From June 24, 2000 

New Scientist  by Alison Motluk

    TIME WAS, fingers were for playing the piano and pulling a trigger. The most a set of hands could betray was how often the dishes got washed. Then scientists announced that our fingers may reveal personal details about our sex lives-whether we're gay; for instance, and how likely we are to be infertile- and suddenly hands are being slid deep into pockets. After all, that's not the kind of information you want to be waving around in public.

Sexual preferences and fertility aren't the only secrets on display, either. Scientists now suspect that fingers may reveal our risk of suffering a heart attack or developing breast cancer. They may shed light on disorders such as autism and dyslexia or gifts such as musical ability.

At first blush, the idea that a person's hands can tell intimate stories about them seems outrageous. But on closer inspection, there might be something in it. Fingers are formed early in fetal development, at around the end of the first trimester a very critical time. Just as hand development is influenced by the unique hormonal bath in the uterus, so is the development of the brain, gonads and heart. Since the relative lengths of our fingers seem to stay fixed throughout our lives, they are robust markers of what our early life was like in the womb-fetal fossils, if you will.

Take a close look at your own hands. Pay particular attention to your index and ring fingers. In women, the two fingers tend to be almost equal in length, as measured from the crease nearest the palm to the fingertip. In men, the ring finger tends to be much longer. The ratio of the lengths of the index finger to the ring finger is called the 2D:4D ratio, and low ratios are considered "masculine", high ratios "feminine".

 

Bodily characteristics that develop in distinctly masculine and feminine ways are usually the product of sex hormones. Some features differentiate at puberty, such as breasts and jaws. But other sex differences are already set by the time we're born, relative finger lengths among them, and seem to be the result of fetal androgens masculinising the males. some-of those hormones come from fetal testes and adrenals, the rest make it across the placenta from the mother. But exactly how much comes from whom-and what alters the balance-are still a bit of a mystery.

"Prenatal development is a black box," says John Manning of the University of Liverpool. He is one of a small number of scientist&: beginning to wonder if fingers could be used as a way of peering into that box.

In a paper just published in the journal Medical Hypotheses (vol. 54, p 855), Manning highlights conditions such as heart disease, breast cancer, autism and dyslexia.  Both heart disease (in men) and breast cancer have been linked with high -levels of the female hormones oestrogen and progesterone. Most of the studies of this link have looked at circulating levels in the adult, but evidence is mounting that too much of the wrong hormone in the womb may be the real culprit.

Oimitrios Tricopou1os, an epidemiologist at  Harvard University, proposed a decade ago that breast cancer may originate in utero (The Lancet, vol. 335, p 939). He suggested that high concentrations of estrogen may create a "fertile soil" for cancer to develop later in life, He also thought that variability in estrogen levels during pregnancy may help to explain why breast cancer rates are generally higher in women born to Caucasian mothers compared with those born to Oriental or younger mothers. Recently he and his colleague Karin Michels showed that high birth weight in girls-another sign of high prenatal estrogen levels-was associated with an increased risk of breast cancer.

If high estrogen levels are indeed to blame, Manning thinks that high 2D:4D

ratios could be used to identify women who are at increased risk of breast cancer. "1 don't know of other sexually dimorphic traits that are so stable," he says. "That's what makes it so exciting." He interviewed 118 women in a breast cancer clinic, measured their finger lengths and noted how old they were when the first tumour appeared. "It was earlier if there was a higher ratio," he says.

The developing brain is also sensitive to hormones in utero; Knowing this; Norman Geschwind and his graduate student Albert Galaburda, now at Harvard Medical School, made a controversial claim in 1985. They suggested that prenatal testosterone slows the growth of certain areas of the left hemisphere and facilitates the growth of corresponding regions of the right hemisphere. At the time they wondered whether testosterone was partly to blame for such things as left-handedness, dyslexia and autism (Archives of Neurology, vol43,p 428).

