We live in a world of improbable bodies; they populate our television screens, magazines and billboards. If you're like most people, you might sometimes get the feeling your body isn't normal. But don't fret - it's all the virtual bodies around us that aren't.
Pick up a Barbie doll and have a good look at her. Which part do you think is the most anatomically improbable? No, you'll have to look lower.
Relative to the average young woman, Barbie's feet (adjusted for height) are 17 standard deviations below the mean. Other parts are almost equally unlikely: her bust-to-waist ratio is 13 standard deviations above the mean.
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It's not just young girls who are presented with improbable bodies: the biceps on the GI Joe doll action figure has almost trebled in girth since the original version in the 1960s. And the ratio of chest girth to waist girth has increased by 40 per cent.
So, here we are, surrounded by images of ideal bodies: actors, sports stars, steroid-pumped bodybuilders, shop mannequins, dolls, dolled-up personal trainers, air-brushed models and digitally-enhanced videogame avatars. And not one of them reflects reality.
Take female shop mannequins. On average, they're 172 centimetres tall. That's about ten centimetres taller than the typical Australian woman. They have much broader shoulders, narrower waists and longer calves.
Shop mannequins. Photo / Thinkstock
Even dress sizes imply a body shape real people rarely match. That's why it's so hard to find a suit or dress that fits you; clothing size templates bear only a passing resemblance to the real shape of Australians.
Australia uses, in principle, the Standards Australia system, which goes down to size eight. Sizes change by regular increments and you can use the size eight measurements to calculate what a size six or four would be. But hardly any retailers follow the Standards Australia system, or any system for that matter, and most label their clothes idiosyncratically.
A woman fitting the Standards Australia dress size eight would have to weigh 40.5 kilograms. In spite of this, 25 per cent of young women report wearing size eight, and I was recently obliged to buy a size four dress for my 54-kilogram daughter.
How does that work? Well, over time there has been an inflationary debasement of dress sizes: what is now called a size eight is actually closer to the original size 12 or 14. And a typical Australian woman actually fits size 16.
So pervasive are images of unreal people that we no longer know what real people look like. Asked to judge their own weight category, about 30 per cent of US adults got it wrong. Women tend to think they're fatter, while men - I'm sure you can guess - think they're leaner.
Parents are even worse when it comes to judging their kids' size. A recent meta-analysis found over half the parents of overweight or obese kids' thought they were normal weight or underweight.
Part of the problem is that there are few key metrics of size and shape. The most common are height, weight, body mass index (BMI, which is your weight in kilograms divided by the square of your height in metres), and waist girth.
While these are good measures across the population, they don't always work well for individuals. BMI, for instance, doesn't distinguish between fat mass and muscle, so almost every member of the current Australian Rugby team would be classified as obese.
In 2008, the average adult Australian male was 176 centimetres tall and weighed 85 kilograms, with a waist girth of 96 centimetres. That's a BMI of 27.5, right in the middle of the "overweight" category.
The average adult woman was 162 centimetres tall, and weighed 70 kilograms, with a BMI of 26.7. Her waist girth was 86 centimetres.
People are definitely getting bigger. For over 100 years, height has been increasing at the rate of one centimetre per decade, and weight by one kilogram every decade. But it's been up to three kilograms per decade in recent years.
About 63 per cent of Australian adults and 25 per cent of kids are overweight or obese. The proportion of overweight people continues to increase in Australian adults, but has plateaued in Australian kids since about 1996.
The variety of body shapes is also increasing. Look at pre-war photos of workers or school kids, and you'll be struck by how similar (and lean) their bodies are. Today, this distribution is skewed with more overweight bodies and more extremely overweight ones.
We're also seeing increasing distribution at each extreme, with separate peaks for the fit and lean, and the overweight. Ethnic diversity is also increasing the spread of body shapes, as are extreme body practices such as steroid use and illnesses such as anorexia.
Athletes are changing size and shape faster than the general population. Top level basketballers have been increasing in height at the rate of two and a half centimetres per decade, and shot-putters have been putting on weight at the rate of seven kilograms every ten years.
New Zealand shot-putter, Valerie Adams. Photo / Getty Images
At the 1928 Olympics, the average weight of shot-putters in the finals was 80 kilograms; today it's 140 kilograms. This extraordinary rate of growth is fuelled by (and fuels) new training techniques, supplements, growth-stimulating drugs and huge salaries to recruit the best and biggest around the world.
Big bodies are a very tradeable commodity.
Nurture and nature
So what kinds of things determine body size and shape? For height, the best advice is to choose your parents wisely. Genes account for about 90 per cent of the variance in height, but both childhood malnutrition and exposure to infections can limit how tall you'll be as an adult.
Weight is much more subject to environmental influences, such as socioeconomic status; about 27 per cent of Australian kids from the poorest quartile of homes are overweight or obese, compared to 19 per cent from the richest quartile.
Body size and shape also vary across ethnicities. The Dutch are the world's tallest people (184 centimetres for men and 171 centimetres for women), while the Indonesians are the shortest (158 centimetres and 147 centimetres respectively).
And the fattest seven countries are all in the South Pacific, headed by Nauru, where almost 95 per cent of adults are overweight or obese.
So I'm not GI Joe, and you're not Barbie, but it's not all bad news. There are a lot more people in this world like you and me than there are like these dolls. Best to aim for good health and get comfortable with your normality
Tim Olds is a Professor of Health Sciences at University of South Australia. He receives funding from the Australian Research Council.