Researchers have determined that women whose upper and lower lips are exactly the same thickness are deemed to have the most desirable pouts. And Scarlett Johansson hits the perfect ratio of 1:1.

The findings come from a study of 560 plastic surgeons and the same number of members of the public, drawn from 35 nations. They were asked to look at photographs and choose those with most attractive mouths.

The study was led by Dr Paul Heidekrueger, of Munich, who has previously conducted similar work into the perfect buttock size and ‘the ideal ear placement in Caucasian females’.

Writing in the Journal Of Cranio-Maxillo-Facial Surgery, he concluded ‘a 1:1 lip ratio can apparently be considered most pleasing in females’. The second most attractive look was to have a slightly plumper lower lip.

But he conceded: ‘Our study represents current trends in aesthetics and may be subject to change, much like fashion.’

What’s in a pout?

Well, quite a lot, as it turns out that full and sumptuous lips are the most preferable lip shape of them all – but according to a global study, there’s one key trait that distinguishes the perfect pouts from their equally plump, yet sub-par, counterparts.

A survey of more than 1,000 people from 35 different countries revealed that the key to the ultimate lip shape actually boils down to symmetry.

In the study, which was published in the Journal of Cranio-Maxillofacial Surgery, more than 60 per cent of respondents chose the 1:1 ratio between the upper and lower lip as the most attractive shape.

The second highest-rated pouts were those with a fuller lower lip, like Kylie Jenner, who has built an entire beauty empire based on her infamous (and surgically-enhanced) pout.

However, lead researcher Dr Paul Heidekrueger added that like with all aesthetic trends, these findings are subject to change.

In fact, according to research conducted by leading Harley Street facial cosmetic surgeon Dr Julian De Silva, the ultimate pout belongs to none other than Taylor Swift.

"Taylor’s lips are perfect,” he said, “she has a gorgeous voluptuous pout and a beautifully-shaped cupid’s bow which so many women want.”

A heavily-defined cupid’s bow is the most-requested lip shape in De Silva’s experience.

"Cupid’s bows can be enhanced and further defined with the use of specific amounts of filler in the upper lip to enhance the natural vertical columns that lie immediately about the upper lip,” the surgeon explained.