Overall trends vs. individual trajectories in the second-to-fourth digit (2D:4D) and metacarpal (2M:4M) ratios during puberty and adolescence

• http://onlinelibrary.wiley.com/doi/10.1002/ajpa.23153/abstract

2D:4D ratio; adolescence; hand development; longitudinal study; prenatal sexual difference

The 2D:4D ratio can be assumed an indicator of sexualdifferentiation, provided that it is stable once it develops, oreventual changes remain uniform in a respective cohort throughoutontogenesis. The main goal of this study was to determine whetherthe metacarpal 2M:4M and the digit 2D:4D ratio change during theperiod of pubertal/adolescent growth. The metacarpals and digitswere measured on radiographs of left hands in the sample of 328individuals (96 pairs of male and 68 pairs of female twins) from theWrocław Longitudinal Study of Twins (1967-1983). Five consecutiveannual measurements were done for each individual within a 4-yearintervalsomewhere between 7 and 18 years of age. Age-relatedchanges in both ratios were studied using a set of mixed-effectslinear models. Three types of correlation coefficients were used forassessment of stability between repeated measurements at differentages. An overall decrease in the average 2M:4M ratio was observed,attributable to larger extent to males than to females. On thecontrary, a slight overall increase in the average 2D:4D ratio wasobserved, attributable to a much larger extent to females than tomales. The rank order of the ratios remained highly stable withinthe monitored period (the correlation coefficient mostly rangedbetween 0.85 and 0.95). In spite of these findings, we recordedsignificant intraindividual changes in both ratios. In someindividuals the 2D:4D ratio can undergo changes comparable toaverage sex differences and much higher than average age-relatedchanges. Relatively slight overall changes in digit ratio in pubertyand adolescence themselves are not inconsistent with the use of the2D:4D ratio as an indirect marker of prenatal sex differentiation.Nevertheless, individual changes in the ratios varied substantiallyin this study and differed from the average trends. Future studiesshould focus on the nature of interindividual developmentaldifferences in the digit and metacarpal ratios.