Is your ring finger longer or shorter than your index finger? The reason for the difference is in our hormones, a new study in mice shows for the first time.

Before birth, the sex hormones estrogen and testosterone control genes that in turn dictate finger length, the study found. Like us, mice naturally have both hormones in their bodies. (Get a genetics overview.)

As a result, most men have ring fingers that are longer than their index fingers, and the reverse is true in women.

Differences in finger length have been repeatedly linked to a range of human traits, from aggression to musical ability to sexual orientation. There are also connections to health problems such as autism, depression, heart attacks, and cancer.

"It's difficult to look at an adult or a newborn child and determine what was happening during their embryonic development," said study leader Zhengui Zheng, a developmental biologist at the Howard Hughes Medical Institute in Florida.

"But the discovery that digit proportions reflect the prenatal hormonal environment helps us to explain all those correlational studies," said Zheng, who works in the laboratory of study co-author Martin Cohn.

First Proof That Hormones Affect Finger Length

Scientists had already suspected that sex hormones played a part in finger length, but "causality had never been demonstrated—no one had ever tested the hypothesis that these hormones directly cause the digits to grow differently in males and females," Cohn said.

Zheng and Cohn took two general approaches for their experiments, which are detailed this week in the journal Proceedings of the National Academy of Sciences.

First, they used genetic tools to deactivate cell receptors—molecules that send and receive signals to and from other cells—for testosterone and estrogen in the developing limb buds of mouse embryos. Second, the scientists boosted the hormone levels of pregnant female mice.

The results proved consistent: Male mice that lacked testosterone receptors in the womb were born with shorter, typically feminine digit proportions, or had index digits longer than their ring digits. Male mice without estrogen receptors had longer ring digits.

Female mice that lacked an estrogen receptor were born with longer digits, and those that did not have testosterone had shorter, superfeminine digits.

The team also found that hormones controlled the rate of cell division in a mouse's developing digits.

In the ring digit, testosterone stimulates cell division, building up cartilage and bone to make the digit longer. Estrogen, on the other hand, slows down cell division, leading to a shorter digit.

But what really surprised the researchers was that it wasn't the absolute level of hormones—say, the total amount of estrogen—that influenced digit length, but rather the balance of hormonal activity in the mouse's limb buds, Cohn noted.

Mouse-Digit Experiment "Elegant"

Scott Simpson, an anatomist at Case Western Reserve University in Ohio, said the study used a "very elegant and well-designed experiment."

That's because the team traced every step in how hormones change mouse bodies, from the gene level to maturity—a still little-understood progression, said Simpson, who was not involved in the research.

"We all know we are the product of our genes, what are all the steps from gene to us?"

What's more, the study is valuable in finding that hormones begin affecting us much earlier than thought—as early as the first trimester of pregnancy. Many scientists have believed that hormones are more active after birth.

"This isn't just an interesting curiosity that's focused on the hand—it really says something about the behavioral and development biology of adults," he noted.

Finger Length a Snapshot From the Womb?

Overall, the results suggest that our finger length can be almost a "readout" of a person's hormonal balance during a very narrow window of development in the womb, Cohn said.

For instance, if a man has a longer index finger than ring finger, he may have had an elevated dose of estrogen at some point during his development—which is not necessarily a bad thing.

But, in some cases, such irregularities can lead to medical conditions in adulthood. Exposure in the womb to synthetic chemicals that mimic or block hormones, for example, has been linked to health problems in children and adults.

Previously, it was difficult to know if a mother had been exposed to a chemical during pregnancy that disrupted hormone activity in her child.