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Fluctuating asymmetry


There are three types of asymmetries:


  • Directional– where a trait normally develops more on one side than the other, e.g., heart.

  • Anti-symmetry – traits develop more on one side, but they are equally common on both sides.

  • Fluctuating asymmetry or FA.


Fluctuating asymmetry (FA) gives a measure of the developmental insults that an organism suffers, with theory saying that the lower the FA, the genetically fitter the organism is. This can be seen where bilaterally symmetrical structures deviate from this pattern. It is not under genetic control, although the magnitude of the FA has a heritable pattern, but not the direction of FA. High levels of FA indicate a degree of maladaptation to the environment and may indicate a relative inability to fight off disease. Higher levels of FA are found in pre-term babies, those with delayed puberty, congenital conditions, etc. The level of FA may increase through life (Kowner 2001). Those high in FA tended to be small, to die earlier, suffer worse health, as well as reduced mating success. Low FA is associated with better health, better cognition, greater musculature in men, more sexual partners, earlier age of first sex and greater number of orgasms of female partners. The effect that FA has on males tends to less than that found in females, probably due to competing evolutionary priorities (Thornhill 1999).


It has been found that in males, FA was negatively related to height, torso volume, shoulder breadth and positively to waist-hip ratio (WHR) and leg length. For females, FA was positively related to height, torso volume and negatively related to WHR and leg length. Those with low FA were found attractive, to have higher ratings of masculinity and femininity (Brown 2008). Symmetrical males are healthier, more fertile and produce better quality sperm, while symmetrical females are healthier and more fertile. There was no difference in attractiveness ratings between normal and symmetric ratings for both men and women. Attractiveness was not related to FA of individual features or female FA (Tovee 2000).


Those who are low in FA are seen as highly desirable partners, as this is a more truthful measure of fitness. Males with low FA tend to have more sexual partners, engage in short term mating and put of long-term mating. Conversely one way for high FA males to increase their inclusive fitness is to gain resources and attract women that way (Takahashi 2006).


FA represents deviation from symmetry in traits that have a right-left population mean of zero and a normally distributed variance. It normally runs at about 1-2% of feature size, with stress increasing this to about 3-5% in some populations. Typically, it is found more often on the right and tends to be non-pathological. It is considered abnormal if the feature deviates by 1-2 SD (Kowner 2001).


What about evidence that links FA to romantic jealousy. University students had 11 measures of various parts of their body to determine levels of FA. All individual measures of FA and a composite score of FA were correlated with self-reports of jealousy (Brown 2003). Heterosexual couples where the females were ovulating normally and were not on hormonal meds were recruited. Women with less symmetrical partners were more attracted to other man and were less attracted to their own partners when they were more fertile (Gangestad 2005). University students who had been infected with toxoplasmosis in the past had facial FA measures taken, as well as other measures. Males previously infected had lower facial FA, whilst previously infected females had a low body mass and BMI. There was a trend for infected females to have lower facial FA, report themselves as attractive and to have more sexual partners. An independent group rated toxoplasmosis infected individuals as more attractive (Borraz-Leon 2022). This might all be to allow greater transmission of the pathogen.


Facial attractiveness


An attractive face may suggest that someone has good genes and there is evidence that it influences mating and is involved in jealousy. There appears to be a global similarity in what is considered facially attractive. Faces with low levels of FA are often judged more attractive even when skin texture and colour symmetry are controlled for. Faces that tend to the average may perform better in terms of various tasks and hence is phenotypically good (Thornhill 1999).


Having feminine facial traits helps males as a masculine face is associated with negative traits e.g., dishonesty. Women favour feminine faces but prefer masculine ones at high fertility. In females who do not use the pill or are in a relationship, there was no relation between masculine preferences and age. So, the desire for masculine faces changes with type of relationship wanted (Little 2002).


