People love to talk about the perils of middle age and mid-life crises. But what really happens in our middle years, asks David Bainbridge.

I have a confession to make. I am 43.

I have a belly, reduced skin elasticity, extra hair where I really don't want it, and a sports car. I, in short, am a middle-age cliche. But I am taking it a day at a time.

I still remember the day I went to buy that aged blue Lotus. I was 41 at the time and only too aware of the vast burden of middle-age stereotypes looming over me as I gazed at the car, which was glistening in the late-spring sun. I could hear a middle-aged devil on one shoulder grunt: "Go on - buy it." Then, after a pause, a middle-aged angel on my other shoulder pipe up: "OK, then. Why not?"

So I bought the car, but was I jumping headlong into the world of the so-called mid-life crisis?

We often think negatively about middle age. It is not a stage of life which we await with excitement. It does not get mentioned much in the media - not in a positive light, at least. Yet as certain suspiciously abrupt changes have overtaken me in recent years, middle age has come to fascinate me.

Find out more David Bainbridge is a clinical veterinary anatomist at Cambridge University and a science writer

He is the author of Middle Age - A Natural History

His episode of Four Thought is on BBC Radio 4 on 23 November 2011 at 20:45 GMT Listen again via the BBC iPlayer Or download the podcast

I am a vet and a reproductive biologist, with a training in zoology, so I suppose I was always likely to end up looking at it in an unorthodox way. Lots of people study childhood, or adolescence, or youth, or old age, but it seemed to be left up to me to study middle age. And because I knew more about animals than people, I studied it as an outsider, as it were.

What I found surprised me.

Middle age does not really exist elsewhere in the animal kingdom. Indeed, it wouldn't make any sense anywhere else in the animal kingdom. We humans usually stop making babies in our early 40s. Any other self-respecting species would take the Darwinian hint and die once that happened.

Yet we humans are exceptional because we don't curl up and die. Far from it. Data from life insurance companies suggests that in the fifth and sixth decades of life you are less likely to die over the coming year than at any other time in your life. Compared with other animals, this seems ridiculous.

The whole process of middle age seems deeply suspicious as it doesn't seem to be about getting old.

People are not demonstrably more stupid and not a whole lot weaker at 50 than they are at 20. And although imaging studies show that middle-aged people may use different brain regions to do the same old tasks, cognitive tests show that apart from brute speed, the brain's abilities are not diminished in middle age. They may be reaching their peak.

Also, although bone mass declines and muscle mass declines, rarely do things get so fragile that they snap. Most important of all, in offices, on construction sites, on football pitches around the world, the great, complex, social, co-operative endeavours which typify our species and make us human, in all of these you find middle-aged people telling supposedly sharper and stronger young adults what to do.

Infantile behaviour

Biologically, the middle-aged human body does not look like something being left to slowly decay. As a counter-example, if machines are left to deteriorate then some of them break down immediately, whereas others function perfectly for a very long time.

Middle-aged humans are simply not like that. They do not vary that much. Almost no-one starts breaking their hip at 40, whereas no-one still looks youthful at 60 - well, not naturally, anyway. If middle age were passive, uncontrolled failure, then it would vary between individuals much more than it does.

In fact, the two examples which buck the trend - the two body systems which do clearly deteriorate in middle age - make my point for me. First, long-sightedness (presbyopia) is almost unknown at 35, yet is universal by 50. I am already running out of arm's length at which to hold my reading matter.

Image caption The media comment when older men seek out younger women

The same is true of reduced skin elasticity. I can almost feel this happening and presumably this process is acceptable as long as I do not look so repulsive or decayed that I actively repel those around me.

So, those changes which do take place in middle age are so precisely controlled and carefully permitted they simply cannot result from creeping failure and decay. In short, the changes of middle age are too abrupt, distinctive of this phase of life, and characteristic of our species for that.

There is a controlling force at work in middle age which allows a few parts of us to suddenly fail, while maintaining the rest of us in good condition.

Indeed, it has become very clear to me the changes of middle age represent a developmental stage of life, as distinct and real as infancy or adolescence. Middle-aged development is programmed into each of us. We each possess the genetic recipe for long, healthy, human middle age. And we owe that genetic inheritance to hundreds of thousands of years of human history, during which - contrary to what you might think - humans frequently lived into their fifth and sixth decades.

Emotional flux

But can this newfound understanding of middle age - its biological basis, its evolutionary origins - help us understand the belly, the inelastic skin, the sports car parked on my drive?

I believe the answer is yes. It is time to take a journey deep into the dreaded male mid-life crisis. It's painfully close to home for some, I know, and perhaps even unpalatable, but this challenge simply must be faced. Men fear it and women joke about it, but what is it exactly?

