Finger length may help differentiate the couch potatoes from the exercise junkies, if new animal research is any indication.

A number of human studies have linked finger length ratio - specifically, the length of the index finger in relation to the ring finger - to certain behaviours and traits, including aggression, athletic ability and academic skills.

Generally speaking, men tend to have a shorter index finger relative to the ring finger, whereas the two tend to be more equal in length in women.

Studies have the linked the "male" pattern - whether it's in men or women - to higher aggression levels and greater athletic prowess, whereas the "female" pattern has been associated with sharper verbal skills.

In this latest study, published in the journal PloS One, co-author Dr Peter L. Hurd and colleagues investigated this relationship in 1000 white mice.

They found that selective breeding to enhance the trait of increased physical activity resulted in mice with a higher digit ratio. Although behaviour was clearly associated with digit ratio, the findings were the opposite of those observed in humans.

Specifically, the scientists found that mice with a lower - more male-like - digit ratio tended to be lazier than their counterparts with more-feminine digits.

That is, the latter group voluntarily took to their running wheels more often than the group with relatively short "index fingers" - in mice, the second digit on the paw.

SO CAN COUCH POTATOES BLAME IT ON THEIR FINGER LENGTH?

Probably not, according to Dr Hurd, an associate professor of psychology at the University of Alberta in Edmonton, Canada.

"The effect sizes for digit ratios on behaviour in humans are really small, making it virtually impossible to tell anything about a single individual from their hands," Hurd said. "The pattern only emerges when a great many people are examined."

Instead, he explained, research on finger length and behaviour is interesting because it suggests that there are things about people's personalities - in this case, a propensity for physical activity - that are "fixed" during early development, probably in the womb.

In the past, researchers have speculated that testosterone exposure in the womb explains the relationship between finger length and certain behaviours in humans. Greater fetal exposure to the hormone tends to result in relatively short index fingers.

However, Hurd said, the situation may not be so simple. He explained that it would be difficult to reconcile the current study's findings with such a "testosterone effect".

Instead, something more complex than a "simple testosterone-driven manliness metric" seems to be at work, according to Hurd.

Prenatal exposure to stress hormones could play a role, he and his colleagues assert - as could other factors that regulate gene expression.

Women tend to have higher levels of corticosterone stress hormones than men, though the effects of such hormones on finger length are not clear.

Whatever the reason for the findings, Hurd and his colleagues conclude, they add to evidence of a connection among the brain, behaviour, personality and the shape of the hand.

Reuters