Contributions to the scientific literature on subtle asymmetries


Fluctuating asymmetry
(small can be beautiful,
but is it sexy?)

| reviews, meta-analyses, disquieting revelations, analytical tools, follies |

Fluctuating Asymmetry: Reviews (back to top)

  • Zakharov, V. M., and H. J. Graham. eds. 1992. Developmental stability in natural populations. Acta Zool. Fenn. 191 (200 pp.).
    (a special issue with contributed papers on a variety of topics; also an excellent introduction to the extensive Russian literature on the subject)

  • Graham, J. H., D. C. Freeman, and J. M. Emlen. 1993. Developmental stability: A sensitive indicator of populations under stress. p. 136-158 in Environmental Toxicology and Risk Assessment, Landis, et al., eds. Amer. Soc. Test. Mater., Philadelphia, PA.
    (a review of methods and applications in ecotoxicology)

  • Novak, J. M., O. E. Rhodes, Jr., M. H. Smith, and R. K. Chesser. 1993. Morphological asymmetry in mammals: Genetics and homeostasis reconsidered. Acta Theriol. 38: 7-18.
    (suggest that the apparent differences in the sensitivity of FA to heterozygosity between poikilotherms & homeotherms may be due to methodological differences rather than biological ones)

  • Markow, T. A. ed. 1994. Developmental Instability: Its Origins and Evolutionary Implications. Kluwer, Dordrecht. 436 pp.
    (contributed chapters on many topics from animals to plants, from genetic to environmental effects; also includes transcripts of discussions at the Tempe meeting of 1993; most papers also were also published in Genetica, vol. 89)

  • McKenzie, J. A., and P. Batterham. 1994. The genetic, molecular and phenotypic consequences of selection for insecticide resistance. Trends Ecol. Evol. 9: 166-169.
    (reviews the genetic basis of pesticide resistance and fitness modifiers that effect the level of fluctuating asymmetry and antisymmetry)

  • Polak, M., and R. Trivers. 1994. The science of symmetry in biology. Trends Ecol. Evol. 9: 122-124.
    ("the deeper study of symmetry in biology promises many more exciting discoveries"; a mini-review of the Tempe meeting, see Markow 1994 above)

  • Clarke, G. M. 1995. Relationships between developmental stability and fitness: Application for conservation biology. Conserv. Biol. 9: 18-24.
    ("Data are presented from a number of studies that establish a clear relationship between developmental stability and fitness in response to both genetic and environmental stress.")

  • Markow, T. A. 1995. Evolutionary ecology and developmental stability. Ann. Rev. Entomol. 40: 105-120.
    (emphasizes contributions on insects)

  • Palmer, A. R. 1996. Waltzing with asymmetry. BioScience 46: 518-532.
    (outlines the major foci of active work on fluctuating asymmetry, as well as possible connections between fluctuating asymmetries and the evolutionary origin of conspicuous asymmetries)

  • Palmer, A. R. and C. Strobeck. 1997. Fluctuating asymmetry and developmental stability: Heritability of observable variation vs. heritability of inferred cause. J. Evol. Biol. 10:39-49.
    (a critique of the companion paper by Moller & Thornhill (1997) 'Heritability of developmental stability')

  • Hallgrimsson, B. 1998. Fluctuating asymmetry in the mammalian skeleton: Evolutionary and developmental implications. Evol. Biol. 30: 187-251.
    (A nice review of hypotheses regarding the developmental origins of subtle deviations from symmetry)

  • Swaddle, J. P. 1999. Visual signalling by asymmetry: a review of perceptual processes. Phil. Trans. Roy. Soc. Lond. B 354: 1383-1393.
    ("In highly developmentally stable signalling systems the magnitude of asymmetry may be too small to be detected accurately and reliably, hence asymmetry signalling is unlikely to have evolved in these situations.")

  • Van Dongen, S., and L. Lens. 2000. The evolutionary potential of developmental stability. J. Evol. Biol. 13: 326-335.
    (An intriguing analysis showing that as 'hypothetical repeatability' (a measure of the quality of the actual FA data) increases, the correlations between individual asymmetries become more pronounced, suggesting that underlying developmental instability does affect all traits similarly.)

