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Fink et al. (2004) found that men with low 2D:4D ratios (indicating high testosterone) and women with high 2D:4D ratios (indicating high estrogen) express greater levels of facial symmetry. [76] The prenatal sex hormone ratio (indicated by 2D:4D) was associated with similar alterations to both male and female face shapes in utero, though the ...
A 2017 meta-analysis of fetal testosterone levels found that fetal testosterone levels (measured by the 2D:4D digit ratio) had only a small effect size of 0.036 on aggressive or violent behavior. The authors further concluded that "the 2D:4D digit ratio is neither strongly nor reliably related to aggressive or violent behavior". [3]
On the one hand, homosexual men were hypothesized to be exposed to high levels of testosterone in utero, which would be associated with a lower 2D:4D ratio than that found in heterosexual men. On the other hand, homosexual men have been hypothesized to have low prenatal testosterone exposure.
The 2D;4D finger ratio, or relative lengths of the 2nd "index" and 4th "ring" fingers, has become a popular measure of prenatal androgen because of accumulated evidence suggesting the 2D;4D ratios are related to prenatal exposure to testosterone. [56]
In mice, prenatal testosterone transfer causes higher blood concentrations of testosterone in 2M females when compared to 1M or 0M females. [6] This has a variety of consequences on later female behavior, physiology, and morphology. Below is a table comparing physiological, morphological, and behavioral differences of 0M and 2M female mice. [1]
Digit ratio (2D:4D, the ratio of index to ring finger length) is a widely used as a proxy measure for prenatal testosterone exposure. Hurd demonstrated that men with more feminine typical-digit ratios showed lower aggressive tendency than males with more masculine-typical digit ratios. [10]
There is also evidence against this theory. For example, one possible biomarker for prenatal testosterone's effect on the brain is a low ratio of the second to fourth finger (the 2D:4D ratio), which has been found to be associated with several male-specific psychological factors. A significantly lower 2D:4D ratio than the general population has ...
Noting that men and women had different finger length patterns, Wilson introduced the 2D/4D digit ratio as a marker of exposure to prenatal testosterone in 1983. [19] [20] His theory that autism represented a hypermale brain [21] was influential in autism studies, [22] but has now been widely discredited. [23]