Fluctuating asymmetry is one way in which an organism can deviate from bilateral symmetry, others being anti-symmetry and directional asymmetry. Fluctuating asymmetry is defined as a random asymmetry about a zero mean value, usually of a low magnitude. It can be measured in the body—as in bilateral symmetry of finger lengths—or in a particular organ. It is related to concepts of symmetry such as facial symmetry, and is believed to measure the ability of the genome to successfully canalize and buffer development to achieve a normal phenotype under imperfect environmental conditions, as implied by Waddington's notion of canalization. As such it is a key concept in evolution and development, and underlies concepts such as resilience or developmental stability—the ability to maintain a normal developmental course under stress. More specifically, disease and infection are examples of environmental stressors associated with FA. FA is positively correlated with the prevalence of parasites and disease in an organism, and negatively correlated with effective immune responses. FA is a common tool to measure developmental stability of an organism, where a large FA suggests a low developmental stability  One specific example illustrating such a positive correlation is the relationship between a mother's morning sickness and the daughter's FA. Morning sickness seems to be an adaptation for voiding and avoiding toxins during early fetal life, creating a positive correlation between morning sickness and thigh girth FA . This interest to the application of FA as a measure of developmental stability has looked at studies on inbreeding and outbreeding depression, studies measuring genetic or environmental stress and measures of mate quality for sexual selection.
In individual differences research, FA has been found to have a negative correlation to measurements of human traits such as social dominance, working memory, and intelligence. In old age, facial symmetry has been associated with better cognitive aging. Symmetry has been shown to affect physical attractiveness, with humans exhibiting a preference for symmetrical faces. Facial symmetry has also been positively correlated with higher occurrences of mating. It has also been shown that FA is negatively correlated with IQ. This is also shown to be the case with FA and facial attractiveness in men. It has been suggested this negative correlation may be due to our perceptions of attractiveness developing based upon developmental quality.  Other studies have also found that the voices of men and women with low fluctuating asymmetry are rated as more attractive, suggesting that voice may be indicative of developmental stability. Additionally, fluctuating asymmetry has been shown to predict atypical asymmetry of the brain  Research has additionally shown that growth rates after birth positively correlate with FA. For example increased FA has been found in people who were currently obese.  It has been shown that women's fluctuating asymmetry of thighs (FAT) can be predicted by their mother's morning sickness during the third trimester of pregnancy.
Research has also shown that the female partners of men with lower levels of fluctuating asymmetry (FA) experience a higher number of copulatory orgasms, compared to the female partners of males with higher levels of FA.
There is no gender difference in the susceptibility of diseases depending on body fluctuating asymmetry (FA).
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