It doesn't matter how sparkling a woman's smile is or how lovely her eyes - a cinched-in waist is the most attractive female attribute, a study has found.

The magic measurement, according to the research, is a fraction under 26 and a half inches - or a NZ size 8.

Singer Beyonce and reality TV star Kim Kardashian are said to have waists around this size.

Kim Kardashian and Beyonce are said to have 26inch waists. Photos / Getty Images
Kim Kardashian and Beyonce are said to have 26inch waists. Photos / Getty Images

The formula for the ideal female figure has long fascinated scientists, with more than 150 studies into the subject carried out in the last 20 years alone.

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Many have concluded the secret lies in the size of a woman's waist in comparison to her hips. A waist measurement exactly 70 per cent of the hip circumference usually scores the highest marks.

But the latest study suggests that hip size is a mere distraction.

The US researchers showed men and women photos of university students and Playboy models. They were asked to rate the attractiveness of the woman in each photo and their answers were compared with details of measurements.

Both sexes agreed wasp waists to be the most appealing. As attractiveness increased, waist size dropped dramatically, but hip size did not.

The average waist size of the "real" women - the university students - was 29in but in the most attractive group it was nearly 26 and a half. The researchers, Dr William Lassek of the University of Pittsburgh, and Professor Steven Gaulin, of the University of California, Santa Barbara, said: "Our results support the hypothesis that waist size is the major determinant of female bodily attractiveness.

When you're like I have nothing to wear LOL

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"It seems that for attractiveness to reach a maximum, waist sizes must shrink, but hip size must not shrink in parallel."

When fictional characters were added to the mix - whose anatomy is not constrained by biology - the results appeared to be confirmed.

When 500 volunteers were asked to name the most attractive imaginary woman they could think of, the hourglass shape of cartoon character Jessica Rabbit topped the poll. Other popular picks included Lara Croft, from the Tomb Raider video games, Wonder Woman and Barbie.

Overall, the 16 most popular fictional females had smaller waists than the 56 who ranked below them on the attractiveness scale, the journal Evolutionary Psychology reports.

Cindy Crawford has a 26-inch waist whereas Victoria Beckham's is on the smaller side at only 23 inches. Photos / Getty Images
Cindy Crawford has a 26-inch waist whereas Victoria Beckham's is on the smaller side at only 23 inches. Photos / Getty Images

The researchers said: "When limitations imposed by biology are removed, preferred waist sizes become impossibly small ... Jessica Rabbit, the most attractive imaginary woman and the sexiest cartoon character, has an estimated waist size of just 10in." The US researchers concluded: "In each case, when more attractive are compared with less attractive women, waist size is the measure that distinguishes."

They said that they weren't sure why waist size is so important - but it may be a signal to our animal instincts that a woman isn't pregnant and is sexually available. Other famous names who are said to have 26in waists include model Cindy Crawford. Kelly Brook is believed to come in just under the magic measurement, with a 25in waist, while Victoria Beckham is thought to have a mere 23in waist.

However, many ordinary women will struggle to measure up. Research shows the average British woman now has a 34in waist - seven inches bigger than 60 years ago.