Facial attractiveness may activate the nucleus accumbens and the medial orbitofrontal cortex. This activity is modulated if people or the attractive other is in a relationship, leading to a negative appraisal of other attractive faces to reduce the risk of straying (Ueda 2022).


From a health point of view, facially asymmetric men tended to be depressed, more neurotic, less optimistic, tended to be angrier. Whilst facially asymmetric females tended to be extravert, less fearful, more impulsive and dated more. Both genders with facial asymmetry tended to experience more jealousy (Shackleford 1997b).


But studies are inconsistent regarding female preference for masculine or feminine faces in males. Some have suggested that there is a trade of between genetically fitter but behaviourally challenged males and genetically less fit males who make better fathers. It has been suggested that females prefer masculine faces in societies with wealth inequality, poor health, etc. One such study found preference for masculine faces was correlated with household income, the murder and fertility rate, though this may have been moderated by disparity in income (Brooks 2011). A study of 30 countries found preference for masculinity rose as health measures declined, independent of differences in female mating practises and cultural wealth (DeBruine 2010). Others have found that preferences for masculine faces.


Body ratio’s


Those with low second to fourth digit ratios tend to be less likely to use aggression against their partners. Men tend to have longer fourth digits and women tend to the opposite (Bendixon 2015), with a low ratio being linked to high prenatal testosterone and low oestrogen, with the opposite being true for women. Differences in the ratio are found in various populations but after the age of two it is generally constant. In males, a low ratio is related to higher dominance, masculinity, a greater number of partners, aggression and being attractive. For women, a low score is associated with masculinity, and higher neuroticism. Also, a low ratio is associated with right-handedness, agreeableness and verbal intelligence, although many studies have shown negative results (Brewer 2009).


Male undergraduates had their 2D:4D ratios measured on both hands, their facial characteristics were rated by females from another institution and salivary testosterone were taken. Those with lower 2D:4D ratios were more likely to be rated dominant by females (Neave 2003). An investigation of this ratio with jealousy was undertaken in UK undergraduates. They found a negative relationship between the 2D:4D ratio and sexual jealousy in the whole sample, but not for each individual gender, i.e., the lower the ratio the greater the distress for sexual jealousy (Fussell 2011).


A good physical marker of attractive women is their WHR ratio. Low ratios are considered attractive due to their association with fertility, while those with high ratios are more likely to be unfaithful and have more sexual partners (Goetz 2005). The heavier the female, the more likely men may mate with them as some studies have shown that the foetuses of heavier women grow quickly and more offspring survive (Baker 1993). As high levels of WHR in females are associated with more sexual partners, I assumed that it will be related to levels of jealousy, with WHR being represented by a standard normal variable, with levels below ‘0’ indicating low levels, values above ‘0’ representing high WHR and zero representing average levels of WHR. I assumed that as female WHR rises, sexual partners increase and so there will greater levels of jealousy. There also appears to be gender-related differences in the generation of jealousy depending on the body shape of a supposed rival. Women with a low WHR are seen as more attractive, while for males higher WHR is seen as more desirable. So for males I assumed that a low WHR would be associated with more jealousy as these people also would have to compensate for genetic difficulties.


When people were asked about potential rivals, jealousy was evoked more by rivals with low WHR, especially amongst women. There was greater jealousy evoked toward rivals with a higher shoulder-hip ratio (SHR), especially for men. It also seems that the effect of the SHR ratio is more significant at higher WHR, i.e. bigger shoulders evoke more jealousy amongst those with higher WHRs. Females tended to see rivals with a lower WHR and higher SHR as more attractive and more socially dominant. When it came to males, rivals with higher SHR and lower WHR were seen as more physically dominant and attractive, with rivals with lower WHR being seen as more dominant if they had higher levels of SHR. In this study females referred more to rivals’ waist, hips and legs while men mentioned shoulders and stomachs (Dijkstra 2001).


