Hold up your hand.

That ring finger is wiggling evidence by itself of how much testosterone or estrogen you were exposed to in your mother’s womb.

In men, it’s usually longer than the index finger; in women, it’s typically shorter.

Dr. Zhengui Zheng and fellow researcher Martin J. Cohn at the University of Florida used mice to determine that the relative length of the index and ring fingers are developed in gestation and stay the same after birth.

Further experiments showed it is specifically the androgen (for example, testosterone) or estrogen levels in the ring finger that determine how long it is.

The more androgen, the longer the ring finger.

Science has long studied the connection between finger length and several factors, including behaviour, fertility, breast and prostate cancer, sexual orientation, and athletic ability.

Now this study has produced genetic evidence for those links, all encoded in your fourth finger, or 4D as scientists call it.

And, as Zheng told the Star, that evidence could also tell doctors if a woman was exposed to hormone-disrupting chemicals during pregnancy that could help trace a disease.

“There are genetic reasons and environmental reasons” for the length of male and female 4Ds, he said.

“A major factor is development of the embryos’ own gonads (ovaries and testes), which produce sex hormones. The mother’s physiological condition or even what the mother eats might affect the sex hormone levels in womb.

“Finally, exposure to endocrine-disrupting chemicals can change sex hormone signalling.”

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What’s also interesting, said Zheng, is that the stage when 4D bone length is influenced by the sex hormones in the womb is the same point when sex steroids “masculinize and feminize the brain.”

“Digit ratios may be simply readouts of androgen to estrogen activity during this period,” he said.