Sex Differences and Age Changes in Digit Ratios: Implications for the Use of Digit Ratios in Medicine and Biology

Abstract

The term digit ratio is often taken to mean the ratio of the lengths of the second and fourth digits (2D:4D). This ratio has been reported to show sex differences such that on average males have longer fourth digits relative to their second digit than do females. The sex difference in 2D:4D appears in the foetus as early as the ninth week, it is found in infants, children and adults, and the 2D:4D of infants is strongly correlated with their 2D:4D when they become adults. The sex difference results from a different male and female growth trajectory of 4D relative to 2D. It has been suggested that the sex dependent growth trajectory of 4D relative to 2D is the consequence of the effects of prenatal testosterone (PT) relative to prenatal estrogen (PE) on finger growth, such that high PT and low PE may result in low values of 2D:4D. Excluding the thumb, the fingers show six ratios and a number of these show sex differences. However, only 2D:4D, 2D:3D and 3D:4D show both sex differences and are relatively stable with growth in children. Therefore, 2D:4D, 2D:3D and 3D:4D are the most likely digit ratios to reflect levels of prenatal sex steroids. Diseases that show a sex difference in their expression may be influenced by PT and PE. Most work has concentrated on 2D:4D and its links to developmental disorders (e.g. autism, Asperger’s syndrome and ADHD), cardio-vascular disorders (e.g. MI), cardiovascular efficiency (e.g. running speed), and cancers (e.g. breast and cervical cancer). Further work is necessary to quantify the strength of such links in order to establish whether 2D:4D may be of predictive value for these diseases. In addition other links with such diseases as prostate cancer are likely to be investigated.