Hormones are chemical substances that are produced by cells or glands in the body. They act as messengers that are transported through the bloodstream towards the receptor cell or tissue in another part of the organism.

Hormones have a variety of effects on the organism, among others:

Stimulating or inhibiting growth,
Mood effects,
Regulating metabolism and behavior,
Controlling the reproductive cycle.

These are only examples, as hormones have many more functions.

Fingers

It has been known for some time that the exposure to male hormones (most notably testosterone) in the uterus influences finger length. So, perhaps it sounds strange, but finger length can actually tell you something about the hormones you were exposed to in the uterus.

Men tend to have a ring finger that is longer than the index finger, whereas with women this is reversed. Between men there are also noteworthy differences. The larger the exposure to male hormones was in the womb, the longer the ring finger is compared to the index finger.

One might wonder why this matters, but remember that hormones have certain profound effects on human behavior.

Behavior

Based on this principle, researchers have studied the social behavior of our ancestors. Male hormones such as testosterone play an important part in the development in male behavior, such as competition and promiscuity. It has already been shown that promiscuous primates have longer ring fingers (compared to their index fingers) than monogamous primates.

By comparing fossils of our ancestors and the relative finger lengths of present day man, it seems that we are less competitive and promiscuous than our early evolutionary relatives.

So, when you want to know how ‘manly’ a man really is, just check his fingers.

Nevertheless, it is important to note here that these findings are based on averages of a large number of people, so don’t go blaming a man for cheating just because he has a long ring finger. There are other factors at play as well, which can produce individual differences.

References

Berenbaum, S.A.; Bryk, K.K.; Nowak, N.; Quigley, C.A. & Moffat, S. (2009). Fingers as a Marker of Prenatal Androgen Exposure. Endocrinology. 150(11), pp. 5119 – 5124.
Loehlin, J.C.; Medland, S.E. & Martin, N.G. (2009). Relative Finger Lengths, Sex Differences, and Psychological Traits. Archives of Sexual Behavior. 38(2), pp. 298 – 305.
Nelson, E. & Schultz, S. (2010). Finger length ratios (2D:4D) in anthropoids implicate reduced prenatal androgens in social bonding. American Journal of Physical Anthropology. 141(3), pp. 395 – 405.
Nelson, E.; Rolian, C.; Cashmore, L. & Schultz, S. (2010). Digit ratios predict polygyny in early apes, Ardipithecus, Neanderthals and early modern humans but not in Australopithecus. Proceedings of the Royal Society of Biological Sciences. Published online before print November 3, 2010, doi:10.1098/rspb.2010.1740.