Women with high levels of the sex hormone oestrogen have prettier faces, research suggests.
Typical female face with high (left) and low (right) oestrogen levels
The findings make evolutionary sense - men are attracted to the most fertile women, the University of St Andrews team told a Royal Society journal.
Oestrogen levels during puberty can impact on appearance by affecting bone growth and skin texture, they said.
But make-up masks this effect, allowing less attractive women to compensate for their lack of natural mating cues.
The team of psychologists at the University's Perception Lab photographed 59 young women's faces aged between 18 and 25 and analysed their sex hormone levels.
They then asked 30 volunteers - 15 male and 15 female - to rate the faces according to attractiveness.
Both male and female volunteers rated the faces of the women with the highest hormone levels as the most attractive.
These faces tended to have classically feminine features, such as larger eyes and lips and smaller noses and jaws.
However, when the women in the photographs were wearing make-up, no relationship between attractiveness and oestrogen was found.
The researchers believe that, while make-up improves facial appearance, it may be masking cues normally seen in the face.
Head of the study, Miriam Law Smith, said: "Women are effectively advertising their general fertility with their faces.
"Make-up can improve appearance across the board, but it will obviously help people who are less attractive more."
For example, eye make-up can be used to make the eyes appear bigger and foundation can make the skin look clearer.
"Our findings could explain why men universally seem to prefer feminine women's faces.
"In evolutionary terms, it makes sense for men to favour feminine fertile women - those that did would have had more babies," she added.
Dr Tony Little, a lecturer in the School of Biological Sciences at the University of Liverpool, said: "It's a very interesting study.
"The findings make sense. There are clear benefits to choosing particular types of female faces. Oestrogen is related to fertility.
"The findings about make-up are also interesting. The implication is that women are employing a deceptive strategy. They can fool the male visual system with make-up."
He said his studies with co-worker Craig Roberts showed female attractiveness also fluctuated throughout the menstrual cycle, peaking at a woman's most fertile days.