The Egyptians crafted eyeliner from kohl, containing lead (definitely not good for you), and at the turn of the last century, people eagerly applied night cream fortified with radium to achieve a healthy glow (even worse).

We know how dangerous these practices are today, but are modern day beauty trends all that different? With acid peels that can permanently damage skin, to injections of the most acutely lethal toxin known to man – under its brand name Botox – the quest for a healthy-looking face has always been fraught with danger.

The reason we put our faces through all of this is straightforward: a healthy looking face brings huge benefits. Healthy looking people are more attractive; we’re more likely to vote for healthy-looking politicians, and a healthy appearance is preferred in faces across the globe.

Scientists have uncovered numerous facial qualities that are linked to health, and have found that people rely on these to judge who is healthy and who isn’t. Some you’re stuck with, like your facial symmetry, or how close your facial shape is to the average shape of the population. These have been shown to be related to health but are fixed aspects of your face.

Others you can change, with a bit of effort. Facial adiposity – the weight that’s carried in your face – for example, is related to your BMI and how many colds you have a year.

More recently, I and others have been interested in facial colouration and its links with health. Carotenoids, found in fruit and vegetables, are only obtained from eating a healthy diet. When deposited in your skin, they make you look much healthier – even more so than if you have a tan. So a change in diet to include more of these can be good for your health and appearance. Similarly, getting enough sleep can get rid of dark circles under eyes, and a good circulatory system from exercise can redden your cheeks.

But there may be an even easier way to look healthier. And this one is totally free, and very quick.

Consider the faces below. These represent averages of 15 faces rated least healthy (left) and most healthy (right). Which one looks healthier to you?

Do you think it is the one on the right, which has a more positive expression than the left face? Well, that might be because we have found strong links between looking healthy and how happy your neutral expression is.

Our first experiment for this research was to try to predict the health rating a face would receive. We used a measure of how happy others thought the face looked, as well as an objective measure of positive expression, obtained by measuring how open the eyes were and how upturned the corners of the mouth were on the faces.

From this, we were able to predict how healthy a face looked based on its expression alone – with a good degree of accuracy. We found that the effect of having a positive expression on the ratings was similar to that from having a good BMI, using makeup, or looking young rather than old.

We decided to investigate further, and look at what the effect on perceived health would be if the expression were a genuine smile, rather than merely looking a little cheerful, and how it would work across different age groups and sexes.

We discovered that faces look healthier when they are smiling, compared to a neutral expression, and that it doesn’t matter whether the faces are male or female. We also found that this effect increased with the age of the face: while younger adults look healthier when they smile, older adults look much healthier. A smile was a more important cue than how old the face appeared to be, and more recent research has shown a happy expression is even more important than cues like adiposity or skin colouration. Scientific proof that a smile really is the best accessory.

But why do we find smiling faces healthier? The answer may be due to a genuine link between smiles and actual, underlying health. Incredibly, the length of people’s lives have been predicted from photographs of them in their youth. Those photographed with genuine smiles lived longer than those photographed with polite smiles or none at all. New university students who were smiling in their Facebook photos report better social lives and satisfaction upon graduating, and smiling during a stressful experience can calm rising heart rates.

These amazing associations are linked to an aspect of our psychology known as “positive affect”. This is a highly desirable trait to have: it means you live a longer and healthier life by virtue of being optimistic and cheerful. People with greater positive affect look happier and smile more often too, and we think our perceptual systems are on the lookout for individuals with this kind of appearance.

There is a simple message here: looking good costs nothing, and need not be dangerous. All it takes is a smile to look healthier, and the more you do it, the healthier you’ll likely be.

This article was written by Alex Jones, College of Human and Health Sciences and was originally published on The Conversation.