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Human Nature Review 2002 Volume 2: 418-423 ( 9 October )
URL of this document http://human-nature.com/nibbs/02/manning.html
Digit Ratio: A Pointer to Fertility, Behavior and Health
by John T. Manning
NJ: Rutgers University Press. 2002
Reviewed by Michael Mills, Psychology Department, Loyola Marymount University, Los Angeles, CA 90045. USA.
Take a look at your right-hand. Which of your fingers is longer: your ring finger, or your index finger? Surprisingly, a passing stranger who noticed a difference in length between these two fingers (and who had handy a copy of John Manning's book Digit Ratio: A Pointer to Fertility, Behavior and Health) might infer some very personal characteristics about you. With no more data than that gleaned from a passing glance at your hands, a stranger might infer whether you are likely to have homosexual inclinations, are highly fertile, may eventually suffer from a heart attack or breast cancer, have musical aptitude or sporting prowess, and a surprisingly long list of other characteristics.
Why do the fingers reveal such a wide spectrum of information (albeit very probabilistic)? Manning reviews evidence to suggest that the ratio of the length between the ring and index finger is somewhat sexually dimorphic, that this ratio is determined during early fetal development, and that it is influenced by sex hormones, particularly testosterone. If this is true, the fingers may provide a permanent, and easily visible, historic marker of important hormonal events that occurred during a critical time of fetal development, the latter part of the first trimester. This is a critical time of sexual differentiation of both the brain and body.
Specifically, it is the ratio of the length of the index finger (digit 2, or "2D") and the ring finger (digit 4, or "4D") that is sexually dimorphic. Generally, males have a ring finger that is longer than their index finger. Females typically have index and ring fingers of about the same length. The ratio of index finger length to ring finger length is called the “2D:4D digit ratio,” or more simply, the “digit ratio.” Manning reports that, for males, the index finger is generally about 96 percent of the length of the ring finger, which gives an average digit ratio for males of .96. The digit ratio would be 1.00 if the ring and index fingers were the same length, and greater than 1.00 if the index finger was longer than the ring finger. Males generally have a digit ratio below 1.00 -- they have what is termed a "low digit ratio." Women generally have a digit ratio of about 1.00 (the index and ring fingers are of about equal length), or a "high digit ratio."
Manning links the proximate causes of digit ratio sexual dimorphism to the effects of sex hormones during early fetal development. He believes the evidence is persuasive, but not yet definitive, that higher levels of testosterone during this critical developmental stage facilitates the growth of the ring finger, while higher levels of estrogen facilitates the growth of the index finger. He states: “In general, it seems that 2D:4D is the most reliable of the predictors of hypermasculinization…” (p. 139). One of the consequences of hypermasculinization, as reflected by the digit ratio, may be higher levels of fertility in men and lower levels of fertility in women. He also suggests that hypermasculinization increases the likelihood of homosexuality or bisexuality, in both males and females.
Manning devotes separate chapters to explore the relationship between digit ratio and a variety of characteristics, including assertiveness and attractiveness (chapter 3), reproductive success (chapter 4), hand preference, verbal fluency, autism, and depression (chapter 5), health and disease (chapter 6), homosexuality (chapter 7), musical aptitude (chapter 8) and sports aptitude (chapter 9). A brief summary Manning’s findings (some of which he notes are quite preliminary) is presented below.
Some Characteristics That May Be Associated with Digit Ratio (from Manning, 2002)
Low 2D:4D ratio
Presumably due to relatively greater fetal exposure to testosterone in the 1st trimester
High 2D:4D ratio
Presumably due to relatively greater fetal exposure to estrogen in the 1st trimester.
Higher lifetime reproductive success
More aggressive and assertive
Greater proclivity toward homosexuality/bisexuality
Higher musical and sports aptitude
Lower SES (?)
Higher risk of early heart disease
Greater proclivity toward homosexuality/bisexuality
More aggressive and assertive
Higher lifetime reproductive success
Higher risk of breast cancer
There is, however, substantial overlap between the sexes with respect to digit ratio. It is not uncommon for a man or woman to have a digit ratio that is typical of the opposite sex. Across various populations, the offset of the distributions of male and female digit ratio is a little less than ½ of a standard deviation – e.g., the digit ratio, collapsed across various populations, has an effect size of about 0.3 – 0.4 (Manning, personal communication, 10/4/02). This is a small to moderate effect size. The sexual dimorphism of height has an effect size of about 1.4. A web-based program is available at my website (at this address: http://bellarmine.lmu.edu/faculty/mmills_fp/software.htm) that will graph the distributions and calculate the effect size for a trait. Using this program, we can see that there is far more overlap between the sexes in digit ratio than there is in the overlap between the sexes in height, as noted in the following diagrams.
Figure 1 – The Sex Difference in Height in inches (American data)
Figure 2 – The Sex Difference in Digit Ratio, Liverpool, UK Data (data from Manning, 2002)
(Note: The lines of graph of the “Proportions of Each Group by Score” are not completely smoothed – this is an artifact of the graphing program.)
