Women whose partners have asymmetrical bodies are turned off by them and attracted to other men at certain times of the month, a US study suggests.
Women look for symmetry in a man when it comes to reproduction
This is not down to conscious choice, but is related to survival of the fittest and a desire to pass healthy genes to offspring, say the authors.
When a woman is fertile near ovulation her partner preference shifts, the New Mexico University team found.
The study of 54 couples appears in the Royal Society journal Proceedings B.
Researchers have already noted that women tend to be more attracted to men who have good symmetry, with the suggestion that this means they have "better" genes to pass on to offspring.
However, it is not always possible for a female to attract the most desirable male and some will settle for less but then seek out the genes they want by sleeping around, animal studies have found.
This so-called extra-pair copulation peaks around the time that the female is most fertile.
Dr Steven Gangestad and colleagues reasoned that a woman's interest around the time of ovulation would depend on the qualities of their partner - if the partner had good symmetry she would not be tempted to look elsewhere for a mate.
The team recruited 54 heterosexual couples to take part in their study.
All of the women were aged between 18 and 44 and were ovulating normally - none were taking the oral contraceptive pill.
The women were asked to report on their sexual attraction to and fantasies about both their partners and other men at different times of the month, including both fertile and non-fertile days.
The researchers then compared the ratings with the symmetrical qualities of the partners' different body parts, such as ear length and width.
The women reported greater attraction to their own partners than to other men overall.
Attraction also peaked around ovulation time.
But at this time many of the women whose partners were asymmetrical tended to fantasise and become attracted to men other than their partner. Women with symmetrical partners generally did not.
Dr Nick Neave, a psychologist at Northumbria University, said the findings made sense and backed what was already known about human attraction.
"Females, like males, are always looking to enhance their reproductive success by trading upwards.
"They are always on the lookout for males with better genes or that have high social status/wealth as both enhance their reproductive success - offspring have better genes or access to better resources, both of which enhance their future reproductive success.
"Males simply go for women younger than their current partner as they are thus also able to enhance their reproductive success by producing healthier offspring."