For thousands of years, palm readers have examined hands in order to predict the future. As it turns out, they may not have been entirely off base: the human hand contains a wealth of information. Because a baby’s hands form early in gestation, researchers like to say that they amount to a “fossilized record” of early development, one that may provide insight into future well-being.
For example, hundreds of studies have shown that the ratio of index-finger to ring-finger length correlates with many traits. Most researchers believe that this is because the ring finger’s length indicates prenatal testosterone exposure, which is known to have a lasting influence. Compared with a man who has a shorter ring finger, a man whose ring finger is longer than his index finger is likely to have a more attractive face , greater athletic talent , a longer penis , and—perhaps not coincidentally—more children . A male whose index fingers are longer, meanwhile, is more prone to schizophrenia  and early heart disease . Then again, he is less likely to be autistic  or to have ADHD . Digit ratio may even reveal something about a person’s propensity for self-control: research inspired by the famous “marshmallow study”—which measured preschoolers’ ability to delay gratification—found that kids with longer index than ring fingers were more likely to resist temptation .
You may have heard that the more symmetrical a person’s face and body are, the more attractive he is considered to be, perhaps because symmetry suggests good genes. And indeed, matching hands bode well: A man whose left and right fingers match up is apt to have faster and more plentiful sperm than one whose fingers don’t match . Another study found that men with mismatched hands reported being depressed more frequently .
The lines on hands may also provide clues about early development. It’s well known that people with Down syndrome and people with fetal alcohol syndrome, as well as children of women who had measles while pregnant, are more likely than other people to have a “simian crease,” a horizontal line stretching from one edge of the palm to the other. In addition, although the overall research is not conclusive, some studies suggest that abnormal fingerprint patterns, such as those with a higher-than-average number of certain whorls, arches, or ridges, appear disproportionately among people with schizophrenia  and people who develop diabetes in middle age , possibly signaling a gestational disruption such as maternal illness. And males are more likely to have abnormal fingerprint patterns than females are—perhaps, researchers have suggested, because male embryos are more vulnerable to environmental influences .
Speaking of fingerprints, one study found that people who consume large amounts of salt and preservatives leave sweatier, saltier prints that are more likely to corrode certain metals, and thus are easier for detectives to pick up off, say, a bullet . For lawbreakers who love junk food, this finding may be the most predictive of all—at least when it comes to where they will be living for the next five to 10 years.