  • Bjorksten, T. A., K. Fowler, and A. Pomiankowski. 2000. What does sexual trait FA tell us about stress? Trends Ecol. Evol. 15: 163-166.
    ("Recent experimental studies show that the relationship between FA and stress is inconsistent, and there is little evidence that sexual traits are especially responsive to stress."; see responses and rebuttal in TREE 15: 330-331.)

  • Hoffmann, A. A., and R. Woods. 2001. Trait variability and stress: Canalization, developmental stability and the need for a broad approach. Ecol. Lett. 4: 97-101.
    ("Published studies that consider multiple environments report little association between the effects of environmental variation on trait canalization and on developmental noise measured as fluctuating asymmetry, suggesting that environmental effects often act independently on these processes.")

  • Hallgrimsson, B., K. Willmore, and B. K. Hall. 2002. Canalization, developmental stability, and morphological integration in primate limbs. Yearbook of Physical Anthropology 45:131-158.
    ("We test three developmentally motivated hypotheses about the patterning of variability components in the mammalian limb.")

  • Lens, L., S. Van Dongen, S. Kark, and E. Matthysen. 2002. Fluctuating asymmetry as an indicator of fitness: Can we bridge the gap between studies? Biological Reviews 77:27-38.
    ("There is growing evidence from both experimental and non- experimental studies that fluctuating asymmetry does not consistently index stress or fitness. The widely held-yet poorly substantiated-belief that fluctuating asymmetry can act as a universal measure of developmental stability and predictor of stress-mediated changes in fitness, therefore staggers.")

  • Polak, M. ed. 2003. Developmental Instability (DI): Causes and Consequences, Oxford University Press, Oxford.
    (A wide-ranging overview of issues relating to DI and FA. Most of the major investigators studying FA have contributed chapters; the book may be ordered here)

Fluctuating Asymmetry: Meta-analyses (back to top)

Meta-analysis is a valuable tool for summarizing results from the literature quantitatively. However, if the literature has been badly biased by selective reporting, meta-analysis will merely reinforce the impact of publication bias.

  • Leung, B., and M. R. Forbes. 1996. Fluctuating asymmetry in relation to stress and fitness: Effects of trait type as revealed by meta-analysis. Ecoscience 3: 400-413.
    ("[W]e found that FA-stress and FA-fitness relations were non-spurious, despite the large number of relations tested. However, FA-stress and FA-fitness relations were fairly weak and highly heterogeneous."; see Palmer 2000 for funnel graphs of these results)

  • Moller, A. P. 1997. Developmental stability and fitness: A review. Amer. Nat. 149: 916-932.
    ("A review of available information from the literature on the relationship between developmental instability and various fitness components such as growth, fecundity, and longevity suggests that there indeed is a general negative relationship."; but see Clarke 1998 American Naturalist 152: 762-766 for a critique)

  • Moller, A. P., and R. Thornhill. 1997. A meta-analysis of the heritability of developmental stability. J. Evol. Biol. 10: 1-16.
    ("This indicates that there is a significant additive genetic component to developmental stability."; however, see the numerous serious critiques also published in the same issue)

  • Moller, A. P., and R. Thornhill. 1998. Bilateral symmetry and sexual selection: A meta-analysis. Amer. Nat. 151: 174-192.
    ("There is considerable evidence that the magnitude of the negative correlation between fluctuating asymmetry and success related to sexual selection is greater for males than for females, when a secondary sexual trait rather than an ordinary trait is studied, with experimentation compared with observation, and for traits not involved with mobility compared with traits affecting mobility."; but see Palmer 1999 for a critique)

  • Thornhill, R., and A. P. Moller. 1998. The relative importance of size and asymmetry in sexual selection. Behav. Ecol. 9: 546-551.
    ("The results lend support to the conclusion that symmetry plays an important general role in sexual selection, especially symmetry of secondary sexual characters.")

