As I approached my 40th birthday, I came across a chart that would – without exaggeration – alter my life’s course. Known as the U-shaped ‘happiness curve’, it plots life satisfaction across adulthood. It suggests that people are, on average, happiest in their 20s, and in old age. The bit that bothered me was the middle: according to the chart, my 40s promised to be my glummest decade.

Shortly after I saw this graph, I made some major life changes – determined to buck the averages. I took a career break, lived abroad for a year, refocused my attention on family, and decided to write a book. Generally, I think it worked: I’m about to turn 45, and now feel pretty content.

So imagine my surprise to learn recently that the chart is more debated than I assumed. While the evidence (600+ papers, apparently) supports the U-shape, its applicability to you or me, well… it depends. For example, in some countries, older age doesn’t necessarily bring greater contentment, particularly if welfare support is lacking (and in some societies, midlife is the peak of wellbeing). There may also be gender differences: the mid-40s slump could be skewed by particularly unhappy men.

This year, the entire shape of the curve has been questioned. It seems the young may be unhappier than previous generations, which is flattening out the classic smile-shape into something more like Mr Spock’s arched eyebrow. In sum, it’s more complex once you dig into the data.

However, I have no regrets about making midlife decisions based on a chart. The real power wasn’t in the curve’s accuracy, but in how it gave me permission to make positive changes. And growing older has taught me that, sometimes, life’s big choices must be made with imperfect information.

Browsing Spotify for music to pull me through the slog of a grey February in New York, I came across the work of Labi Siffre, a 1970s artist I had never heard of. I was immediately taken with his delicate voice and simple, intimate musical arrangements.

Shortly after my precious discovery, Labi Siffre turned up in random places. I heard his crooning in coffee shops, a friend put him on at karaoke, and the singer-songwriter Lucy Dacus mentioned him as an artist who epitomised yearning. Labi Siffre was following me. Or at least that’s how I’d think about it if I didn’t know better. Instead, I thought: Baader-Meinhof.

The Baader-Meinhof phenomenon, also known as the frequency illusion, is a type of cognitive bias where, once you learn about something – such as a word, person or concept – you start to notice it more frequently. Your best friend clues you in on a slang term, and suddenly you see it endlessly in your feed. Your brother recommends a supplement, and you start repeatedly hearing ads for it. It feels eerie, especially if you don’t have a name for what’s happening.

Terry Mullen recalled feeling similarly when, after learning about a 1970s terrorist group called the Baader-Meinhof Gang, he encountered another mention of them the next day. He wrote to a newspaper about it in 1994. About a decade later, a Stanford professor proposed that a mix of selective attention and confirmation bias explains why something you are newly aware of might seem to happen all the time.

I first learned about this phenomenon in a psychology class at university. Today, when I feel it in action, I mutter ‘Baader-Meinhof’ under my breath. It helps me notice what I’m noticing, and the knowledge that my brain is primed for recognition dispels the feeling that I’m being sent some coded, mystical message – musical or otherwise.

As you read, do you scribble around the edges? Over the years, writers have sworn by the practice. Edgar Allan Poe once wrote: ‘In getting my books, I have been always solicitous of an ample margin … for the facility it affords me of pencilling suggested thoughts, agreements and differences of opinion …’

I tend to make notes digitally nowadays. And if I include magazines and newspapers, most of my reading is online, where I probably could annotate with the right software, but can hardly doodle. That’s a shame. Apart from the fact that annotating boosts recall, we’d lose something if freeform margin-writing faded away. One of the meanings of ‘marginalia’ is ‘non-essential’, yet it’s anything but.

For one thing, scribbled-upon books have helped reveal the web of influences between writers and thinkers. Herman Melville’s annotated copy of John Milton’s Paradise Lost shows how it fed into Moby-Dick. And Charles Darwin’s comments in his books – from Charles Lyell’s Principles of Geology to his grandfather Erasmus Darwin’s The Temple of Nature – inform how they shaped his theory.

Marginalia can reveal everyday insights too. Historians occasionally find scrawls in manuscripts that speak of long-dead readers’ lives. A recent exhibition in Dublin features scriptures doodled on by medieval Irish monks. One described feeling ‘ale-killed’ (hungover); another complained about his materials (‘New parchment, bad ink. O I say nothing more’). Turns out nuns did similar. In 2022, researchers found surprisingly modern cartoons of people around the edges of an 8th-century Act of Apostles. A nun etched them, perhaps mischievously.

Reading about these examples made me think I should annotate more – not everything, but the writing I care about. Or perhaps I’ll take a leaf from a wholesome TikTok trend I discovered while writing this: gifting an annotated book. This 21st-century practice suggests digitalisation may not be annotation’s death knell after all. Poe might have approved.