Align your four fingers. Now compare your ring and index fingers. Are they about the same length? If so, you’re probably a woman. Is the ring finger longer? If so, you’re probably a man.
The ratio of the lengths of the second digit (index finger) and fourth digit (ring finger) is called the 2D:4D ratio. Men typically have a lower 2D:4D ratio than women. Prenatal testosterone exposure is thought to account for this subtle sexual dimorphism. There are more testosterone receptors in 4D (ring finger) before birth. Thus, higher levels of testosterone cause 4D (ring finger) to grow longer than 2D (index finger) i.e. more male sex hormones causes longer ring fingers.
If the testosterone theory is true, then could the finger length ratio correlate with masculine behavior? A 2005 study published in Biological Psychology suggests that a lower 2D:4D ratio correlates with more aggressive behaviors in men, and that women perceive men with lower 2D:4D ratios to be more masculine.
And, because I know that there are some men out there who are fretting because their 2D:4D is feminine, don’t worry… your ratio doesn’t tell anybody about the size of your “equipment.” The size of that particular “finger” is determined pubertal (not prenatal) testosterone.