Palm readers profess to see the future in a person's hands but fingers might offer clues about a person's prenatal past, a developmental period that possibly influences behavior and health later in life, scientists say.
Despite the discovery of several genes known to cause the rare hereditary form of the deadly disease that killed Yankee slugger Lou Gehrig at 37, the underlying cause of the common form of ALS remains a mystery. But a team of British researchers thinks prenatal testosterone exposure might be involved.
"We know that ALS is about 1.5 times commoner in men than in women, and we know that men are exposed to more testosterone during development than women," said Dr. Ammar Al-Chalabi, professor of neurology and complex disease genetics at London's Institute of Psychiatry and director of the Motor Neuron Disease Care and Research Center at King's College London in the U.K. "We also know that nerve cells have many testosterone receptors on them, and if you disrupt those receptors, those cells die. They need testosterone for some reason."
Because few pregnant women would be willing to offer up their baby-bearing bellies for prenatal testosterone testing, researchers have come up with a proxy: finger length. During development, the male hormone is thought to influence the ratio of index finger to ring finger length, called the 2D:4D ratio (second digit-to-fourth digit). A relatively long ring finger and therefore low 2D:4D ratio is thought to reflect high levels of prenatal testosterone. That's why men, on average, have a lower ratio than women.
Based on the careful calculations of four independent measurers who were unaware which subjects had ALS, the researchers found that the 2D:4D ratio among 47 people with ALS was significantly lower than in 63 controls. The study was published Monday in the Journal of Neurology, Neurosurgery, and Psychiatry.
But Al-Chalabi cautioned that having a relatively long ring finger doesn't boost a individual's risk for ALS.
"It's purely averages," he said. "Just as you couldn't look at a hand and say definitively whether someone was a man or woman based on finger length, you can't look at one particular hand and say that's someone's going to have ALS. But if you looked at 1,000 hands, you'd be able to say there's a tendency."
A low 2D:4D ratio has also been implicated in aggression, sexuality, intelligence and even high-stakes stock trading ability. It has also been linked to athleticism, which is thought to be a risk factor for ALS.
Nevertheless, Al-Chalabi stressed that the finger length test is no more a predictor of ALS than it is of being male. If replicated, however, the finding could open new doors for ALS research.
"It's surprising that something that happened during development could influence something that happens to you much later in life," Al-Chalabi said. "I think it's intriguing, and if it can be shown again I think it could become much more important."