Sperm competition – Intrasexual competition between males seems to continue after sex, in that the sperm compete with each for access to the ova. This may have something to do with female infidelity, as there would be sperm from several males in her reproductive system. Infidelity could have been selected for if there was a history of extra-pair copulations in our evolutionary past and there is abundant evidence of this. If female’s mate with other males, within two to three days after having sex with their partner, then sperm from different males will have to compete to reach the ova (Shackleford 2007). In fact, some feel that jealousy is a result of this behaviour pattern (Goetz 2006b).


The penis may displace semen that is already present by pushing it under the frenulum, if it is inserted about 75% of the way. Males tend to push harder and deeper, if there are allegations of unfaithfulness. After separation, males find their partners more attractive and desirable (Goetz 2005; Shackleford 2007; Shackleford 2004b). This might explain the male refractory period as it reduces the chance of a man removing his own sperm (Goetz 2006b). About a third of sperm are lost after about 30 minutes, but retention is higher if the female orgasms within a minute before to 45 minutes after insemination. During orgasm, via sex or masturbation, results in sperm or acidic vaginal fluids rising up. This means that non-copulatory orgasms may have an antibiotic function, as well as killing sperm (Baker 1993).


Some say that males tend to copulate with women who present the lowest risk of being unfaithful, i.e., when sperm competition will be minimal. Whilst for females’ short-term mating’s allowed access to healthy males, additional resources, potential long-term mates, protection, etc. Suggesting that women prefer unattached males. Mate retention behaviour was associated with the total time since the last copulation (Starratt 2007). Both men and women were less likely to have sex if they were married, to be slightly more likely to stray if they were involved, and both were most likely to stray if they were not in a relationship (Shackleford 2004b). This was in keeping with men mating with women with the lowest sperm competition and for women trying to access long-term partners, as both choose partners with the lowest risk of other sexual contact (Wilson 1992; Barrett 2002b). 


Testicular size is positively related to sperm competition within different primate species, with humans having sizes within the middle of the range. More monogamous primates tend to have smaller testes, than primates where mating’s are more flexible (Simmons 2004). Cues associated with sperm competition may activate sexual arousal in men, presumably to enhance sperm competition (Shackleford 2007; Shackleford 2004b). This variation of testes size may be related to female cuckoldry, with the scrotum providing a reservoir of sperm just in case (Wilson 1992; Barrett 2002b). There may be a balanced polymorphism in testes size between those that are and are not faithful. But testicular volume was not significantly different in those who engaged in extra-pair copulations or not, and in those who did stray there was no correlation between the number of extra-pair partners and testes size (Simmons 2004).


A further test of sperm competition surveyed couples who provided estimates of copulations and masturbations, with ejaculations being collected for males and flow back for women (Baker 1993). The results were as follows:


  • The greater the time spent apart, the greater chance of the women having more than one partner.

  • Males inseminated less sperm as time spent together and number of copulations rose.

  • Median time between copulations was about 90 hours, with the time since the previous copulation being associated with the amount of sperm produced.

  • Number of sperm deposited was related to the proportion of time spent together, time since last sex, and possibly amount of sperm deposited for each hour since last copulation.

  • No association between number of sperm deposited and female orgasm, timing of menstrual cycle or whether the woman was on the pill.

  • Female weight was positively correlated with sperm deposition.

  • The greater the time between sex, the more likely masturbation occurred. Copulation at any point afterwards did not alter observed and expected sperm levels.

  • Female sperm retention was higher if more sperm had been deposited, with more time since the last act of copulation (but not masturbation) and younger sperm were more likely to be retained.


Men in a committed heterosexual relationship from the USA and Germany, found their partner to be more and more attractive the longer they stayed away from them. They also thought that other men might find their partners more attractive, they were more likely to desire sex with their partner and to think that their partner is more sexually attracted to them. These psychological adaptations would result in their sperm being deposited inside the female, so as to compete with sperm from other males (Shackelford 2002). It seems that the number of sperm deposited during the first copulation, after returning to the partner, was positively correlated with the amount of time the two had spent apart from each other (Goetz 2006), so skewing the race to that man’s sperm.


