How male sex hormones can influence sexuality
In women index (second) and ring (fourth) finger tend to have the same length, in men the index finger is the shorter one.
In case you are looking at your hands now, the evidence for masculine non-straight finger length differences is rather tenuous, but this paper finds a clear correlation for gay or bisexual women.
The study involved pairs of twins of both sexes with differing sexual orientation, that is, one member of the pair was straight and the other one non-straight (either gay or bisexual).
For the 14 male pairs that took part of the study, the non-straight twins had "contrary to our prediction, more masculinized finger length rations than straight co-twins", but the difference was not significant.
For the 18 female pairs, the lesbian or bisexual twins had more masculinized index to ring finger ratios than their straight co-twins, but this was only on their left hands
The masculine ratio means that these women's index finger is shorter than their ring finger.
The study used identical twins because they genetic makeup is identical so the differences in their sexual orientation is not genetic and must be due to some other external factor.
Finger length ratios are due to exposure to a higher level of androgen (male sex hormones) in the womb; so it is also probable that this intrauterine hormonal exposure may also influence sexual orientation.
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