  • Palmer, A. R. 1999. Detecting publication bias in meta-analyses: A case study of fluctuating asymmetry and sexual selection. Amer. Nat. 154: 220-233.
    (A funnel-graph analysis "revealed that selective reporting appears to be widespread in studies of FA and sexual selection. It was noticeable among both published and unpublished studies. It was more pronounced among studies conducted by the authors of the original meta-analysis (Moller and Thornhill) than among other authors and, among studies by these two authors it was more pronounced where correlations with asymmetry were predicted by them to be stronger, such as for secondary sexual traits compared to ordinary traits. Finally, among all studies, the prevalence of large effects among studies of small sample size for secondary sexual traits suggests that conclusions about differences in the correlation between asymmetry and attractiveness for secondary sexual traits compared to ordinary traits are likely overstated.")
    (download full-text pdf file, 269K)

  • Vollestad, L. A., K. Hindar, and A. P. Moller. 1999. A meta-analysis of fluctuating asymmetry in relation to heterozygosity. Heredity 83: 206-218.
    ("Our analysis suggests, at best, only a weak association between H and FA, and heterozygosity seems to explain only a very small amount of the variation in developmental instability among individuals and populations (r^2 = 0.01 for the total material)")

  • Palmer, A. R. 2000. Quasireplication and the contract of error: Lessons from sex ratios, heritabilities and fluctuating asymmetry. Ann. Rev. Ecol. Syst. 31: 441480.
    ("Funnel-graphs of published results, including: (a) sex-ratio variation in birds, (b) field estimates of heritabilities, and (c) relations between fluctuating asymmetry and individual attractiveness or fitness, suggest selective reporting is widespread and raise doubts about the true magnitude of these phenomena.")
    (abstract and pdf download available here; data files used in the analyses are available here)

  • Moller, A. P., and M. D. Jennions. 2002. How much variance can be explained by ecologists and evolutionary biologists? Oecologia 132:492-500.
    ("Analysis at the level of individual tests of null hypotheses showed that the amount of variance key factors explained differed among fields with the largest amount in physiological ecology, lower amounts in ecology and the lowest in evolutionary studies. In all fields though, the hypothesized relationship (e.g. main effect of a fixed treatment) explained little of the variation in the trait of interest.")

  • Jennions, M. D., and A. P. Moller. 2002. Relationships fade with time: a meta-analysis of temporal trends in publication in ecology and evolution. Proceedings of the Royal Society of London Series B-Biological Sciences 269:43-48.
    ("We examined the relationship using effect sizes extracted from 44 peer-reviewed meta- analyses covering a wide range of topics in ecological and evolutionary biology. On average, there was a small but significant decline in effect size with year of publication.")

Fluctuating Asymmetry: Disquieting revelations (back to top)

Concerns are rising that the apparent widespread support for associations with fluctuating asymmetry (FA) may be overstated. In particular, as is often the case in science, published studies are not a random sample of all studies that were conducted. Where the signal is small, and enthusiasm for preferred hypotheses high, selective reporting becomes a potentially serious problem. These concerns do not invalidate the use of FA as a measure of developmental precision, but they do suggest the literature may not be a reliable guide to the biology.

  • Houle, D. 1998. High enthusiasm and low R-squared. Evolution 52: 1872-1876.
    (A thought-provoking but damning review of Moller & Swaddle's 1997 book Developmental Stability and Evolution. Essential reading for anyone interested in FA. Draws attention to a fundamental contradiction: subtle departures from symmetry can not simultaneously be a "reliable signal of individual quality or fitness", as claimed by Moller & Swaddle, and yet uncorrelated with other subtle asymmetries on the same individuals, as Moller & Swaddle also freely acknowledge. Also documents a case of unethical behavior by Moller.)

  • Palmer, A. R. 1999. Detecting publication bias in meta-analyses: A case study of fluctuating asymmetry and sexual selection. Amer. Nat. 154: 220-233.
    (Illustrates how funnel graphs of published results can help reveal evidence of selective reporting, and one way in which published results become biased: exclusion of data inconsistent with a preferred hypothesis.)
    (download full-text pdf file, 269K)

  • Simmons, L. W., J. L. Tomkins, J. S. Kotiaho, and J. Hunt. 1999. Fluctuating paradigm. Proc. Roy. Soc. Lond. B 266: 593-595.
    (A startling review showing that as more authors conducted formal tests of the significance of FA relative to measurement error, fewer and fewer detected significant associations between FA and sexual selection.)

