Letter to the editor |
Reply: A study of finger lengths, semen quality and sex hormones in 360 young men from the general Danish population
1 University Department of Growth and Reproduction, Rigshospitalet and 2 Department of Biostatistics, University of Copenhagen, Copenhagen, Denmark
3 To whom correspondence should be addressed at: University Department of Growth and Reproduction, Rigshospitalet, GR-5064, Blegdamsvej 9, DK-2100 Copenhagen, Denmark. E-mail: niels.joergensen{at}rh.hosp.dk
Sir,
We thank Drs Voracek and Dressler for their interest in our publication ‘A study of finger lengths, semen quality and sex hormones in 360 young men from the general Danish population’ (Bang et al., 2005
). We agree that our findings are interesting. It is somewhat peculiar that the average second to fourth digit ratio (2D:4D) found in the Danish men is higher than that found in other studies. There is no reason to suspect that this is a consequence of the different measuring procedures adopted in different studies, which is also the conclusion of the comparison of methods described in the letter by Voracek and Dressler. Thus, it is possible that young Danish men from the general population are feminized in comparison with men from other countries. Such an interpretation would be in line with our previous findings of a lower semen quality among young Danish men than that of men from the eastern part of the Nordic-Baltic area (Jørgensen et al., 2002), and more generally in line with a high frequency of testicular dysgenesis syndrome (TDS) among Danish men (Skakkebæk et al., 2001; Sharpe, 2003). TDS is assumed to arise, at least partly, due to a disturbed development of the testicles during fetal life. This distubance is most likely due to endocrine disruption via exogenous substances having an estrogenic and/or anti-androgenic action. However, it is important to note that this finding has no bearing on the validity of our primary analysis, namely that our relatively large study of young, normal Danish men did not show any reliable association between 2D:4D finger ratio and current testicular function.
As pointed out by Drs Voracek and Dressler, it is conceivable that single measurements of sex hormones might be unreliable to capture subtle associations between 2D:4D and, for example, testosterone. It is worth noticing that in the statistical analysis of the data presented in Bang et al. (2005), the diurnal variation in the secretion of testosterone was investigated as a potential confounder, because the effect of hour of the day of the blood sampling was considered in calculations of relations between 2D:4D and the hormones. However, it is evident that a more precise estimate of the 2D:4D and testosterone association would be achieved by allocating resources to fine tuning the measurement procedures. This technical point should not deflect attention from the fact that the associations found in Bang et al. (2005) did not even indicate supporting evidence for the hypothesis raised by Manning.
Of the many correlations we calculated in our study, we only found two to be statistically significant, but mutually contradictory. An association between 2D:4D finger ratio and an increased FSH level in the group of men having 2D:4D >1. The increased FSH level was—as stated in Bang et al. (2005)—not found to coincide with altered semen parameters, hence indicating a possible compensated reduced spermatogenic capacity. From a biological point of view, this finding supports the Manning theory that the increased 2D:4D finger ratio correlates with reduced male testis function. Contrary to this, an increasing 2D:4D finger ratio was associated with increasing total sperm count in the group of young men having 2D:4D finger ratios 
1. This finding contradicts the Manning hypothesis, because an increased total sperm count should accordingly not coincide with an increased finger ratio. These two statistically significant correlations, of the 14 we made between 2D:4D and testicular function parameters, may be ‘false-positives’ (type 1 errors) rather than representing true associations.
It is worth noticing that the publications following Manning’s original study (Manning et al., 1998) have been unable to support Manning’s hypothesis of an association between 2D:4D and semen quality/testicular function (Firman et al., 2003; Manning et al., 2004; Bang et al., 2005). Therefore, we still conclude that a true biological relationship between finger length and testicular function cannot be excluded, but measurements of finger lengths do not have the power to predict the testicular function of adult men.
References
Bang AK, Carlsen E, Holm M, Petersen JH, Skakkebæk NE and Jørgensen N (2005) A study of finger lengths, semen quality and sex hormones in 360 young men from the general Danish population. Hum Reprod 20,3109–3113.
Firman RC, Simmons LW, Cummins JM and Matson PL (2003) Are body fluctuation asymmetry and the ratio of 2nd to 4th digit length reliable predictors of semen quality? Hum Reprod 18,808–812.
Jørgensen N, Carlsen E, Nermoen I, Punab M, Suominen J, Andersen AG et al. (2002) East–West gradient in semen quality in the Nordic-Baltic area: a study of men from the general population in Denmark, Norway, Estonia and Finland. Hum Reprod 17,2199–2208.
Manning JT, Scutt D, Wilson J and Lewis-Jones DI (1998) The ratio of 2nd to 4th digit length: a predictor of sperm numbers and concentrations of testosterone, luteinizing hormone and oestrogen. Hum Reprod 13,3000–3004.
Manning JT, Wood S, Vang E, Talton J, Bundred PE, van Heyningen C and Lewis-Jones DI (2004) Second to fourth digit ratio (2D: 4D) and testosterone in man. Asian J Androl 6,211–215.[Web of Science][Medline]
Sharpe RM (2003) The oestrogen hypothesis – where do we stand now? Int J Androl 26,2–15.[CrossRef][Web of Science][Medline]
Skakkebæk NE, Rajpert-De Meyts E and Main KM (2001) Testicular dysgenesis syndrome: an increasing common development disorder with environmental aspects. Hum Reprod 16,972–978.
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