Ovulating women unconsciously buy and wear sexier clothes when they are most fertile to beat off competition for a mate — and don't even realize they're doing it.
Research shows women unconsciously dress in sexier clothes when they are more likely to become pregnant.
The team from the University of Minnesota's Carlson School of Management said it was the first study into how hormones influence shopping habits.
"The desire for women at peak fertility to unconsciously choose products that enhance appearance is driven by a desire to outdo attractive rival women,” said Kristina Durante, who headed the study.
"If you look more desirable than your competition, you are more likely to stand out," she said.
The results showed women look at those around them to judge how high to set the bar.
When shown a group of pretty local women, participants opted for sexier clothes and accessories than those shown less beautiful girls who lived more than 1,000 miles away.
"In order to entice a desirable mate, a woman needs to assess the attractiveness of other women in her local environment to determine how eye-catching she needs to be to snare a good man," Durante said.
The study showed women who were not ovulating and not likely to conceive at the time of the tests did not behave in the same way.
"For about five to six days every month, normally ovulating women may be especially likely to purchase products and services that enhance physical appearance,” she said. "Such products include not only clothing, shoes, and fashion accessories, but also cosmetics, health supplements, fitness products, medical procedures, and more."