I don't feel old. Not the way I see my parents getting old. But I'm not a young person, either. (I wince at being someone who uses the words "young person.") I'm on the cusp of 40, part of a generation stuck between the ever-shifting focus on millennials and baby boomers. Hands up if you're with me, hovering between the one and the other and largely irrelevant to marketers and ignored by pop culture.

But however much it might damage our self-esteem to not be wooed directly by so much of advertising or make us feel like ghosts to be absent from so much of pop culture, an even more worrying state of affairs is our absence from so much scientific research. It was a Canadian psychologist, Elliott Jaques, who coined the term "midlife crisis" in a study published in the International Journal of Psychoanalysis in 1965. A little more than a decade later, the concept became cemented in the popular imagination through influential books, such as psychologist Daniel Levinson's The Seasons of a Man's Life and Gail Sheehy's Passages: Predictable Crises of Adult Life.

Together, they established our understanding of midlife and the midlife crisis – an anxious time of reassessment when we must leave behind youthful illusions in order to find renewed purpose. That's sort of where we left it, though. As a subject of prolonged research, midlife is barely out of its adolescence.

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There is so much we still don't know. Why are some people leading satisfying lives throughout their 40s while for others it's a decade that feels like Edvard Munch's The Scream? If a drop in happiness is inevitable for most people, how do they hurry its return? How do we maintain the areas of cognitive function that are peaking and how do get back whatever we have lost? Can we regain any of it? Is the desire for reinvention that seems to underlie so much of midlife's anxiety even possible?

"Scientists and psychologists and neuroscientists have really studied people when their brains are developing and that early part of life, and then if you happen to live a long time they study you and wonder why you lived such a long time. But especially in the 40s, it is this invisible decade," says Barbara Bradley Hagerty, author of Life Reimagined: The Science, Art, and Opportunity of Midlife, published earlier this year, which makes the case that midlife is not a period of inevitable decline. "It's the best of times and the worst of times, but no one really studies it."

It's true that for a long time, researchers ignored midlife altogether, and even those who poked and prodded it neglected or ignore the 40s entirely. The Stanford Center on Longevity, for example, look at people over 50.

On the flip side, a University of Alberta study from earlier this year, which looked at how people got happier as they aged, claimed to prove the midlife crisis is a myth didn't look at anyone over 43. Thankfully, no one seems to be having a midlife crisis at 38.

But while the 40s have long been a largely invisible decade, researchers are beginning to fill in the picture of its importance. Here's what we do know, thanks to British researcher Andrew Oswald: that over the course of a lifetime, happiness follows a u-shape – and your 40s are rock bottom. Oswald, a professor of economics at the University of Warwick, made the discovery in the early 1990s. It has since been reiterated in more than 80 countries by multiple researchers.

"You dream big when you're young," Oswald said in a phone interview. You're going to be a CEO, you'll be a surgeon, you'll star in movies. "By your mid-40s, for most people, they realize they haven't attained that, and it's painful."

But while happiness reaches its nadir, cognitive functioning peaks in your 40s. Yes, memorization skills and perceptual speed both decline, but verbal abilities, spatial reasoning, abstract reasoning skills and simple math abilities will never be better, according to the Seattle Longitudinal Study led by Sherry Willis, a developmental psychologist at the University of Washington, and that has tracked thousands of adults for more than 50 years.

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Other researchers have been telling us what we already knew, in our soft, midlife guts: that lifestyle choices in your 40s can affect a person much later in life. In one, researchers at the Boston University School of Medicine found that a lack of physical activity in your 30s and 40s was linked to having smaller brain volumes two decades later.

While more research is beginning to focus attention on the decade, it is only just beginning. We are still a long way off from saying definitely "This is 40," a phrase that's stuck thanks to one of the few pieces of recent pop culture that address the decade.

"The happiest period in people's lives is from age 40 to 60," says a completely distraught Leslie Mann in the 2012 film This is 40, in which she plays a wife and mother of two girls looking down the barrel of her 40th birthday.

It's what we all want to think, that in our 40s we will have it together enough – stable jobs, a loving family of our own, plenty of distance between us and the tumult of youth and, on the other end, the decline of old age – that we can enjoy a solid contentment.

The fact is, no one knows for sure.