Finger length ratios

What are finger length ratios?

Scientists have been interested in the differences in human finger lengths since at least 1888 [1] Jump to reference section: [1] . Over time, the prevailing area of interest became the difference between the length of our index finger (2D) and our ring finger (4D). This is what is known as a 2D:4D ratio, which is simply the length of your index finger divided by the length of your ring finger. What became most apparent during early research into the 2D:4D ratio was that, on average, women would demonstrate a higher 2D:4D ratio than men [2] Jump to reference section: [2] .

So why were researchers interested in finger ratios anyway?

While understanding finger ratios does not have any immediate medical or scientific application, understanding it more may result in a deeper fundamental understanding of human development. This kind of 'blue skies' research without a clear aim may not amount to an immediate tangible outcome, but in some cases can result in unexpected breakthroughs.

The 2D:4D ratio is usually demonstrated as a decimal. A low 2D:4D ratio (a value lower than 1) means your ring finger is longer than your index finger, whereas a high 2D:4D (a value greater than 1) means your index finger is longer than your ring finger.

For example, researchers found that the average ratio in a group of men they sampled was 0.947 [2] Jump to reference section: [2] . This means that the average man's index finger in the group was 94.7% the length of their ring finger (their ring finger was longer than their index finger).

The average ratio in the female group was 0.965 [2] Jump to reference section: [2] , meaning that, while on average the female group also had longer ring than index fingers (the average index finger length was 96.5% the length of the ring finger), there was a smaller difference between the lengths of the two fingers than in the male group.

What factors influence my finger lengths?

The interesting observation that the female 2D:4D ratio was higher than in men led to the idea that male and female sex hormones; testosterone and oestrogen could be playing a role in influencing our 2D:4D ratio. Since then, research has been conducted and some scientists argue that the level of testosterone and oestrogen you are exposed to whilst you are in the womb is significantly associated with your 2D:4D ratio [3] Jump to reference section: [3] . However, this finding has been hotly debated across the research world, with others arguing that it has not been possible to confirm the validity of this discovery.

What about genetics?

 However, geneticists have been able to establish a large genetic component to our 2D:4D ratio. About 60% of variation in 2D:4D variation can be explained by our genetics [4] Jump to reference section: [4] . Here we’ll evaluate some of the genetic variants shown to be influencing your 2D:4D ratio.

A key variant associated with the 2D:4D ratio is on the SMOC1 gene. The SMOC1 gene carries the instructions for a protein called secreted modular calcium-binding protein 1. This protein regulates molecules called growth factors that stimulate the growth and development of bone tissues throughout the body, importantly including the formation of limbs. Researchers from the University of Cambridge have found that each copy of the ‘C’ variant at the rs4902759 takes away 0.76% of your 2D:4D ratio [5] Jump to reference section: [5] .

In real terms, this equates to a less than 1mm difference in most cases. In fact, we decided to do some of the maths ourselves. Theoretically, if you took a person with an index finger length of 66mm and a ring finger of 69mm and suddenly gave them a ‘C’ variant at rs4902759, each ‘C’ copy you gave them would increase their ring finger length by about half a millimetre!

Another variant geneticists have discovered influences our 2D:4D ratio is on the LIN28B gene. The LIN28B gene carries the instructions for a protein of the same name. What this protein does is not yet fully understood, but experiments on the same gene in worms has shown it has something to do with the timing of development. This gene has been shown to be very active in the placenta during pregnancy, so it’s likely it plays a role in our very early development. Researchers have shown that each copy of the ‘A’ variant at rs314277 adds 0.63% to your 2D:4D ratio [6] Jump to reference section: [6] . 

A third gene indicated in 2D:4D finger ratios is GLIS1. GLIS1 carries the instructions for a protein of the same name and involves the regulation of the organs of the reproductive system. It is thought to have a critical role in regulating specific stages of embryogenesis (the development of an embryo). Geneticists have recently discovered that a ‘T’ variant at rs4927012 takes off on average 0.36% of your 2D:4D ratio [7] Jump to reference section: [7] .

What does this all mean for me?

Accurately calculating your exact 2D:4D ratio would be tough, and the number you’d get from measuring and calculating it yourself is unlikely to vary very much from the general population. However, whether male or female, a 2D:4D ratio greater than 1 (or 100%), where the index finger is the longer of the two, is quite rare. Some research has also indicated that having a longer index finger may predict a range of traits - but it’s important to remember, as mentioned earlier, these findings remain a topic of fierce debate within the scientific community and further research is required to successfully validate such claims [8] Jump to reference section: [8] .