Fluctuating asymmetry (FA) is a type of biological asymmetry, referring to the extent to which small random deviations occur from expected perfect symmetry in different populations of organisms.[52] In humans, for example, FA can be demonstrated through the unequal sizes of bilateral features such as the eyes, ears and breasts. FA acts as an index for measuring developmental instability as it provides a clear indicator of the possible environmental and genetic stressors affecting development.[53] It is thought that having a preference for a symmetrical face offers some adaptive value as such symmetry may signal an individual's ability to cope with environmental challenges.[54] FA shares an inverse relationship with certain desired traits; a low FA is correlated with higher stress tolerance, larger body size in males, smaller body size in females, and higher facial attractiveness.[55] FA is detectable through the olfactory senses and it has a measurable effect on sexual attraction.[40] Significant cues may be found through body odour relating to a potential mate's health, reproductive status and genetic quality and FA is one such cue as it is considered to be a marker of genetic and developmental stability.[56]
During their fertile phase, females have repeatedly been shown as being more attracted to the body odours of more symmetrical males[57] and of males whose faces they rate as attractive.[58] The scent of symmetrical males provides an honest indicator of the male's phenotypic and genetic quality.[57] This may explain why women who are highly fertile find the scent of low FA as attractive and yet this scent is not necessarily as attractive to other women.[40] For example, it has been found that normally cycling women near their peak fertility tended to prefer the odour of shirts worn by symmetrical men and yet women at low fertility in their menstrual cycle or those using contraceptive pills showed no preference for the odour of shirts of symmetrical men compared to those of asymmetrical men.[59] These findings support the good genes hypothesis such that when women are in the fertile phase of their menstrual cycle, they should prefer markers of genetic benefits or 'good genes'.[39] In other words, fertile women who prefer the scent of men with low FA are demonstrating a preference for the genetic benefits associated with those symmetric men.
It is widely accepted that men prefer the scent of women in their most fertile period. For example, in one study, men rated the body odours of T-shirts worn by women during their most fertile phase as more sexy and pleasant than T-shirts worn during their least fertile phase.[4] Despite this, much of the research in the area concludes that the effect of the scent of symmetry appears to be sex specific such that men do not find the scent of symmetrical women more attractive than the scent of asymmetrical women.[40][57] It therefore appears that attraction to symmetric body odour of the opposite sex appears to apply exclusively to women, and specifically fertile women, as non-fertile women and men do not display this preference.
TL;DR
"During their fertile phase, females have repeatedly been shown as being more attracted to the body odours of more symmetrical males[57] and of males whose faces they rate as attractive"
During their fertile phase, females have repeatedly been shown as being more attracted to the body odours of more symmetrical males[57] and of males whose faces they rate as attractive.[58] The scent of symmetrical males provides an honest indicator of the male's phenotypic and genetic quality.[57] This may explain why women who are highly fertile find the scent of low FA as attractive and yet this scent is not necessarily as attractive to other women.[40] For example, it has been found that normally cycling women near their peak fertility tended to prefer the odour of shirts worn by symmetrical men and yet women at low fertility in their menstrual cycle or those using contraceptive pills showed no preference for the odour of shirts of symmetrical men compared to those of asymmetrical men.[59] These findings support the good genes hypothesis such that when women are in the fertile phase of their menstrual cycle, they should prefer markers of genetic benefits or 'good genes'.[39] In other words, fertile women who prefer the scent of men with low FA are demonstrating a preference for the genetic benefits associated with those symmetric men.
It is widely accepted that men prefer the scent of women in their most fertile period. For example, in one study, men rated the body odours of T-shirts worn by women during their most fertile phase as more sexy and pleasant than T-shirts worn during their least fertile phase.[4] Despite this, much of the research in the area concludes that the effect of the scent of symmetry appears to be sex specific such that men do not find the scent of symmetrical women more attractive than the scent of asymmetrical women.[40][57] It therefore appears that attraction to symmetric body odour of the opposite sex appears to apply exclusively to women, and specifically fertile women, as non-fertile women and men do not display this preference.
TL;DR
"During their fertile phase, females have repeatedly been shown as being more attracted to the body odours of more symmetrical males[57] and of males whose faces they rate as attractive"
