Two studies have also reported feminised 2D:4D in 47XXY men with Klinefelter syndrome 27, 28. Notably, the effect sizes observed were ~ 50% smaller than those reported in a meta-analysis published a decade previously 9, implying that early studies may have overestimated the magnitude of these effects. A recent meta-analysis detected significant effects for R2D:4D in males and L2D:4D in females (i.e., lower ratios in people with CAH compared to controls), but not for R2D:4D in females or L2D:4D in males 10. Studies of 2D:4D have also been conducted in relation to congenital adrenal hyperplasia (CAH), a suite of conditions characterised by elevated prenatal androgen exposure. However, although an early small-scale study 23 reported that low frequencies of CAG repeats, indicative of high sensitivity to androgens, were associated with low (male-typical) R2D:4D and D, subsequent meta-analyses have not confirmed a meaningful association 24, 25, 26. Researchers have also tested for associations between 2D:4D and variations in the trinucleotide CAG repeat sequence located on exon 1 of the androgen receptor gene, a genetic polymorphism believed to influence individual differences in androgen sensitivity 22. If differential prenatal exposure to the physiological effects of testosterone influences the development of 2D:4D, lower variance should be expected in the latter group than the former 21. However, both relied on small samples, and, notably, the variance in 2D:4D did not differ between 46XX and 46XY women. Two studies 19, 20 have reported feminised digit ratios in women with CAIS. A female-typical phenotype therefore develops in presence of a male-typical (46XY) karyotype and prenatal hormonal environment. Despite the presence of normal (or even elevated) androgen levels, this condition results in testosterone being unable to exert physiological effects on the developing tissues. Complete androgen insensitivity syndrome (CAIS) is an X-linked recessive condition characterised by defective or absent androgen receptors. Some studies have explored the 2D:4D validity question by examining samples of individuals with medical conditions that affect endocrinological pathways. As there is a vast and rapidly growing literature examining 2D:4D in relation to an extensive range of variables across multiple research fields (e.g., psychiatry 12, social science 13, 14, cancer research 15, criminology 16, sports science 17, 18), it is important to consider this lack of consistent evidence rather than rely on the assumption that 2D:4D is a valid and reliable proxy for the prenatal hormonal environment. Despite enduring popularity, results from studies that have tried to validate digit ratio measures have been equivocal 10, 11. However, it should be noted that this effect size estimate for D may be attenuated this is due to it having been derived from self-measured finger lengths, which are known to be associated with high levels of random measurement error 8. Small-to-medium sized sex differences (male < female) are reliably observed for R2D:4D ( d = 0.457) and L2D:4D ( d = 0.376) 9, but that for D appears to be much smaller ( d = 0.065) 8. Low R2D:4D relative to L2D:4D has been suggested to reflect high prenatal androgen exposure 6, 7, 8. Researchers typically measure digit ratios for the right hand (R2D:4D) and/or left hand (L2D:4D), though sometimes also examine directional asymmetry (D ). More specifically, it has been suggested that a high concentration of testosterone 1, 2 or high ratio of testosterone-to-estradiol (T:E) 3, 4, 5 during prenatal development results in low 2D:4D. The ratio of second (index) to fourth (ring) finger length (digit ratio or 2D:4D) has been hypothesised to reflect individual differences in prenatal exposure to sex hormones. Taken together, the results of our study suggest research with larger samples is required to determine whether digit ratios are valid proxies for maternal sex hormone exposure. However, the first of these effects is in the opposite direction to that predicted by theory. Nevertheless, the multivariate level of analysis did reveal that T correlated positively with left hand digit ratio (L2D:4D) and negatively with D. Although the testosterone (T) to estradiol (E) ratio correlated negatively with right hand digit ratio (R2D:4D) and directional asymmetry (right-minus-left) in digit ratio (D ), neither effect remained statistically significant once demographic and obstetric covariates were controlled for. We report the first pre-registered study to investigate mothers’ early pregnancy sex hormone concentrations in relation to their children’s digit ratios measured at 18–22-month follow-up. The ratio of index to ring finger (2D:4D) has been hypothesised to indicate prenatal androgen exposure, yet evidence for its validity is lacking.
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