Jun 12 2009
Fingerprints and Grip – Wrong vs Incomplete
Everyone knows that the purpose of fingerprints is to increase friction and therefore improve our grip. New research calls this common belief into question – sort of.
The media, and it seems the researchers themselves (if they are being presented accurately) are presenting this as scientific evidence debunking a myth – but the real story is more complex.
Dr. Roland Ennos did a number of experiments looking at the friction between a finger pad and a plate of glass. He found that the friction did not increase by much when the force between the finger and glass increased – less than predicted. However, the friction did increase in proportion to the surface area of the finger pad that was in contact with the glass.
This means that finger pad skin behaves more like rubber – where friction is mainly a function of surface area, not the force of contact.
What this further means is that the ridges of fingerprints actually decrease the surface area of contact, and therefore decrease friction – rather than increase it, as was expected. That’s interesting, and certainly counter-intuitive.
However, that is not the whole story, and it does not mean that fingerprints are not useful for grip. Ennos only studied smooth surfaces, and it is possible that fingerprints may increase grip with rough surfaces. Other hypotheses are that fingerprints increase grip when wet – the channels allow for water to be carried away from the finger pads. Or that they increase sensitivity.
What struck me, and what the article did not mention, is that glass is a very artificial material. It is unlikely that our ancestors would have encountered such smooth material often in their day-to-day lives. Therefore there would not have been much selective pressure to develop a good grip on glass or similarly smooth material. Tree branches, rocks, fur, bones, and other materials that might find their way into the grasp of a hominid or ape are much rougher than glass.
Clearly follow up research is needed. How do fingerprints behave when applied to other materials, and how does wetness affect their utility?
What interested me most about this story is how the media channels science news stories into a few themes with which they feel comfortable. Debunking a commonly held myth is one of those themes. While this story hold a kernel of that theme – it is more accurate to say, in my opinion, not that the grip hypothesis is wrong but that the story is more complex.
That is a much more useful theme for science reporting – because the story is almost always more complex – more complex than the typical publish understanding, and even of our previous scientific understanding.
Likewise, it is more meaningful in many cases to portray our prior models and theories not as “wrong” but as incomplete. Sometimes they are wrong, but that needs to be distinguished from ideas that are oversimplified and therefore incomplete, but not wrong.
A recent relevant example of this is the New Scientist article on Darwin’s tree of life. The theme of the reporting, boldly declared in the headline, was that the tree of life is wrong – a debunked idea. This was greatly misleading, however, and lead to a misunderstanding and a great deal of mischief at the hands of the boobs over at the Discovery Institute.
The evolutionary tree of life is the basic concept that all life on earth is related through common ancestry and branching descent. This pattern includes only “vertical” genetic transfer – from parent to child. What the New Scientist article was discussing was emerging information about the extent of horizontal gene transfer – such as the swapping of genes between strains of bacteria. It turns out there is much more horizontal transfer, even among vertebrates, than was previously known. Viruses, for example, may act as vectors for carrying genes from one species to another.
This information, however, does not invalidate the tree of life. Horizontal transfer may be greater than previously known, but it is still a minor factor compared to the branching descent that dominates the tree of life, especially for multi-celled organisms. The tree of life is not wrong – it is still mostly correct.
It is as correct as saying that the Earth is a sphere. The spherical Earth theory is very useful, correct for most purposes, and is sufficient for working with other theories, such as gravity and plate tectonics. But it is not completely accurate – the Earth is really an oblate spheroid and slightly larger in the Southern hemisphere.
Similarly, the tree of life is true enough, and supports the underlying theory of evolution, and is sufficient for most purposes. But it needs to be tweaked with an understanding of the contribution of horizontal transfer. Saying the tree of life is “wrong” is to profoundly misunderstand the science, which the ID goons happily did. And New Scientist was lambasted for this botching of science reporting, as they should have been.
The difference between wrong and incomplete is very important to the proper understanding of the history and progress of science. I wish the science reporting media would getter a better “grip” on this idea (hey, I had to bring it home somehow). In fact, they need to expand and improve their repertoire of science writing themes – so that they more often tell the real story rather than just the story they want to tell.
![[del.icio.us]](http://images.del.icio.us/static/img/delicious.small.gif){kind=small}
Print This Post
15 Responses to “Fingerprints and Grip – Wrong vs Incomplete”
There was a recent paper with some data suggesting that the spacing of finger ridges matched the spacing necessary to optimize touch sensitivity to certain types of surface roughness.
http://www.sciencemag.org/cgi/content/abstract/323/5920/1503
I find this idea more compelling than data based on friction between skin and artificial surfaces such as glass. Other primates have similar patterns of finger and toe ridges and there has been a fair amount of research into the firing of sensory nerves from tactile stimulation in monkeys.
