It seems that you can tell whether a person is promiscuous or monogamous just by looking at the fingers! Well, maybe it doesn’t work for individuals but researchers curious about human evolution have used finger length to make an educated guess about some of our distant ancestors’ mating habits.
According to evolutionary theory male characteristics such as aggressiveness and competitiveness are more likely in promiscuous species than monogamous ones. Looking at fossil skeletons we can therefore suggest that Australopithecus, which lived three to four million years ago was monogamous whereas an even earlier group, Ardipithecus, was highly promiscuous – all because of their finger length!
Of course it isn’t the finger length that causes sexual behaviour – it is levels of the male hormone androgen. Higher aggression and competitiveness in males is related to intrasexual selection (the more androgens the more competitive) and is also related to higher levels of androgens. An effect of higherlevels of androgens is that they cause a short forefinger and longer ring finger. This means that males high in androgens have a low forefinger to ring finger ratio whereas males low in androgens have a high ratio.
Finger length ratios have been linked to lots of other things, such as numeracy and literacy (see The A2 Complete Companion page 288).