  • Palmer, A. R., and L. M. Hammond. 2000. The Emperor's codpiece: A post-modern perspective on biological asymmetries. Int. Soc. Behav. Ecol. Newsl. 12: 13-20.
    (Ever feel that some claims about associations between FA and quality or fitness have been, well, a bit overstated? Learn how the Traumweber brothers convinced the Emperor of the mythical kingdom of Glucklichtal to introduce a National Health and Education Plan to promote symmetry.)
    (download full-text pdf file, 104K)

  • Palmer, A. R. 2000. Quasireplication and the contract of error: Lessons from sex ratios, heritabilities and fluctuating asymmetry. Ann. Rev. Ecol. Syst. 31: 441-480.
    (Illustrates how selective reporting may be a serious problem in other areas of ecology and evolution.)
    (abstract and pdf reprint available here; data files used in the analyses are available here)

  • Helm, B., and H. Albrecht. 2000. Human handedness causes directional asymmetry in avian wing length measurements. Anim. Behav. 60: 899-902.
    (A fascinating study showing how statistical support for subtle deviations from symmetry may arise entirely as an artifact of measurement biases introduced by human handedness.)

  • Palmer (posted 2005). Irregularities in data reported by A.P. Moller (unpublished MS)
    (Scatterplots of tail asymmetry versus tail size in barn swallows in early papers by A.P. Moller differ from those in a later paper and the differences correlate with the intent of the study.)
    (download full-text pdf file: 128K)
    (view entire exchange regarding this document on the EvolDir listserv)
    Additional questions have also been raised about questionable data from, or conduct by, Anders Moller.

Fluctuating Asymmetry: Analytical Tools (back to top)

DOWNLOAD TIPS. Netscape users: (a) click and hold on the link, then choose "Save this link as ...", or (b) opt-click on the link, then (c) choose "source" from the resulting dialogue box. Internet Explorer users: consult your friendly Microsoft tech support staff.

  • FA Primer: Palmer, A.R. (1994) Fluctuating asymmetry analyses: A primer. Pp. 335-364 in Developmental Instability: Its Origins and Evolutionary Implications, T. A. Markow, ed. Kluwer, Dordrecht, Netherlands.
    (a review of methods and common pitfalls, plus a checklist and collection of statistical recipes; download a pdf version of the original MS with figures; 201K).

  • FA Analysis review: Palmer, A.R. & C. Strobeck (2003). Fluctuating asymmetry analyses revisited. In Press, in Developmental Instability (DI): Causes and Consequences, M. Polak, ed. Oxford University Press, Oxford, pp. 279-319 (the book may be ordered here)
    (a updated overview of methods, applications and common pitfalls; download a pdf version of the original MS with figures; 217K; download a pdf version of an appendix that illustrates a worked example of an FA calculation in detail; 103K).
  • FA Spreadsheet: Download an Excel spreadsheet (binary Excel'98 file, 27K) to conveniently compute various fluctuating asymmetry statistics and their significance levels (revised May 15, 2002).

  • Sample FA Dataset: Download the sample data used for the examples in the FA spreadsheet above (binary Excel'98 file, 42K). The sample data file includes 3 replicate measurements of each of the four distal-most limb segments of the right and left, posterior-most walking leg of 40 female shore crabs (Hemigrapsus nudus). A complete analysis of these data is published in Chippindale & Palmer. 1993. Persistence of statistical departures from symmetry over multiple molts in individual brachyuran crabs: Relevance to developmental stability. Genetica 89:185-199 (see Table 1, pg.189).

  • Other published FA data: Data from three published meta-analyses of FA variation are available here.

Fluctuating Asymmetry: Follies (back to top)
("When the going gets weird, the weird turn pro" Hunter S. Thompson)

Because of the ease with which fluctuating asymmetry (=FA) may be measured, and its undeniable appeal as a measure of developmental precision, some remarkable claims have been advanced regarding its power as a predictive tool. I welcome submissions of other examples (editorial commentary in italics).

