Let’s think about the various sorts of people we have some relationship with. I’ll start: there’s my wife, brother, parents, other family members, friends, coworkers. But there are also former coworkers, my doctor, the people I talk to only on social media, the owner of the vegan café…

Taking stock of my social life, I could sort these into crude groups such as ‘family’, ‘friends’ or ‘business’. But relationships are complicated, and there are surely more revealing ways to compare and contrast them. Much as personality models like the Big Five offer a distilled language for describing individuals – slightly introverted, highly conscientious, etc – a model for relationships might be useful for thinking and talking about the shades of difference.

So I was intrigued to read about a proposed ‘unified framework’ for capturing how people see relationships. Researchers asked people from 19 world regions to rate the features of various types of relationships, ranging from siblings to leader and follower to fans of opposing sports teams. They found that relationships could be described in terms of five main dimensions:

• Formality: roughly, how formal and public a relationship is vs informal and private;
• Activeness: how close and involved vs distant;
• Valence: how friendly vs hostile;
• Exchange: how much it involves trading concrete resources like money vs intangible things like affection; and
• Equality: how equal each person’s power is in the relationship.

While the researchers say this model is ‘far from conclusive’, it does give scientists – and the rest of us – a new lens for considering our relationships and what they mean to us. Was the local bartender with whom I chatted about TV and movies for years a ‘friend’? Maybe, maybe not. But relationship traits like formality and exchange help me think about why there seemed to be more to it than ‘business’.

In 1917, France was on the brink of calamity. The great incinerator known as the Western Front consumed young men as voraciously as ever; supplies dwindled; mutinous unrest percolated in the ranks. Desperate for American intervention to tip the balance against the Central Powers, the French prime minister Aristide Briand pled for the US president Woodrow Wilson to commit his huge country to the war. But Wilson, messianic and haughty, bided his time.

Briand huddled with his ministers. They needed a new tactic, something unexpected. What if, someone bravely suggested, we send a philosopher to Washington, DC and see what he can do?

That philosopher was Henri Bergson, an extraordinarily popular thinker whose public lectures on time, memory, and the élan vital packed auditoriums with swooning fans: he appeared to imbue life with the creativity and mystery that science seemed to diminish. Passionately patriotic, he accepted the mission and took a liner to Washington.

Bergson was granted an audience with the president, whose self-love was most intense when it came to his own intellect. They talked for hours. Bergson played his cards well: he appealed to Wilson’s desire to strut the world stage as the bringer of peace, the founder of the League of Nations. They plumbed Bergson’s philosophy, which he said implied that the Germans were the enemies of civilisation. A few weeks later, the United States was at war.

I don’t want to give too much credit to Bergson – and I don’t think the US should have joined the war – though at least one Wilson adviser said that Bergson gave the president the push he needed. This little-known story of the philosopher and the president is remarkable: it is perhaps the most important diplomatic intervention by a philosopher, and a reminder, as if we needed it, of the extraordinary consequences of unlikely meetings.

My twins turned 11 the other day. I’m thankful the endless nappy changing and interrupted nights of their infancy are in the distant past. But there are new stresses, such as becoming an on-call chauffeur – to parties, sleepovers and sports clubs. Then, as now, the end result is similar: I’m frazzled most of the time. Ask me to guess and I’d say that being a parent has probably accelerated my brain age. So you can imagine my surprise when I stumbled upon a new paper in the journal Social Cognitive and Affective Neuroscience by James K Rilling and colleagues that claims caregiving – including being a dad – is associated with having a brain that’s young for your age.

The study builds on past research that found mothers and fathers in their 50s had a younger ‘brain age’ compared with their childless peers – by about half a year. This was based on measures of their brain structure, such as white- and grey-matter volume, as judged against databases of hundreds of brains of various ages. The new study extended this pattern to grandmothers and people caring for someone with dementia, and the findings suggested the brain age benefit might be even greater later in life. Compared with controls, grandmothers and caregivers had brain ages that were between four and six years younger, on average (after adjusting for other factors such as income and BMI). Both the old research and the new suggest there’s a sweet spot – too many (grand)kids or too much stress and the brain benefits are reduced.

As to why caregiving is associated with lower brain age, the researchers propose various reasons, such as being more mentally and physically active, the emotional connection, and the sense of meaning and purpose that comes from being a caregiver. My brain feels tired, but it’s nice to know the responsibilities of parenthood might be keeping it youthful!