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Conversely, In Females The Second Digit Has A Tendency To Be The Same Length ...

Conversely, in females the second digit has a tendency to be the same length or even a little longer than the fourth digit. In this sense the 2D:4D ratio is said to be sexually dimorphic, although some experts prefer to use the term ‘sexually differentiated' to express the relatively small gap in digit ratios between sexes. To a large extent the distribution of both high and low 2D:4D ratios overlaps in both sexes. Certainly other traits that are considered sexually dimorphic such as height have larger effect values.

A Brief History of Digit Ratio Research
Research on finger length, digit ratio and its effects can be traced back to the 17th century and through to the present time. In the 1800s it was found through experiment and stated in a number of scientific works that women had longer index fingers than men. In 1952, Phelps demonstrated that the 2D:4D ratio could be identified after just seven weeks of pregnancy, and that the digit ratios of children often correlated with the digit ratios of their parents. Then, in 1975, the findings of Garn et al supported this conclusion. The researchers found that their results confirmed these findings when they studied a number of embryos and foetuses. Next, in 1983 Dr. G Wilson published the first study to examine the relationship of digit ratios to psychological characteristics between members of the same sex. His study explored the relationship between the digit ratio of females and the level of assertiveness they displayed. Wilson hypothesised that the sex hormones present in the uterus had an impact on both personality and bone structure. Despite growing support for the study of digit ratio over the last several decades, the idea that digit ratio is influenced by prenatal hormones was treated with scepticism for some time. It was thought that digit ratio theories were comparable to phrenology and other psychological disciplines that were vigorously pursued and then discarded.
In recent years, digit ratio research has received an ever-increasing amount of attention. John Manning and Mark Breedlove are two experts who have established a long-term, pro-active research programme in this field. In 2002 John Manning released a work containing the sum of digit ratio research up to that point in time, and in it he affirmed that prenatal testosterone does indeed affect psychological traits and digit ratios.
Research on the Effect of Androgens on Digit Ratios
There are now multiple studies that confirm the effect of prenatal androgen exposure upon digit ratio. One commonly cited study by Brown et al (2002) examined females suffering from congenital adrenal hyperplasia (CAH). CAH causes androgen levels in the uterus to become significantly augmented. The results of the study showed that the females in the study had lower 2D:4D ratios, that is, ratios that are more similar to those recorded for men.

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