Could the length of your fingers predict a predisposition to breast cancer? Or musical genius? Or homosexuality? The author posits that relative lengths of the second and fourth digits in humans (2D:4D ratio) provides a window into fertility- and sex-related traits. It has been known for more than a century that men and women tend to differ in the relative lengths of their index and ring fingers, which upon casual observation seem fairly symmetrical. Men on average have fourth digits longer than their second digits, while women typically have the opposite. Digit ratios are unique in that they...