One of the especially interesting theoretical discussions in Manning’s book is his speculation (starting on page 54) regarding why there is any overlap between the sexes in sexually dimorphic traits in the first place, including digit ratio. If being taller than women is adaptive for men in general, why aren’t all men taller than all women? And why don’t all men have a lower digit ratio than all women? Why the overlap? Manning interprets this as an evolutionary stable equilibrium point in a conflict between sexually antagonistic genes. Genes that tend to masculinize the fetus will increase when there is an advantage to having male offspring (e.g., when the operational sex ratio favors men, or there is a polygynous mating system). The benefits of more masculinized male fetuses (increased adult sperm count, higher libido, inclination toward sexual promiscuity, etc.) will compensate for the reduced fertility of female offspring due to their relatively higher than normal fetal exposure to male hormones. When the mating system pendulum swings to the other side, and a monogamous mating system is in place, or operational sex ratio favors females, genes that tend to feminize the fetus will be favored. The result is an overlap in male/female distributions for many sexually dimorphic traits – a somewhat middle ground as parents hedge their bets regarding the relative reproductive potential of male or female offspring.
Somewhat surprisingly, the effect size for digit ratio between the sexes varies substantially as a function of geography and race. For example, among the Zulu the effect size was found to be as low as about .2, while the effect size for a German sample was .56. Surprisingly, the females in some cultures may have a lower digit ratio than males of other cultures, although men have a lower digit ratio than women within populations in all cultures for which there is data. It is unclear why the effect size of the digit ratio of the sexes varies between different populations. This is a curious fact, one for which Manning provides little in the way of definitive conclusions -- and the reader may be left to wonder whether some of Manning's interpretations are threatened by this between population variability in effect sizes. However, the fact that the average height of men of some populations is lower than women of other populations doesn't negate the sex difference in height, nor does the fact that the gender effect size of height varies in different populations. Height, like digit ratio, is still sexually dimorphic. But the causes of between population variation in sexually dimorphic traits, such as digit ratio, is certainly puzzling, and it is a fertile area for future research.
One might be tempted to speculate that racial variation in digit ratio may correspond to Ruston’s (1997) theory that populations closer to the equator are more r-selected, and thus are relatively more masculinized in utero and have higher adult testosterone levels. By this reckoning, one might expect to find low digit ratios near the equator, and progressively higher digit ratios in populations farther away from the equator. Indeed, Caucasians tend to have higher digit ratios than do Blacks. Data for Asian populations has not yet been published, however, some preliminary data suggests that Asians tend to have low digit ratios (Manning, personal communication, 10/5/02). Manning suggests that populations in middle latitudes may have higher digit ratios compared to populations nearer to the equator or nearer to the poles. This is inconsistent with Rushton’s theory, and Manning, so far, has not speculated about a possible theoretical explanation for this curvilinear pattern. The curious relationship between digit ratio and geographic latitude may require much additional research to disentangle what may be some complex determinants of these population variations.
It is clear why men and women have sexually dimorphic reproductive organs. But why did they evolve a sexually dimorphic digit ratio? Manning notes that it has been suggested that the male digit ratio pattern may be functional -- a longer ring finger may help to stabilize the third digit (the middle finger) when throwing objects, thus increasing throwing accuracy. This implies that the throwing accuracy required for successful hunting and/or tribal warfare was of sufficient importance to drive the evolution of this sexually dimorphic trait. While gathering, ancestral women presumably did not need this extra stability for the third finger. Today, this sex difference may be seen in male superiority in throwing darts. And, it would be interesting to know if men with lower digit ratios were better dart throwers and men with higher digit ratios.
Another hypothesis for the origin of this sexually dimorphic trait was that it was driven by direct sexual selection -- female choice. If so, it is surprising that women today are not conscious of being particularly attracted to men with low digit ratios. However, it is interesting that women sometimes comment that they were attracted to a man's "masculine looking" hands, albeit without commenting directly on digit ratio. One wonders if the appearance of "masculine looking hands" includes an (unconscious?) assessment by females of male digit ratio? If so, this would lead more credence to the direct sexual selection hypothesis.
Manning's book summarizes a cutting-edge area of research. He provides an outstanding (and currently the only) review and synthesis of the literature on digit ratio. This is a book that will serve as a valuable resource for researchers conducting studies in this area. He provides a wealth of statistical data (right down to the number of subjects and F test values) from a variety of studies. Many of these studies he has conducted himself. However, to a casual reader, lay or professional, trudging through statistical information will likely be an obstacle. Manning may have been able to capture a far wider readership if much of the detailed statistical data from specific studies had been relegated to footnotes or end notes. Indeed, this is a fascinating topic – what other easily visible physical trait (other than sex itself) is likely to be associated with such a variety of behavioral, reproductive, and health characteristics?
In the final chapter, Manning provides an interesting discussion about possibilities for further research that may help to further disentangle the consequences of early fetal hormonalization. Given the results from such future research, a brief glance at the hands may be even more revealing.
Don't wish others to glean too much personal information about you from a brief glance? Manning might suggest that you may wish to keep your hands in your pockets…
Manning, J. T. (2002). Digit ratio: A pointer to fertility, behavior and health. NJ: Rutgers University Press.
Rushton, J. P. (1997). Race, evolution and behavior, 2nd Ed. New Brunswick, NJ: Transaction Publishers.
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© Michael E. Mills.
Mills, M. E. (2002). Review of Digit Ratio: A Pointer to Fertility, Behavior and Health by John T. Manning. Human Nature Review. 2: 418-423.