We humans are an 'information economy' and middle age is the time when we pass on most of that information - this is why middle-aged people like being listened to

Well, the concept is only a few decades old but it has always been an elusive thing to define. Common versions of the "crisis" frequently involve a variable mix of three phenomena occurring some time in the male 40s.

First, a degree of emotional flux and uncertainty - an "intrapsychic" reorganisation. Second, humiliating urges to seek out the romantic attentions of younger women. Third, a tendency to revert to childish behaviours, interests and recreational activities.

All in all, it is remarkable how undefined the mid-life crisis seems when we all think we know exactly what it is. Let us take those elements one at a time.

First, the emotional flux and uncertainty. As it turns out men are no more likely to become depressed in their 40s than at any other time. If anything, questionnaire studies suggest that our mood seems to improve slightly. Also, we are no more likely to experience what we think of as "life-turning points" at this stage of our lives.

Similarly, we are less likely to get divorced than earlier in life - what a reliable, constant bunch we are. And finally, we, unlike women, consistently underestimate our proximity to our own death. This, of course, makes us even more happy. In summary, there is little evidence of a crisis here, certainly no specific "intrapsychic reorganisation".

Crisis myth

Next, the lusting after inappropriately young women. Did I really buy that Lotus to lure young women attracted to chubby, greying men in tiny blue cars?

In fact, studies of age preference in lonely hearts ads and dating websites have told us a surprising amount.

At 16, boys seek women who are roughly two years older than them. This age gap narrows to zero by the time we are 24 and thereafter we prefer younger women, with the age gap progressively increasing to only 12 years by our own old age.

Thus, there is no obvious evidence of middle-aged men suddenly hunting down young women, although it is certainly true that men who remarry do tend to marry partners younger than their first spouses - as women who remarry also do.

Image caption Middle-aged men like to be listened to

And there is good evidence that marrying a younger woman extends men's lifespan. Although this, strikingly, is not true in reverse. And finally, it is of course a simple arithmetical fact that there are no much-younger women in men's 20s - they do not yet exist.

Third, the infantile behaviour. I am afraid this is the point where I will go all anecdotal and unscientific - with two assertions.

First, having watched a son grow up, I believe that, on average, boys often have fundamentally different interests from girls - wheels, running around, machines, making things, breaking things, hitting each other - generally "doing stuff".

Second, I claim that men's interests do not change fundamentally between the ages of eight and 60 - with the exceptions of romance and sex. Instead, all that happens in middle age is that we become once again free to indulge ourselves. We have more money, some time and less fear of ridicule by others.

All I know is that when I play Lego with my son I am not enjoying it in some ironic, post-modern way, I am enjoying it in exactly the same way I did when I was 10. So, these pastimes and preferences of middle age are not new-found, they are our same old pastimes and preferences.

Complex lives

And, having demolished these three pillars of the so-called mid-life crisis, we can see the final nail in its coffin comes when people are asked if they have experienced such a crisis. They turn out to be just as likely to say they experienced the crisis in their 30s or 50s as in their 40s. And worst of all, women are just as likely to report a mid-life crisis as men.

How vague and lacking in evidence does a phenomenon have to be before we accept that it is not real?

So, we have a dichotomy between middle age and the mid-life crisis.

Middle age - those two healthy decades after the babies stop - is very real. Only humans have it, we evolved it, and we have enjoyed it for much of our species' history. And why? We evolved middle age because we have always lived more complex lives than other animals - in the ways we acquire resources, socially and technologically.

Image caption Often men have the same passions throughout life

Many animals are pre-programmed with almost all the information they will ever need, yet a human child is almost a blank slate. Acquiring huge volumes of information is essential if any of us are to cope in the complex world which we humans create for ourselves.

So unlike most animal parents, we don't just give our children genes and calories, we give them our culture. That takes time, and quality time, too, which we cannot dilute by churning out yet more babies. We humans are an "information economy" and middle age is the time when we pass on most of that information - this is why middle-aged people like being listened to.

So middle age is a very real and distinctive phenomenon, one central to the success of our species - which places it in stark contrast to the mid-life crisis, which turns out not to exist at all.

So why do we persist with the male mid-life crisis, talk about it, believe in it? I believe we retain, even love the crisis myth because it is a great story.

It is a narrative on which we can hang part of our lives. It is inherently funny and women like making fun of men, and men like distracting attention from real issues by making fun of themselves. And of course, I play along with this too. I often call the car my "malemenopausemobile" as a way of avoiding having to explain why I spent so much good money on it.

In other words, the mid-life crisis is a fable - but we all love fables, don't we?

This is an edited version of David Bainbridge's Four Thought broadcast.