Since as early as the 19th century(1), people have noticed that men tend to have a lower 2D:4D ratio than women. Since then the ratio has been linked to a variety of sexual, psychological, and behavioral traits. The number tends to be lower for men, for people with greater athletic prowess, assertive women, engineers, mathematicians, and lesbians. So they call a lower number a "masculinized" 2D:4D ratio.
But it's different in mice. Mice with a HIGHER ratio are more aggressive, more likely to bite when handled, and have a higher daily level of activity.
In the study cited here, the investigators selectively bred mice to be more active; they selected the mice that liked to run in their wheels more and bred them to each other. Then they measured their fingers. Voila. They had a HIGHER 2D:4D ratio.
So does this mean that finger length has any biological implication for behavior, physical prowess, or sexuality? Here's what the authors said:
"Given the many factors that have the ability to affect digit ratio, it is clearly more complicated than a simple testosterone-driven manliness metric."
I couldn't have said it better. Yeah, there might be some hormonal thing going on, but as it stands, it's no more scientific than trying to guess what's going on in a person's head by mapping the bumps on the skull.
(1) Anthropological Notes on the Human Hand