Does your finger length predict sexual behavior? A rat study says yes

Sexual behavior and preference – whether in humans or animals – isn’t random. It’s shaped early on, during fetal development, by how the brain responds to hormones like androgens (male sex hormones).

This early exposure is known to affect adult traits like sex drive and cognition. But there’s been one big obstacle: how do you measure that influence without invasive methods?

Rat fingers hint at sexual behavior

Researchers from Okayama University in Japan may have found an answer – by looking at rats’ fingers.

In a recent study published on May 14, 2025, a team led by Professor Hirotaka Sakamoto and Dr. Himeka Hayashi found that a simple finger measurement – the ratio of the second digit to the fourth digit, known as 2D:4D – can predict sexual behavior and preference in rats.

This digit ratio has been studied in humans for years. On average, men tend to have a lower 2D:4D ratio than women, meaning their index fingers are shorter relative to their ring fingers.

That difference has been linked to prenatal androgen exposure. It’s also been tied to traits like cognition, personality, and sexual orientation. But surprisingly, there hasn’t been much research testing whether this holds true in other animals. That’s what drew the researchers to rats.

Shorter fingers, stronger sex drive

“We found that the 2D:4D ratio serves as a reliable biological marker for predicting sexual activity in rats,” said Sakamoto. “Rats with shorter second digits were not only more sexually active but also showed a clear preference for female scent.”

Building on their initial measurements, the team then ran mating trials to observe behavior.

The results were clear. Male rats that ejaculated during their first encounter had shorter second digits compared to those that didn’t.

These males also showed signs of higher sexual drive: they ejaculated more often, did so more quickly, and had stronger erectile function.

The scent of sexual behavior

To go beyond behavior and look at preference, the researchers ran a simple but telling test. They placed male rats in a cage with two kinds of bedding: one scented by females, and the other by males.

All the male rats initially explored the male-scented bedding. But only those with shorter second digits stayed interested in the female-scented bedding. They sniffed, explored, and spent more time with it – indicating a clear sexual preference.

“While most studies have only linked the digit ratio to morphological sex differences, we noticed a surprising lack of research connecting digit ratio to the biological basis of sex itself – especially sexual activity and preference,” notes Hayashi. “This curiosity marked the start of our study.”

Fingers mirror brain wiring

Beyond offering insight into rat behavior, the study points to a deeper question: how early hormone exposure shapes the brain in lasting ways.

Before birth, hormone exposure might wire our brains in ways that the 2D:4D ratio later reflects – potentially shaping complex behaviors like sexual orientation or emotional attachment.

“Our results suggest a profound link between the body and mind, with significant scientific and clinical implications,” said Sakamoto. “Simply examining our fingers could one day help identify aspects of our behavioral tendencies.”

The findings also offer a practical model for exploring biological questions that are difficult – or impossible – to study directly in humans.

Because 2D:4D is also connected to cognitive traits and mental health conditions, this research could help explain sex-based differences in disorders like autism, depression, or attachment issues.

Biology is not the whole story

While the findings are compelling, they also raise important questions. For one, it’s still unclear how well results from rats translate to humans.

Rats provide a useful model for controlled experiments, but human sexuality is influenced by many layers – cultural, social, psychological – that go far beyond biology.

The researchers also focused only on male rats. Future studies will need to explore whether similar patterns hold true for females, and how other factors like environment or stress interact with digit ratios to shape behavior.

Another key question is causation. Rats with shorter 2D:4D finger ratios tend to show more sexual activity and stronger preferences, but scientists aren’t yet sure whether the digit ratio drives this behavior – or if both stem from shared prenatal influences.

None of this takes away from the value of the study. But it highlights the importance of treating biological markers like 2D:4D as part of a much larger picture.

The full study was published in the journal Experimental Animals.

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