  • FA, scent, & human attractiveness: Smell the symmetry! "This study examined whether women's olfactory preferences for men's scent would tend to favour the scent of more symmetrical men, most notably during the women's fertile period. College women sniffed and rated the attractiveness of the scent of 41 T-shirts worn over a period of two nights by different men. Results indicated that normally cycling (non-pill using) women near the peak fertility of their cycle tended to prefer the scent of shirts worn by symmetrical men. Normally ovulating women at low fertility within their cycle, and women using a contraceptive pill, showed no significant preference for either symmetrical or asymmetrical men's scent. A separate analysis revealed that, within the set of normally cycling women, individual women's preference for symmetry correlated with their probability of conception, given the actuarial value associated with the day of the cycle they reported at the time they smelled the shirts." (from Gangestad &. Thornhill. 1998. "Menstrual cycle variation in women's preferences for the scent of symmetrical men." Proc. Roy.Soc. Lond. B 265: 927-933)

    "The current study replicated these [earlier] findings with a larger sample and statistically controlled for men's hygiene and other factors that were not controlled in the first study. The current study also examined women's scent attractiveness to men and found no evidence that men prefer the scent of symmetric women. We propose that the scent of symmetry is an honest signal of phenotypic and genetic quality in the human male, and chemical candidates are discussed." (from Thornhill & Gangestad. 1999. The scent of symmetry: A human sex pheromone that signals fitness? Evol. & Hum. Behav. 20: 175-201)

    "The results showed a significant positive correlation between facial attractiveness and sexiness of body odour for female subjects. We found positive relationships between body odour and attractiveness and negative ones between smell and body asymmetry for males only if female odour raters were in the most fertile phase of their menstrual cycle." (from Rikowski & Grammer. 1999. Human body odour, symmetry and attractiveness. Proc. Royal Soc. Lond. B 266:869-874)

    Remarkable! Hence even blind women should be able to recognize more symmetrical males, and thus be more likely to achieve orgasm with them . . . see 'FA & human female orgasm' below. Pretty damned puzzling, though, that a blind male would not be able to 'smell' a more symmetrical female. After all, by definition, symmetrical females bear better genes.

  • FA & human female orgasm. "Women with partners possessing low FA and their partners reported significantly more copulatory female orgasms than were reported by women with partners possessing high FA and their partners, even with many potential confounding variables controlled. The findings are used to examine hypotheses for female orgasm other than selective sperm retention." (from Thornhill, Gangestad & Comer. 1995. "Human female orgasm and mate fluctuating asymmetry", Anim. Behav. 50:1601-1615)

  • "Results support the hypothesis that women mated to more attractive men are more likely to report an orgasm at the most recent copulation than are women mated to less attractive men, after statistically controlling for several key variables." (from Shackelford, Weekes-Shackelford, LeBlanc, Bleske, Euler, & Hoier. 2000. Female coital orgasm and male attractiveness. Human Nature. Interdisc. Biosocial Persp. 11: 299-306)

    Upon reading these, many wives will no doubt encourage their husbands to visit a plastic surgeon to have their face straightened up; and since presumably the reverse is true as well, husbands should encourage similar plastic surgery for their wives, in place of Viagra and its potential side effects . . . what a deliciously simple solution.

  • FA & human health. "We offer this paper as a 'wake-up call' to the health professions on the importance of developmental stability as a marker of good health. It is proposed that developmental stability is an important marker of human health. Our goal is to initiate formally the integration of the sciences of evolutionary biology, developmental biology and medicine. We believe that this integrative framework provides a significant addition to the growing field of Darwinian medicine. Developmental instability may be the best measure of phenotypic and genetic quality available. Evidence suggests that fluctuating asymmetry may be comparable to the physician's thermometer -- both are sensitive indicators of departure from homeostasis." (from Thornhill & Moller. 1997. "Developmental stability, disease and medicine." Biol. Rev. (Cambr.) 72: 497-548)

    One wonders if Thornhill & Moller have now abandoned trips to their doctor in preference for simple measurements that they can conduct in the privacy of their own home?

