No association between fluctuating asymmetry in highly stabilized traits and second to fourth digit ratio (2D:4D) in human fetuses

No association between developmental instability as a general measure of stress and 2D:4D digit ratios in a non-western sample

OBJECTIVE

Random deviation from perfect symmetry of organisms is defined as fluctuating asymmetry. Fluctuating asymmetry is widely considered to be a marker of developmental instability, developmental noise and phenotypic quality at the population level. In this study, we investigated hypothesized relationship between 2D:4D digit ratios as a proxy measure of prenatal sex hormones and developmental instability levels in young adult humans.

MATERIALS AND METHODS

In the study, seven bilateral traits were measured for fluctuating asymmetry, as well as second and fourth digit lengths on both hands of 185 individuals, 87 men (mean age: 22.42 ± 3.37) and 97 women (mean age: 22.88 ± 3.87). The second digit length was divided by the fourth digit length and the resulting value was used as the digit ratio (2D:4D). Composite fluctuating asymmetry was calculated using five bilateral traits displaying fluctuating asymmetry.

RESULTS

Only the right hand 2D:4D ratio is lower in men than in women. However, this difference disappears when the effect of digit lengths is controlled. Composite fluctuating asymmetry results reveal that men are more asymmetrical than women. There was no significant relationship between digit ratios and composite fluctuating asymmetry.

CONCLUSION

In this study, the 2D:4D digit ratios appear to be influenced (at least in humans) by different digit lengths. This study also confirms previous studies that digit ratios as a proxy measure of exposure to either high testosterone or estrogen levels during early development does not appear to affect levels of developmental instability.

Anthropometric fluctuating asymmetries in living humans through the eyes of an anthropologist

There are many environmental and genetic factors that disrupt the stable structure of development in organisms. Although the strength of these vary, they leave certain signs in the body structure. Fluctuating asymmetry is a widely used population-level index of developmental instability, developmental noise, and robustness. Many bilateral traits are used in fluctuating asymmetry studies in humans. These traits include dermatoglyphics, limb lengths and widths, bilateral facial characters, and teeth. In this review, I evaluate the measurement methods of many bilateral anthropometric characters, taken from the bodies of living individuals with classical digital calipers.

Fluctuating asymmetry and masculinity/femininity in humans: a meta-analysis

Theory predicts that if particular morphological features correlate with "genetic" quality--i.e., mating with individuals bearing those characteristics increases fitness--selection favors preferences for these features. Both developmental instability (DI)--which emerges morphologically as small random deviations from perfect symmetry, i.e., fluctuating asymmetry (FA)--and sexually dimorphic traits due to variation in sex hormones, like facial masculinity and digit ratios, may reflect mate quality and/or relate to attractiveness. Therefore, FA and morphological expressions of sex hormones may represent interrelated measures/signals of individual quality. This article presents a meta-analysis of these associations, including 44 effect sizes from 16 studies. Mean effect size (Pearson correlation) was nearly zero (r = .04, SE = 0.05). Largest effect sizes occurred for the few studies measuring sex hormone levels directly. This result was, however, unexpected since there is little evidence that hormone levels determined at a single time point would accurately reflect masculinity/femininity. Facial masculinity/femininity, which contains information on the functioning of the endocrine system, did not relate to DI (average effect size: r = .01, SE = 0.04). In spite of a substantial body of literature, there is little evidence for a robust association between DI and sexual dimorphism. Nevertheless, estimates of average effect sizes in DI showed wide confidence intervals. It is, therefore, premature to draw general conclusions in any direction. Further research is clearly required both to study associations between DI and sexual dimorphic morphological traits and the link between these dimorphic traits and levels of reproductive hormones.

Is the Index Finger and Ring Finger Ratio (2D:4D) Reliable Predictor of Semen Quality?

PURPOSE

The purpose of this paper is to test the validity of hypothesis that the index finger and ring finger ratio (2D:4D) is related with men's semen quality.

MATERIALS AND METHODS

We set two groups of healthy young male student (mean age 23.9) who have different 2D:4D ratio from different departments of Chosun University. One group consists of 26 men whose 2D:4D ratio is higher than 1 (mean 1.06), and the other group consists of 33 men whose 2D:4D ratio is lower than 1 (mean 0.94). Their semen was collected by masturbation and examined.

RESULTS

A comparative analysis revealed that there is no relation between semen quality and the 2D:4D ratio. The group of 2D:4D>/=1 showed a semen volume of 3.66+/-1.64 ml, on the other hand, the group of 2D:4D<1 showed a semen volume of 3.73+/-1.40 ml. No statistical correlation was found (p=0.82). The view of sperm count and motile sperm in single ejaculated semen also showed no statistically significance with 2D:4D ratio (p=0.84, p=0.43, respectively).

CONCLUSIONS

The 2D:4D ratio has no statistically significant correlation with the semen quality of health young male. Thus, measurements of finger length cannot be a reliable indicator of semen quality and testicular function.

Testosterone, facial symmetry and cooperation in the prisoners' dilemma

Recent research has analyzed how individual characteristics, like the exposure to different hormones and symmetry, affect decision-making and strategic behaviour. The present article investigates the effect of symmetry, of exposure to testosterone (T) in utero and during puberty and of current T on cooperation in a Prisoners' Dilemma Game (PDG). T is a hormone with well known effect on males' behaviour, and that promotes activities that seek to increase reproductive success. Fluctuating Asymmetry (FA) reflects the ability of the organism to maintain a stable development and it is usually employed as a variable reflecting genetic quality (low FA values are thought to signal higher genetic quality). Our results show that subjects with intermediate levels of second to fourth digit ratio (a proxy of exposure to T in utero) and with high FA cooperate more often in the PDG. We also observe that the latter effect is due to the fact that FA has an impact on subjects' expectations about the behaviour of their counterpart in the game. These results reinforce the described link between markers related to genetic quality and cooperative behaviour. This possible linkage of individual condition and pro-social behaviour in humans clearly merits further attention.

Scientometric analysis and bibliography of digit ratio (2D:4D) research, 1998-2008

A scientometric analysis of modern research on the second-to-fourth digit ratio (2D:4D), a widely studied putative marker for prenatal androgen action, is presented. In early 2009, this literature totalled more than 300 publications and, since its initiation in 1998, has grown at a rate slightly faster than linear. Key findings included evidence of publication bias and citation bias, incomplete coverage and outdatedness of existing reviews, and a dearth of meta-analyses in this field. 2D:4D research clusters noticeably in terms of researchers, institutions, countries, and journals involved. Although 2D:4D is an anthropometric trait, most of the research has been conducted at psychology departments, not anthropology departments. However, 2D:4D research has not been predominantly published in core and specialized journals of psychology, but rather in more broadly scoped journals of the behavioral sciences, biomedical social sciences, and neurosciences. Total citation numbers of 2D:4D papers for the most part were not larger than their citation counts within 2D:4D research, indicating that until now, only a few 2D:4D studies have attained broader interest outside this specific field. Comparative citation analyses show that 2D:4D research presently is commensurate in size and importance to evolutionary psychological jealousy research, but has grown faster than the latter field. In contrast, it is much smaller and has spread more slowly than research about the Implicit Association Test Fifteen conjectures about anticipated trends in 2D:4D research are outlined, appendixed by a first-time bibliography of the entirety of the published 2D:4D literature.