I just heard about this recently but, apparently, it's been known for a while. If you're male, the difference between the length of your index finger and your ring finger says something about how much testosterone you were exposed to in the womb. In most men, the ring finger is longer than the index finger and the greater the difference between the two, the more testosterone you were exposed to in the womb -- and, presumably, the more masculine you are. Wikipedia has an article on it here
. They call it the 2D:4D ratio, with a lower ratio referring to how much shorter your index finger is than your ring finger.
A 2003 study found that men with a lower 2D:4D ratio tend to be more physically aggressive but the ratio in women didn't seem to make any difference in aggression. That study abstract is here
At first, I thought, well, that's interesting. Then I thought, oh great. Now guys are going to get competitive about digit ratios. They're going to start checking out each others' fingers. Some guys are going to look surreptitiously while others are going to come right out and ask you about your fingers. Some guys are going to wear mittens so you can't see while others will wear gloves that expose only the index finger, making it look smaller. I suppose it's only a matter of time until plastic surgeons start offering ring finger implants.
And, yes, my index finger is shorter than my ring finger.