    "FA was significantly associated with two health measures: body mass index (BMI), but only for females (more asymmetric = greater BMI), and number of medical conditions (asymmetric subjects were more likely to report they had two or more medical conditions), FA was not associated with waist/hip ratio, systolic blood pressure (BP), blood cholesterol, cardiorespiratory fitness, and periodontal disease." (from Milne et al. 2003. " Fluctuating asymmetry and physical health among young adults". Evolution & Human Behavior 24:53-63)

    Ah well, no one really meant that FA was a perfect tool, just a 'sensitive' one.

  • FA & human IQ. Furlow, Armijo-Prewitt, Gangestad, & Thornhill (1997, Proc. Roy. Soc. Lond. B 264:823-830) "claim that fluctuating asymmetry correlates between 0.20 and 0.25 with psychometric intelligence. Moreover, this is probably an underestimate -- values of 0.30 to 0.35 might be expected in experiments using a better IQ test criterion and subjects from the full range of IQ, rather than the undergraduates from the University of New Mexico studied by Furlow & colleagues." "they suppose that there is a real, common, causal link between bodily asymmetry and lowered IQ. Indeed, they are prepared to estimate that anything between 17 and 50 per cent of the variability in IQ is attributable to [the causes of higher fluctuating asymmetry]." "Expect many attempts at replication -- low-life demonstrations that blacks/half-breeds/less-favoured races suffer more from fluctuating asymmetry than white, middle class golden youth. Also expect, in general, a failure to recognize that Furlow and his colleagues may just have glimpsed a way of reconciling the long-standing antagonism between chiropodists ['biological determinists . . . who have their feet firmly planted in biology'] and sky pilots ['artificial-intelligence researchers and constructors of psychometric tests']." (from Blinkhorn, 1997. "Symmetry as destiny -- taking a balanced view of IQ", Nature 387:849-850)

  • FA & detection of human fertility. "We present evidence from studies of non sexually selected traits (ear and digit size) and a sexually selected trait (breast size) that, in characters made up wholly or in part of soft tissue, CA [cyclical asymmetry] varies across the menstrual cycle in women. It is highest at the beginning and end of the cycle, when women are generally infertile, and low in mid cycle, when fertility is highest. Furthermore in mid cycle there is an indication of a transitory (24 hour) increase in CA followed by a substantial decrease, which may indicate ovulation. Temporal changes in CA could therefore be used by males to indicate a female's position in the cycle." (from Manning, Scutt, Whitehouse, Leinster, & Walton. 1996. "Asymmetry and the menstrual cycle in women." Ethol. & Sociobiol. 17: 129-143)

    Closer examination revealed that FA was an even more precise predictor of the day of fertility (after all, we know that FA is an 'absolutely honest' signal):

    "This paper presents evidence that symmetry in four paired soft tissue traits showed a marked increase on the day of ovulation. The difference (i.e. the asymmetry) between the size of the left and right trait in ears, 3rd, 4th and 5th digits was measured. The timing of ovulation was confirmed by real time pelvic ultrasonography and trait measurements were made without knowledge of scan results. Asymmetry was lowest on the day of ovulation (day 0), decreasing by about 30% from day 1, and significant within subjects changes occurred from days 2 to day 1, and day 1 to day 0." (from Scutt & Manning. 1996. "Symmetry and ovulation in women." Hum. Reprod. 11: 2477-2480)

    Clearly, messy tests of vaginal mucous and tiresomely repetitive measurements of body temperature are no longer needed; just rush your local hardware store for a pair of (very accurate!) calipers.

  • FA & human drug sensitivity. "Objective: To determine the mediating effects of developmental instability on individual differences in response to caffeine. Background: Individual variation of drug effects might reflect broad genomic factors as well as the direct effects of specific alleles. The current study tested the hypothesis that individual differences in developmental instability, in part determined by genomic characteristics, would predict individual variation in the magnitude of caffeine-induced verbal memory deficits. . . . Results: Consistent with predictions, a composite measure of developmental instability predicted the magnitude of caffeine-induced memory decrements. Conclusions: These results may have important implications for the genetic underpinnings of individual differences in drug effects." (from Jung, Yeo & Gangestad. 2000. "Developmental instability predicts individual variation in verbal memory skill after caffeine ingestion." Neuropsychiatry Neuropsychol. & Behav. Neurol. 13:195-198)

    Best to check your symmetry and switch to herbal tea if you think you might be a bit too asymmetrical.