Galaburda and his colleagues have since developed a way to induce selective brain damage to the frontal lobe of newborn rats to mimic some of the symptoms of dyslexia. Curiously, while male rats with this kind of damage have  trouble responding to rapidly changing sounds – much like dyslexic humans – females don’t. “We induce the malformations in males and females,” he says, “but only the males have trouble.”

It is clear that there is a "genetic, component to dyslexia. But Galaburda thinks fetal  testosterone plays a role too by reducing plasticity in the young brain, making males, susceptible to brain malformations that females manage to overcome.

Intriguingly,  when female rats are given extra testosterone, they too show signs of dyslexia.

Manning hasn't yet checked the finger lengths of human dyslexics to see whether they also point to a testosterone link. But he has already checked out the handedness idea, using a dexterity test. People are not always straightforwardly right or left- handed: many have been trained to use their right hand for writing, even if they are more skilled with the left. So Manning and his colleagues tested how quickly 285 children could move 10 pegs from one row of holes to another row five inches away, using one or the other hand.

Children with low 2D:4D ratios suggestive of high exposure to testosterone in the womb-are more likely to be quicker with their left hands than the kids with higher ratios. This, he says, suggests that our degree of left-handedness and more generally the way the brain divides up tasks between left and right hemispheres - may be influenced by hormone levels in the womb.

Manning has begun examining autism too. He teamed up with Simon Baron- Cohen and Svetlana Lutchmaya from the University of Cambridge, who have used samples of amniotic fluid to directly measure the levels of hormones that babies are exposed to in the womb. When the children reached their first birthday, the researchers measured their vocabularies and ability to make eye contact. Poor language skills and an unwillingness to make eye contact are early hallmarks of autism. They found that babies who'd been exposed to high levels of testosterone in the womb fared the worst.

"What we're hoping to look at is whether finger ratios can be used as a proxy for hormones," says Lutchmaya. Amniocentesis is a risky procedure that only a few mothers choose to undergo, she says. But by measuring finger lengths instead, researchers can assess a random sample of children for possible early signs of impaired language and social skill development. Currently, they are checking the fingers of children for whom they have amniotic samples. Meanwhile, Manning and Baron-Cohen have looked at the finger ratios of 49 children with firm diagnoses of autism, 23 with a mild form of the disorder called Asperger's syndrome, and their families. The researchers found that autistic children tended to have very low 2D:4 ratios. Interestingly, children with Asperger's syndrome had ratios that fell between those of autistics and unaffected children. "It fits exceptionally well with the theory," says Manning.

Clearly genes playa role too in these conditions. But could fetal hormone levels explain other cognitive differences between the sexes? Janel Tortorice at Rutgers University in New Brunswick, New Jersey, thinks they may. She has measured finger ratios in 4D gay women and found that their hands were significantly different from those of heterosexual women-in fact, they tend to resemble those of heterosexual men.

But she has also found differences in the way these women's brains work. "They have more masculine fingers and more masculine cognition," she says. On tests of spatial and verbal ability, lesbian volunteers perform more like men than heterosexual women, she says. If this can be confirmed by further studies, perhaps Manning's most recent suggestion is not as outrageous as it sounds. He claims that musical talent, too, is nurtured in the womb.

Manning recruited 54 male musicians from a British symphony orchestra. He discovered that these men had significantly lower 2D:4D ratios than controls-they had a very "masculine" ratio. Even more striking, when he compared the top-ranked "first" musicians with their lower-ranked colleagues--a measure of their  relative ability-the former had significantly lower 2D:4D ratios. Could testosterone really predispose the brain to be more tuned in to music? Manning thinks so.

Musicians with short ring fingers and lesbians with long index fingers needn't lose heart, however. Even if fingers win a place in the pantheon of diagnostic medicine, it's  unlikely that prospective employers or partners will ever be able to predict our fortunes , from our hands. Tortorice reminds us that  males tend to be taller than females. "But," she says; "we don't use height to determine whether you're a man or a woman."

next: autism >

 

 ( with special thanks to The New Scientist)