是实的，那个叫做 2D:4D ratio, wiki上面有解释
一般来说男性的食指要比无名指短、而女性的食指和无名指长度差不多，以至食指更长一些。原因是在胚胎发育的过程中有一个重要的手指骨骼生长期间，无名指的生长遭到雄性激素的调控，呈正相关。一个显著的例子就是当以卵双生儿产生的时候，女性婴儿就会遭到男性婴儿的雄性激素影响，2D：4D ratio降低。但是那个比值跟成年人的荷尔蒙凹凸没有关系。详细见http://en.wikipedia.org/wiki/Digit_ratio
The digit ratio is the ratio of the lengths of different digits or fingers typically measured from the bottom crease where the finger joins the hand to the tip of the finger. It has been suggested by some scientists that the ratio of two digits in particular, the 2nd (index finger) and 4th (ring finger), is affected by exposure to androgens e.g. testosterone while in the uterus and that this 2D:4D ratio can be considered a crude measure for prenatal androgen exposure, with lower 2D:4D ratios pointing to higher androgen exposure. The 2D:4D ratio is calculated dividing the length of the index finger of the right hand by the length of the ring finger. A longer index finger will result in a ratio higher than 1, while a longer ring finger will result in a ratio of less than 1.
The 2D:4D digit ratio is sexually dimorphic: in males, the second digit tends to be shorter than the fourth, and in females the second digittends to be the same length or slightly longer than the fourth.
A number of studies have shown a correlation between the 2D:4D digit ratio and various physical and behavioral traits.
A 2011 paper by Zhengui Zheng and Martin J. Cohn reports that "the 2D:4D ratio in mice is controlled by the balance of androgen to estrogen signaling during a narrow window of digit development". The formation of the digits, in utero, is thought to occur at 14 weeks, and the bone-to-bone ratio is consistent from this point into an individual’s adulthood. During this period if the fetus is exposed to androgens, the exact level of which is thought to be sexually dimorphic, the growth rate of the 4th digit is increased, as can be seen through the analyzing the 2D:4D ratio of opposite sex dizygotic twins, where the female twin is exposed to excess androgens from her brother in utero, and thus has a significantly lower 2D:4D ratio Importantly, there has been no correlation between the sex hormone levels of an adult and the individual’s 2D:4D, which implies that it is strictly the exposure in utero that causes this phenomenon. A major problem with this research on this topic comes from the contradiction in the literature about the testosterone level in adults cannot be predicted by the 2D:4D ratio, but male sexual traits that are stereotypically attributed to testosterone levels have been found in correlation with the 2D:4D. So there should either be a correlation with one or the other but not both.
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