  • Human breast FA & fecundity. "Here we show that (1) large breasts have higher levels of fluctuating asymmetry than small breasts, (2) breast fluctuating asymmetry is higher in women without children than in women with at least one child, (3) breast fluctuating symmetry is a reliable predictor of age independent fecundity, and (4) breast fluctuating symmetry appears to be associated with sexual selection." "Choosy males that prefer females with symmetrical breasts may experience a direct fitness benefit in terms of increased fecundity and an indirect benefit in terms of attractive or fecund daughters." (from Moller, Soler, & Thornhill. 1995. "Breast asymmetry, sexual selection, and human reproductive success" Ethology & Sociobiology 16: 207-219; see also Manning, Scutt, Whitehouse, & Leinster. 1997. "Breast asymmetry and phenotypic quality in women." Evol. & Hum. Behav. 18: 223-236)

    "No need to worry, ma'am, we've made sure the calipers are warm."

  • Breast FA & human attractiveness. "Results from [two] tests show that figures with low WHRs [waist-to-hip ratios] were judged to be more attractive than figures with high WHRs, regardless of their degree of breast asymmetry. The figure with low WHR and symmetrical breasts was judged to be most attractive and youngest of all other figures. It appears that men use both WHR and breast asymmetry in judging attractiveness and being willing to develop romantic relationships. It is proposed that WHR and breast asymmetry may signal different aspects of overall female mate quality." (from Singh, 1995. "Female health, attractiveness, and desirability for relationships: Role of breast asymmetry and waist to hip ratio." Ethol. & Sociobiol. 16: 465-481.)

    Once again, plastic surgery or breast implants are recommended for those unfortunately asymmetrical, but at least a simple and reliable solution is at hand.

  • Breast FA & risk of breast cancer. "There was evidence that breast cancer patients had more breast asymmetry and larger breasts than age-matched healthy women. The former observation is the first evidence that high breast asymmetry may be a risk factor for breast cancer. Breast asymmetry is likely to be a predictor of, rather than the effect of breast cancer." (from Scutt, Manning, Whitehouse, Leinster, & Massey. 1997. "The relationship between breast asymmetry, breast size and the occurrence of breast cancer." Brit. J. Radiol. 70: 1017-1021)

    Further evidence that sophisticated medical diagnostic methods may be replaced with simpler and more reliable caliper measurements; see FA & human health above.

  • FA & human sociosexuality. "Men who were more symmetrical and who had a more unrestricted sociosexual orientation were more likely to use direct competition tactics than were less symmetrical and restricted men. Restricted men accentuated their positive personal qualities, presenting themselves as 'nice guys.' Structural equation modeling revealed that fluctuating asymmetry (FA) was directly associated with the use of direct competition tactics. However, the link between FA and presenting oneself as a nice guy was mediated through sociosexuality. No effects were found for women." (from Simpson, Gangestad, Christensen, & Leck. 1999. "Fluctuating asymmetry, sociosexuality, and intrasexual competitive tactics." J. Personal. & Soc. Psychol. 76: 159-172)

    What a pity, that 'nice guys' must also bear the unfortunate burden of higher FA, and therefore poorer health (see FA & human health), lower ejaculate quality (see FA & human male ejaculate), and less sexually satisfied partners (see FA & human female orgasm). Jeez . . . nice guys never win. Maybe the association in women was overwhelmed by the confounding effects of menstrual cycle on FA (see FA & detection of human fertility).

  • FA & human male ejaculate. "We show that, in a sample of 53 men from an infertility clinic, a measure of overall absolute fluctuating asymmetry (FA) in digits 2 to 5 was negatively related to sperm number per ejaculate, sperm speed, and sperm migration, and overall relative FA was negatively related to sperm number and sperm speed. Subjects who had few or no sperm in their ejaculates (azoospermics) tended to have high FA, particularly when obstructive azoospermia was the likely diagnosis." (from Manning, Scutt, & Lewis-Jones. 1998. "Developmental stability, ejaculate size, and sperm quality in men." Evol. & Hum. Behav. 19: 273-282)

    One wonders whether those with more asymmetrical digits also had more asymmetrical sperm. Should be true, of course, since symmetry is such a reliable predictor of genetic quality.

