistic to be happy all the time, which sounds obvious, but the message has become that if you are not happy, you are not doing life right. Similarly, there is an idea that happiness is something you can achieve and then relax. “The good life is a complicated life for everybody,” says Waldinger. “We study thousands of lives. Nobody is happy all the time – no one person on the planet that I’ve ever encountered. The myth that you could be happy all the time if you just do all the right things is not true. Happiness waxes and wanes.”

Happiness “happens” to us, he says (assuming – and it is a big assumption at present – that your basic needs are met). “But there are things we can put in place in our lives that make us more likely to feel happiness more of the time.” Taking care of your health, diet, sleep and exercise are big ones: “If you are in better health, you are more likely to be happy.” But so is taking care of your relationships. “That’s partly because they help us with the flip side: they don’t just make us happy; they also help us weather the unhappy times, the challenges.”

In a world ravaged by Covid and economic crisis, we might feel that we are in particularly challenging times, but so did the first participants of the Harvard study, who had grown up in the Great Depression and, when the study started, were months away from the outbreak of the second world war (many participants fought in it).

“We asked them what got them through it and everybody said something about people. Soldiers said: ‘It was the people writing to me from home, and fellow soldiers.’ When people were asked about the Great Depression, it was the neighbours pulling together and sharing what limited resources they had,” says Waldinger.

“What we find is that if people maintain a network of good relationships, they’re more likely to weather the storms and they’re more likely to be happy.”

Every generation feels that the world is “going to hell”, he says, “but there are some unique things happening to us”. Economic inequality is rising. “It really matters. We know that collective wellbeing goes up when more people have their needs met.” There is increasing social disconnection. “Loneliness is on the rise, but also tribalism, and that is fuelled by the digital revolution.” The study is starting to ask questions about social media usage and its effect on wellbeing. “Other research is showing that, if we use social media actively to connect with each other, that’s more likely to enhance wellbeing. But if we passively consume, that often lowers our wellbeing.”

The study has made him pay more attention to his own behaviour, he says. “I don’t just let my wife run our social life. I used to say: ‘Just tell me where to be.’ Now, I’m more careful about my own relationships and making sure that I keep them up.”

He describes it as “social fitness”: you don’t go to the gym once or twice and then assume your physical fitness has been addressed, he says. The same applies to friendships. “Good relationships wither away from neglect. There doesn’t have to be a problem of any kind, but if you don’t keep them up they fall out of your life. We find that the people who maintain vibrant social networks are the people who make an effort.” It doesn’t have to be big or time-consuming – a regular text, a coffee, a walk. “These can be tiny actions, but if you do them repeatedly it keeps those networks vibrant.”

The quality of the relationship is important, regardless of who it is with – friend, partner, sibling, neighbour. “We asked people at one point: ‘Who could you call in the middle of the night if you were sick or scared?’ We believe that everybody needs at least one or two people like that,” says Waldinger. “If you don’t have that, you’re probably hurting.

“But then, beyond that, it really varies – a good relationship could be somebody you go to the pub with. Maybe you don’t talk about anything personal, but you don’t need to. Maybe you talk politics and it helps you feel connected and like you belong.”

Casual connections – a smile or a short conversation with the cashier in the supermarket or the bus driver – can also bring benefits. Ultimately, it comes down to connection and belonging. Join that club, don’t use the self-service checkout, text a friend and meet them, read that story again to your child – your health and happiness depend on it.