Height – as a characteristic height seems to indicate dominance and there is a female preference for taller men. Height is associated with intelligence, greater resources, and health (Buunk 2008).


Height is a variable that can predict ratings of jealousy. Tall men tend to have more replies to dating adverts, their girlfriends are more attractive, to have better cognitive abilities, higher wages and having more children. Women prefer taller men in the fertile part of the cycle and when looking for short-term relationships. In one study, as people got taller self-reports of jealousy reduced. In terms of how people rated their partner’s interest in others, there was no connection with male height, while short women were more likely to feel their partners were interested in others. Taller men may be less jealous than short men when rivals were socially / physically dominant and physically attractive, but not when rivals were of a higher social status or behaving seductively. Women of average height were more jealous of dominant or high-status rivals than those women who were tall or short. There was an opposite relationship with physical attractiveness, i.e., women of average height were less likely to be jealous of physically attractive rivals than women who were very tall or very short (Buunk 2008).


Some suggest that women who could not get a tall man got a short man and then engaged in copulations with taller men when the opportunity arose. Amongst heterosexual men, there was greater relationship satisfaction, and less cognitive and behavioural jealousy in tall men. They may also be less likely to use love / care to keep a partner but instead use vigilance, using most of the partner’s time and inducing jealousy to keep partners. Because of female preference for taller men, shorter men may be at greater risk of cuckoldry, so it may be advantageous for smaller men to experience cognitive / behavioural jealousy and use more but different mate retention devices and encourage longer-term relationships as a result (Brewer 2009). This may have relevance to the fact that when people perceive others to be better in some characteristic, then jealousy may be aroused (Wieling 2006).


Breasts - Breast milk contains nutrients vital for the baby’s development (Sila-vila 2005). However, breast fat is not used to make milk, thus making the formation of large breasts an evolutionary puzzle (Bateman 2006). Breasts are a source of immune cells and antibodies to help the baby fight infection, etc (Sabbaj 2005). Presumably, changes to the breast may damage its many functions and hence would be expected to cause distress. Women with symmetrical breasts tend to be more fertile and have a greater chance of their children surviving labour (Bereczkei 2000). Some have hypothesised that large breasts signal good quality breast milk, but if large breast size is due to fat deposition this may be a deceptive signal (Low 1986).


Men, who prefer large breasts men tend to date more, are more masculine and tend toward heterosexuality, but this only seems to hold true for slim women. Women who are slender, with low Waist Hip Ratios (WHR) and large breasts were deemed attractive, healthy and preferred for long and short-term mating’s. Those with low ratios were judged more attractive, judged to have higher health rating, were judged as more feminine, and as more understanding and kind. Heavy figures were judged to older, especially if the ratios were high, as were those with larger breasts. In terms of relationships, slender women were preferred, especially if the ratio was small and they had large breasts. Figures with large ratios were more preferred if they had large breasts, as opposed to small breasts (Singh 1995). Low levels of asymmetry in breasts may signal good health in that women with asymmetric breasts may be more likely to have lower IgA in their saliva (Locke 2021).


Could male preference for large breasts, provide an evolutionary drive for bigger breasts and against any genes that encouraged developmental instability? Several have hypothesised that breasts are associated with large amounts of FA. Hence if symmetry is preferred it might be selected for via sexual selection. In women with children, some of whom were breast-feeding, the amount of variation in breast size varied by a magnitude of up to 10%. Absolute FA was related to breast size. Women with children tended to have lower absolute FA than those women with higher FA. In a multi-regression model, as the number of children born increased, the FA was lower (Moller 1995). However, another study found the opposite (Penton-voak 2001).


