We've all heard those phrases that denote a certain blindness to the passage of time. "She looks as young as the day I met her" husbands say of their wives 50 years into married life, or "haven't they grown", people tell me of my children. How about "it wasn't even hot" said the frog, realising too late that he had sat unawares in the pot while the water slowly crept up to boiling point.
The thing is, we don't tend to notice change if it's gradual. And according to a recent study from Georgia Southern University and published in Paediatrics, parents don't recognise when their children have become obese.
Slow changes over time in anything we see every day become invisible and can be ignored – which is great for the ageing wives among us but not so helpful for frogs or children whose parents who should be taking notice so something can be done about it.
But is it just a matter of timing and what should parents do when they do eventually realise that something is wrong?
The new normal
Fat children may be invisible to their parents not only because the weight gain has been gradual but because their point of reference has changed. The term "obesity" not only means excess body weight but it also implies disease, illness, difference and a "problem".
But since the prevalence of obesity has increased dramatically over the past 40 years, and as populations get fatter, the new normal has become overweight and therefore invisible. By comparison fatter children are now thin. And as seats on aeroplanes, buses or in stadiums get wider and clothes get bigger, being obese is no longer a problem, particularly for children who may only experience symptoms in their later years. So if nearly everyone is obese then bizarrely no one is.
And so the "obese" become only those we see on TV being airlifted out of their houses – not the now "chubby", "well covered" or "big boned" children living in our own homes.
Frogs boil because the change is gradual. And if all frogs were boiled, a boiled frog would be the norm.
Taking positive action
Parents need to be encouraged to see that their child is overweight so they can do something about it, or be aware that it could happen.
But what should they do so as not to make matters worse? We don't want our children to be overweight because it can cause them to be teased, having low self-esteem or develop illnesses in childhood such as asthma, diabetes, heart disease or cancer in later life.
But neither do we want them to develop an eating disorder and become anxious about their weight in ways that prevent them from losing weight in a healthy way. In my recent book, The Good Parenting Food Guide, I outline some evidence-based tips for helping a child lose weight without making food into a problem.
Central to this is the art of subtle (even underhand) control that children aren't even aware is going on. So have active weekends (rather than nagging them to be more active), buy healthier food (rather than telling them to eat better), give them a smaller portion on a smaller plate (rather than expecting them to leave food) and be a good role model for eating well and being active (rather than eating what they like in front of them and expecting them not to want it).
Eating behaviour and exercise are governed by processes such as availability, modelling and association. At its simplest, we do what we like, if others do it and if it's easy to do.
Parents need to recognise that their child is overweight, and when they do they need to manage it in a ways that does good not harm, seeking to change their behaviour in ways that won't make a bad situation worse.
As I've argued before, health professionals can feel uncomfortable mentioning when someone is obese for fear of upsetting them or preventing them from coming back. But health professionals – and teachers, and good friends – who are perhaps less blinded by love can act as another pair of eyes, to show parents that something is wrong and, crucially, to offer positive ways to make things better.
Jane Ogden is Professor of Health Psychology at University of Surrey. She does not work for, consult to, own shares in or receive funding from any company or organisation that would benefit from this article, and has no relevant affiliations.