    "Comparison of our data with previous studies suggests that the putative relationship between semen quality and 2D:4D [ratio of second to fourth digit] may have been driven by the inclusion of severely oligozoospermic men within the original subject group. Our sample included men with equally high 2D:4D ratios but who had normal semen. Thus, the 2D:4D ratio may not reliably indicate poor semen quality although FA might." (from Firman, Simmons, Cummins & Matson. 2003. Are body fluctuating asymmetry and the ratio of 2nd to 4th digit length reliable predictors of semen quality? Human Reproduction 18:808-812.)

    Pity, the 2D:4D ratio seemed so promising; good thing we still have FA.

  • FA & human depression. "We found the [Beck Depression Index, BDI] was positively related to fluctuating asymmetry in men but not women, i.e., asymmetric men reported more depression. Digit length (divided by height), particularly digit 4, was positively related to the BDI in men but not women. In men a tendency toward depression may be one cost associated with high prenatal testosterone concentrations" (from Martin, Manning, & Dowrick. 1999. "Fluctuating asymmetry, relative digit length, and depression in men." Evol. & Hum. Behav. 20: 203-214)

    Many psychiatrists must now be incorporating caliper measurements to improve the reliability of their diagnoses. It's just soooo simple.

  • FA & human running ability. "Deviations from perfect bilateral symmetry were measured in seven traits; ear size, nostril width, 2nd to 5th digit length and wrist width. After measurements were made the subjects were ranked for athletic ability and they reported their best 800 metre and 1500 metre times. Symmetric subjects had higher rankings for athletic ability (nostrils, p<0.001 and ears, p<0.001 ), lower best 800 metre times (nostrils, p<0.05 and ears, p<0.01) and lower best 1500 metre times (3rd digit, p<0.01 and ears, p<0.05) than asymmetric subjects." "We conclude that symmetry in traits such as nostrils and ears indicates good running ability. It may therefore be useful in predicting the future potential of young athletes." (from Manning & Pickup. 1998. "Symmetry and performance in middle distance runners." Int. J. Sports Med. 19: 205-209)

    Good thing they didn't measure asymmetry in the long bones of arms and legs, as these exhibit significant directional asymmetry and would have messed up a mighty nice correlation (see Jolicoeur 1963. "Bilateral symmetry and asymmetry in limb bones of Martes americana and man" Revue Canadienne du Biologie 22: 409-432); and there's also this curious business about why foot races and horse races are always run counter-clockwise 'round the track.

  • FA & human metabolism. "It is shown that in males (but not in females) resting metabolic rate (RMR) is positively related to FA. This is explained in terms of the balanced energy equation of males. Sexual selection for large body size (resulting from male-male competition) and low FA (a consequence of mate choice) results in a stress on the provision of energy for growth and the maintenance of symmetry. . . . High-quality males are better able to withstand the stress of sexual selection than low-quality males. The former have ''energy-thrifty genotypes'' and are able to allocate more energy to growth and reducing FA than the latter." (from Manning, Koukourakis, & Brodie. 1997. "Fluctuating asymmetry, metabolic rate and sexual selection in human males" Evol. & Hum. Behav. 18:15-21)

    "We conclude that some men show changes in [soft-tissue] asymmetry over periods as short as 24 h and, although the mechanism driving these asymmetry changes is unknown, the temporal pattern of asymmetry is related to changes in a number of hormones." (from Manning, Gage, Diver, Scutt, & Fraser. 2002. Short-term changes in asymmetry and hormones in men. Evol. & Hum. Behav. 23:95-102)

    These studies overlook another obvious way to improve your fitness: simply by eating more you can increase the energy available for "growth and reducing FA". Perhaps Macdonalds should add this attractive possibility to their advertising literature: eat more, get more symmetric, and therefore get more dates (see Moller & R. Thornhill. 1998), at least if you are a guy (sorry ladies, doesn't seem to work if you have two X chromosomes).


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(revised Feb. 18, 2024)