Breast volume as determined from mammograms for several hundred women were found and the difference between each breast was used to give a measure of fluctuating asymmetry. Now as mean breast volume rose the level of breast FA rose, but instead of one-to-one relationship, larger breasts have lower levels of FA than would be expected from a 1-1 relationship. Also greater breast FA was associated with fewer children and later age of first labour. But the regression coefficients were small. A multiple regression analysis (in women who had at least 1 child) found breast FA was related to breast volume and negatively related to number of children, but again the regression coefficients were small. This suggests that women with more symmetrical breasts tend to more children (Manning 1997)


Pheromones - Pheromone detection occurs via the Vomeronasal organ next to the nasal septum, which projects to the accessory olfactory bulb and then the hypothalamus. Pheromones are produced by apocrine glands in the axillae and pubic areas. Male pheromones are related to 16-androstenes which are converted by coryeform bacteria to form odours. Aliphatic acids production varies with the menstrual cycle.  Pheromones can enhance mood, increase skin conductance, affect sexual behaviour, may have short term attraction / repulsion effects and long-term effects via the HPA axis. Androstenone may alters sexual arousal in men (up) and women (down). People given pheromones tended to have more sex. Interestingly, girls raised by stepdads, reached puberty earlier (the Vandenburgh Affect) (Grammer 2005).


From an evolutionary point of view, some feel that only those with good genes can afford the costs of producing odour signals, especially if they are intense as the cost is greater.  Our body odour is related to number and type of skin glands, hairs which allow bacteria to grow and hence modify smells and our own perception, etc. Women are more sensitive to smells, and this varies with the menstrual cycle and age. The Major Histocompatibility Complex (MHC) seems to be involved e.g., there is correlation between pleasantness of a smell and the similarity of the MHC between the smeller and the person who produced the smell, with the intensity rising as MHC differences rises. Men who were HLA – A/B/DR wore t shirts for 2 days, which were then given to women, who were similar and dissimilar in terms of their MHCs to the males, to smell. The odours of the homozygous and heterozygotes were similar. Also, smells were more intense to MHS dissimilar women, MHC homozygotes did not affect intensity scores for MHC similar women and overall pleasant odours tended to be less intense (Wedekind 2006). As the similarity between women and men’s MHC increases, preferences for male odours decreased. As similarities decreased odours were judged more pleasant and likely to remind women of former mates. There is no gender difference in pleasantness of smells and MHC similarity. There is some inconsistent evidence that married couples tend to be MHC dissimilar. In couples with similar MHC loci, women tended to be less sexually responsive (Wedekind 2007). So there is tentative evidence that the interpretation of body odour is influenced by the MHC complex, and presumably how we respond to our own smells. So, is it possible that how we respond to smells depends on our MHC status? Of course, there are many types of smell, beyond body odour so there are still many questions.


There is further evidence for the involvement of D4 and D2 in delusional processes. The former has been linked to sexual desire and arousal, with D4 48bp VNTR being linked to sexual novelty and D4 48bp VNTR 3R+ possibly being related to age of first sexual contact. D2 intron5/exon6 2 haplotype is associated with early first sexual contact. D2 TaqI A1 may be related to the number of children born. 195 participants were genotyped for D4 48bp VNTR and D2 TaqI A and were given the sociosexual orientation inventory. D4 7R+ wanted children slightly earlier and was little related to marriage age and virgins tended to be 7R- (though this may be age related). Those with A1+ were less likely to want children. Although far from conclusive, it shows that the dopamine system may be involved in jealousy and erotomania (Eisenberg 2007).


Other - Various physiological measures, e.g., skin conductance, heart rate and temperature, all increase when women imagine emotional infidelity and men imagine sexual infidelity. We have seen from Thornhill that a scent associated with low functional asymmetry can be attractive to females during the fertile pat of the cycle. So, could a combination of gender-specific responses to betrayal, attachment style, timing of cycle and functional asymmetry levels be involved in delusional jealousy (as this often involves delusions of infidelity)?

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