If a primate needed to get a better grip on something, increasing muscle strength would seem to be an easier thing to evolve, and could be achieved via normal muscle strength regulation. If other primates have skin ridges, that puts the time for their evolution much earlier than humans.
I think there is a great compulsion to find a single and simple explanation, rather than look at the complexity of reality.
Dog bites man is not news. Man bites dog is news.
Similarly, the headline ‘Theory X Found To Be Somewhat More Complicated Than Previously Thought, But Still Essentially Correct’ would not sell as much copy (and hence ad space) as would the headline ‘Theory X Proved Wrong’.
The media in general struggles mightily with having to serve two masters (profit vs. factuality).
I read something a while ago saying that the ridges of fingerprints didn’t significantly increase grip, however it did significantly increase the resolution of one’s sense of touch on a surface.
This wasn’t it, but it seems to be saying the same thing http://www.oandp.com/articles/NEWS_2009-02-05_02.asp.
I agree with your frustration with simplistic science news coverage. I found that whole business about materials which channelled light around them was pretty irritating to me, having to explain to all my (non-physics) friends who asked me about it the ridiculous limitations of the technology.
Speaking of evolution, I recently read a book on bird species which stated that white swans lived in the northern hemisphere of the globe and multiple types of black swans developed in the southern hemisphere.
Why are we convinced that fingerprints must be the product of a selective advantage? Why couldn’t they be a neutral, chance morphological character?
Fingerprints, more accurately, finger ridgings, serve the primary purpose of tactile discernment. They allow the fingers to act as a probing mechanism, which was arguably the first type of sensory apparatus developed by life forms. Like all sensory apparatuses, they help us uncover important information about the nature of our surroundings. For humans, they help the fingers tell us something about what a substance consists of that our eyes, ears, and noses, etc., cannot completely do.
Why are they all necessarily different? I suppose partly because there’s no necessity for them all to be the same.
A great essay dealing with the whole concept of wrong vs. incomplete is “The Relativity of Wrong” by Isaac Asimov. Highly recommended.
(I am a reformed headline writer.)
It is important to note that many headlines are written by ‘headline writers’- not the author of the article.
The job of the headline writer is to come up with some short phrase that will provoke emotion in the reader. The headline writer may or may not actually read the article that he is writing the headline for.
Anytime a story has complexity (as with economics, science, politics…) the difference between the headline ane the article can be large.
Facts to aid in the evaluation of information in the modern age.
The Isaac Asimov article mentioned by Bigfrozenhead was succinctly summarized in a recent episode of The Big Bang Theory, when someone said, “It’s wrong to say that a tomato is a vegetable, but it’s REALLY wrong to sat that it’s a suspension bridge.”
The last time I read an article on nanotribology, several years ago (I think in “Scientific American”), it was presented that in fact friction is always poportional to contact area, rather than contact force, but that in harder materials, such as steel, an increase in contact force compresses microscopic ridges, increasing the actual contact area. So the high-school physics rule of thumb that friction force equals a coefficient times the normal force is just a convenient shortcut, not the primary physical relationship.
Which has nothing to do with the main point, other than perhaps as something else the media got wrong or glossed over.
“the Earth is really an oblate spheroid and slightly larger in the Southern hemisphere”
It also pulsates with the tides.
i haven’t read the original paper… but, (and i know i dredge up the past…) anyone who works in this area (i.e., adaptation and evolution) should read, digest, then read-again gould&lewontin’s classic paper.
http://www.ncbi.nlm.nih.gov/pubmed/42062
do it; you’ll feel better
Here’s a link to the original paper:
http://ethomas.web.wesleyan.edu/wescourses/2004s/ees227/01/spandrels.html
And a good follow-up link:
http://en.wikipedia.org/wiki/Spandrel_(biology)
And to really feel good:
http://www.online-literature.com/voltaire/candide/
Or it could just be that finger prints do nothing useful and are just a leftover of evolution that didn’t make enough of a difference to bother getting rid of. Just because we have something doesn’t mean it has to be useful. Sure I can keep spare change in my appendix and wag my tail nub when I am happy but that’s just me.
Steve, where did you get the bit about “…behaves more like rubber – where friction is mainly a function of surface area, not the force of contact.”?
I’m pretty sure that friction is simply a measure of force times coefficient of friction, rubber or not.
http://www.worsleyschool.net/science/files/tires/andfriction.html