Tuesday, May 26, 2015

Finger Length Ratio and Personality, Part 2: Getting Specific

Here is a follow-up to my recent post on Finger Length Ratio and Personality in which we attempt to get more accurate about measuring the ratios, a bit more descriptive about the differences between men and women (on average) and slightly more detailed about what any of this might mean. 

Figure 1. A technique for more accurately measuring and calculating the 2D:4D ratio

Don't just look at the back of your left hand, as suggested in the popular Higher Perspectives article. Put both hands palms down on a copier and make a life-sized photocopy. Then, on the copy, mark the base and the tip of the 2nd and 4th digits of both hands as shown in the example. Measure and tabulate the length of all four fingers. Calculate the finger length ratios by dividing 2D by 4D.

Perhaps your left and right hands are symmetrical, as they appear to be in the above example. In this case, the left- and right-hand ratios will be identical. If your subject's hands are not symmetrical, use the ratio with the largest absolute distance from 1. Some studies suggest that the right hand is more accurate, but I suspect that this is because most people are right-hand dominant. Use which ever hand has the more pronounced effect.

I used data from Bailey and Hurd (March 2005) to make the figure below. In a sample of  134 people, the distribution of men's finger length ratios has a mean of 0.947 and a standard deviation of 0.029. For women, the statistics are 0.965 and 0.026.

Figure 2. Finger Length Ratios of Men and Women

Given these probability density functions, I set out to construct the corresponding cumulative distribution functions, so that we could more accurately estimate the percentages of men and women who have a 2D:4D ratio in defined ranges. Recall that in our previous study, Group A had a ratio less than 1, Group B had a ratio greater than 1, and Group C had a ratio equal to 1.

Fig. 3. The Higher Perspectives categories

To operationalize the Higher Perspectives model, I define Group A as less than 0.98, Group B will have ratios over 1.02, and those in Group C will have ratios that fall between 0.98 and 1.02.

Figure 3. Cumulative density functions of the Bailey and Hurd data for men and women

Using the cumulative density functions of the Bailey and Hurd data for men and women, I made the table shown below.  Approximately 85% of men and 70% of women will fall into Group A with a finger length ration less than 0.98. Twice as many women (28%) as men (14.4%) will have approximately equal ring and index fingers, placing them in Group B. Only a very small percentage of the population (2% of women and only 0.6% of men) will have longer index fingers, indicating very little androgen exposure in utero, and landing them in Group C.

Table 1. Groups A, B, and C Defined in terms of Finger Length Ratios for Men and Women

Obviously, we could opt to make Group C wider at the expense of Groups A and B and then adjust the population proportions. However, any difference between 2D and 4D that is perceptible to the unaided eye will cause a reasonable person to place the result into Group A or B. Group C should be narrowly defined within the limits of human perception, or the group divisions lack credibility. The eye can easily detect an eighth of an inch of difference at arm's length, but 1/16th is tough. Increasing the subject's index finger by 1/16th of an inch changes the ratio to 3.1875:3.25 or 0.9808--apparently equal to the unaided eye and nearly equal in real terms.

OK, now you are asking, "So what? Where are we going with this?"

I am glad you asked. The finger length ratio is determined in the womb, based on the amount of the androgen testosterone present in utero. Men are more likely to have longer ring fingers and a 2D:4D ratio less than 1. A lower 2D;4D ratio is associated with exaggerated masculine traits in both men and women. I wondered myself, is this useful information, or just some sort of genetic parlor trick like seeing who among your friends can curl their tongue or cross only one eye? A longer ring finger is no excuse for inappropriate behavior, but it may help explain why some folks are more naturally aggressive and risk-seeking than others. Simple markers such as finger length ratios may help us identify people who will need additional training and reinforcement to, for example, remain calm while driving.

And speaking only for myself, I find that kind of information very useful.


Bailey, A,A,, and Hurd, P.L.. (March 2005). "Finger length ratio (2D:4D) correlates with physical aggression in men but not in women." Biological Psychology. 68 (3): 215–22.