Take a look at your feet…

What do you see? Most of us will see 5 toes … and very likely your inner toe will be the longest, the largest, AND the biggest! As a matter of fact, this a typical characteristic of the human foot!

Now, let’s take a look at the feet of primates… (see the picture below):

The primate foot and the human foot.

Do you see the difference? The foot of these primates show quite a different picture: for, the inner toe is always the lowest set toe! This is a typical characteristic of the primate foot. And often the length of the inner toe does not pass the length of the other toes. 

Another rather remarkable aspect is the fact that the ‘big toe’ of primates (the inner toe) shows quite some similarities with the thumb of our hands.

These typical characteristics in human and primate foot were already described in 1920 in the work PRINCIPLES OF ANATOMY presented by Frederic Wood Jones. Wood Jones described these characteristics through the concept: DIGITAL FORMULA.

The typical ‘digital formula’ for the human foot is: 1>2>3>4>5 – in the typical human foot the inner toe (toe 1) is usually the longest, and the little toe (toe 5) is usually the shortest.

However, the typical primate foot is characterised by the so-called ‘simian digital formula’: 3>4>2>5>1 – in the typical primate foot the middel toe (toe 3) is usually the longest, and the ‘inner toe’ (toe 1) the shortest!

Notice: Jones’ describes on page 30:

 “… the digital forumula*, and it must be remembered that such a formula does not express the relative length of the digits, but the relative projection of the tip of the digits from the extremity of the limb”.

The ‘human digit formula’ for our hands…

Interestingly, the typical ‘digital formula’ for the primate foot resembles the typical ‘simian digital formula’ for the primates hand: 3>4>2>5>1 – you see the confirmation in the picture below!

How to measure digit ratio?

As a matter of fact, the ‘digital formula’ of the human hand often resembles the typical ‘simian digital formula’: 3>4>2>5>1 – especially in the hand of human males! But also the hands of human females can often be characterized by the ‘simian digital formula’, however more often the hands of women can be described with the ‘human digital formula’: 3>2>4>5>1.

The difference is that in human males the ring finger is more often larger than the index finger (or: pointer finger) – which resembles the  ‘simian digital formula’! In human females more often the index finger is longer than the ring finger – which resembles the ‘human digital formula’! These principles are visible in the picture below:

Digit ratio in the male and the female hand

The ‘digit ratio’ formula!

However, both in human males and females the length difference between the index finger (digit 2) and the ring finger (digit 4) is often very small. And therefore the ‘ratio’ between the length of those fingers is often close to ‘one’ (1.00) – when both fingers have the same length, the ‘ratio’ between the 2 fingers is exactly ‘1.00’.

Because of this situation scientists around the world are using the concept ‘digit ratio’ to study the relative length difference between the index finger (digit 2 = 2D), and the ring finger (digit 4 = 4D), which results in the formula: ‘digit ratio’ = 2D:4D.*

* One should notice here that the concept of Wood Jones ‘digital formula’ is related to “… projection of the tip of the digits from the extremity of the limb” -see the 3th picture in this blog post). However, the ‘digit ratio’ is related to the absolute length difference between the index finger and the ring finger – see the picture below.

How to measure digit ratio?
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