What your hands really say about you

Rembrandt instinctively understood it. In many of his portraits, he painted the hands of his subjects with as much care as the face. Hands speak volumes about the sitter's status, age and lifestyle, be they the gnarled, wrinkled hands of a poor woman or the smooth, confidently folded ones of a nobleman.

Now, science is catching up, and it's clear that your hands say all sorts of things about you. They hold clues to your development, personality, health and fitness -- if you just know where to look.

Hands define us as a species, too.

"People think language makes us human, but it's the hand," says surgeon Simon Kay, who recently led the UK's first hand transplant operation at St. James' University Hospital, Leeds. Hand gestures may have been the forerunner of language, and they remain central to communication, even if we don't always notice them.

Some cultures have made gesticulation an art form. Sicilian people, for example, have a huge range of hand signals, and the traditional dances of India and Bali are characterized by precise gestures, each with a specific meaning.

Above all, the fiendish design of our hands allows us to interact with the world like no other animal. From writing, drawing and making music to building nuclear power stations, almost everything we do with them is peculiarly human.

We also developed flattened tips to our finger bones. These support fleshy finger pads, providing a greater sensing area and making our grip more stable.

David Carrier, of the University of Utah in Salt Lake City, argues that the ability to form a fist, with shorter fingers curling into the palm, buttressed by a longer thumb, would have given an advantage to our male ancestors when competing for mates. The shape of our hands is a trade-off between dexterity and fighting, and in general women are more dexterous than men, he suggests.

Whatever forces shaped the human hand, we are not born dexterous. Children only gradually learn to manipulate objects. Aged around 1, they go from grasping with their whole hand to using their thumb and index finger, and fine coordination takes at least 10 years to develop fully. Handedness is mostly down to genetics, but children can learn to exploit their less-preferred hand if necessary.

And we're always learning. Until recently, people were used to typewriter-sized keyboards, says Lynette Jones, who studies tactile interfaces at the Massachusetts Institute of Technology, but we have quickly adapted to the tiny keypads on smartphones.

Still, there are some things our hands just cannot do.

"You can't really move one finger at a time," says neuroscientist Marc Schieber at University of Rochester Medical Center in New York. You think you can, he adds, but you only have to look at your hands as you type to realize that's not the case. The reason is that our fingers are linked to each other by neural circuits and physical ties. For example, the ring and little fingers share a tendon, making it very difficult to move them independently.

With around 17,000 receptors of various types, each hand has a sensory capacity similar to that of the eye. The ability to discern tactile sensations varies, though. Touch sensitivity is highly heritable and appears to have genes in common with hearing. Research published last year showed that it tends to be less acute in people with poor hearing, perhaps because hearing and touch both involve mechanosensory receptors.

By contrast, blind people are often able to recognize an extraordinary variety of different sensations, Kay says. In addition, women generally have greater sensitivity than men, by virtue of their smaller fingers.

Male and female hands differ in another way, too. In men, the index finger tends to be shorter than the ring finger, whereas in women, they're more equal. A study in 2003 by John Manning, now at Swansea University, UK, found that the ratio of these two fingers' lengths reflects the level of testosterone a fetus was exposed to in the womb.

Recent research questions this conclusion, but hundreds of studies reveal that the ratio is linked with a variety of characteristics, including sexuality. More "masculine" ratios correlate with traits ranging from risk-taking and financial acumen to athleticism and autism.

One very personal feature of your hands is definitely laid down in the womb: Your fingerprints, which take shape between weeks 10 to 16 of gestation. Although genes influence their general patterns -- the whorls, loops and arches -- the details are affected by factors such as the position of a fetus and its hands. As a result, even the fingerprints of identical twins are different.

Why we have fingerprints at all is another matter. They may increase touch sensitivity, protect the fingertips by allowing the skin to stretch, or help with drainage in wet conditions. Perhaps surprisingly, the idea that they increase friction and so improve grip does not stand up.

While fingerprints are unchanging, the appearance of your nails reflects health and nutrition. Changes in their color, smoothness, shape, and whether they develop any ridges are not usually the first sign of disease, but can indicate a host of problems, ranging from vitamin deficiency to heart disease and cancer.

For example, yellow nails may signify diabetes, while half-white, half-dark ones suggest liver and kidney disease. Deep, horizontal ridges are nothing to worry about, though; they're usually a natural consequence of aging.

It's no wonder we identify so strongly with our hands. They're both highly personal, yet always on display. Whether you adorn them with rings, watches and nail paint, or simply embrace their lines and wrinkles, there's no other body part more familiar. You might even say you know them...like the back of your hand.

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